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6. Natural selection occurs at the level of the individual but is measured in a population. Explain why this is so?

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6. Natural selection occurs at the level of the individual but is measured in a population. Explain why this is so?

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Natural selection in populations (article) | Khan Academy

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How natural selection works at the level of genes, alleles, genotypes, & phenotypes.

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7. What are life-history traits, and how are they related to natural selection?

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7. What are life-history traits, and how are they related to natural selection?

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Life History Strategy

Life history traits includes such factors as the number, size and sex ratio of offspring, the timing of reproduction, age and size at maturity and growth pattern, longevity, and so on. All of these are heritable to some degree and thus subject to natural selection. These characteristics are not mutually exclusive but interconnected. Organisms must allocate the total energy available among survival, growth, and reproduction and the trade-offs are inevitable. In behavioral ecology life history traits are vital to understanding evolutionary decisions of organisms, i.e., life history strategies. In the early 1990s the three books - Roff (1992), Stearns (1992), Charnov (1993) - came out and laid out the theoretical grounds for the study of life histories.

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F. Stuart ChapinIII, in Scaling Physiological Processes , 1993

G Community Change

Life-history traits such as dispersal ability, reproductive potential, and degree of incorporation into the seed bank are probably the most important traits governing community change, both through succession (Walker and Chapin, 1987) and during long-term vegetation change (Davis, 1981). In early primary succession, presence or absence of nitrogen fixers also strongly influences the rate and pathway of succession (Vitousek and Walker, 1989).

In summary, plant size is the trait that most frequently influences ecosystem processes. RGR, tissue chemistry, phenology, and storage are other traits of major importance. Any growth-form classification that links ecophysiological and ecosystem characteristics should, therefore, incorporate these traits (e.g., Table 16.1). However, to predict community change we need to know much more about life-history traits that determine dispersal and establishment. Perhaps even more important, we need to know how the species respond to human activity (e.g., harvest, disturbance, and dispersal) because changes in human activity probably will be more important than climate in determining future vegetation composition over the coming decades.

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Life History Characteristics

Life history characteristics, such as synchronized generations or life stages (perhaps dormant) that are not available for sampling (e.g., the seed bank or the nymphs of the 21-year cicada), can lead to apparently major changes in density. Arguably, the variation is the result of a particular definition of density. Whether such a definition is useful will depend on the question under investigation.

Some species have reproductive strategies that amplify fluctuations in density. Physiological and/or morphological changes (usually triggered by habitat characteristics) can lead to dramatic increases in population growth, for example, the shift from sexual to asexual reproduction when conditions are good.

An organism's mobility (or its ability to disperse propagules) can also influence its temporal variability. If it has an aggregated, patchy, spatial distribution (e.g., metapopulation structure) the degree of connectedness between the parts of the population and the degree to which they fluctuate independently can influence the extent of the variation of the whole population.

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All life history traits can be related to a fundamental dimension, or derived units. For instance, gestation and incubation periods, weaning or hatchling periods, age at first reproduction, and lifespan are all measured in units of times (i.e., days, weeks, months, or years), antler size of deer is measured in units of length (centimeters or meters), and body mass has a dimension of volume measured in units of mass. Such an heterogeneity in dimension has obvious practical consequences. For instance, the dimensional analysis implies an allometric exponent to one-third (not 1) when regressing antler size against body mass (on a log-log scale) to assess the type of allometry (i.e., positive, negative, or isometry). Indeed, physical constraints lead us to expect isometric relationships among traits sharing the same dimension. The dimensional analysis thus provides the theoretical background for constructing dimensionless numbers (e.g., ratio between age at first reproduction and lifespan) that are expected to be size-independent (see Charnov, 1993).

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Evolution of Mating Systems

Among life-history traits, reproductive characters that influence mating are of profound adaptive significance because they govern the character of genetic transmission between generations, the fitness of offspring and the amounts and distribution of genetic diversity in populations. The diversification of many angiosperm families (e.g., the orchid and phlox families) has been attributed to adaptive radiation of pollination and mating systems, often accompanying changes in the ecology of populations. Reproductive versatility is therefore a hallmark of angiosperm evolution and this is manifested by considerable inter- and intra-specific variation in mating systems. This variation implies frequent evolutionary transitions between mating systems, which are usually associated with changes to floral characters, pollination systems, and sex allocation in concert with life-history evolution. Ecological factors play a key role in driving the evolution of reproductive traits and in the maintenance of different mating systems. For example, long-lived tree species of stable plant communities are usually predominantly outcrossing whereas weedy colonists that occupy ephemeral environments are more commonly highly selfing. A major challenge is to determine the specific environmental, demographic, and genetic factors promoting changes in mating system.

The evolution of predominant self-fertilization (autogamy) from outcrossing represents the most important reproductive transition in angiosperms (Stebbins, 1974). There is evidence from numerous herbaceous families (e.g., the mustard and tomato families: Brassicaceae and Solanaceae, respectively) of multiple independent origins of autogamy and these transitions are often associated with the evolutionary breakdown of self-incompatibility, the primary genetic mechanism preventing selfing in plant populations. Investigation of evolutionary transitions from outcrossing to selfing is an active area of research today using comparative, experimental and genomic approaches, and theoretical models (e.g., Lloyd, 1992) on the selective mechanisms governing this transition have been particularly influential in guiding empirical research.

Why has the evolution of selfing from outcrossing been the focus of sustained interest for over a century? First, multiple independent transitions to selfing provide valuable opportunities to study convergent evolution, particularly the genetic and developmental basis of floral traits (e.g., small flowers, reduced floral display, and pollen production) that constitute the selfing syndrome (Sicard and Lenhard, 2011; Figure 1). Second, because selfing enables single plants to found colonies following long-distance dispersal, the shift to selfing can have significant biogeographical consequences (Figure 1), for example, by facilitating island colonization or migration to range margins where plant density may be low, a phenomenon known as Baker's Law (Baker, 1955). Third, selfing leads to restricted gene flow and reproductive isolation from ancestral outcrossing populations, potentially influencing speciation and lineage diversification (Wright Igic and Busch, 2013). Finally, because predominant selfing reduces the effective rate of recombination and effective population size there has been much recent interest on the genomic consequences of transitions to selfing, and the demographic and genetic processes causing genome-wide reductions in diversity in populations (Wright et al., 2013). A persistent theme concerns the extent to which the evolution of selfing is an 'evolutionary dead end' as a result of the ephemerality of selfing lineages compared to those that are outcrossing ( et al., 2008).

Diverse reproductive, demographic, and genetic factors influence mating system evolution in plants. Of particular importance is the relative fitness of progeny that results from cross- versus self-fertilization, a phenomenon known as inbreeding depression. Darwin (1876) conducted controlled pollination studies on 57 species from 30 families and, in virtually all cases, discovered that selfed offspring performed less well compared with outcrossed offspring. Darwin used this observation to explain the function of numerous floral adaptations in angiosperms; they were outcrossing mechanisms limiting the harmful effects of self-fertilization. Today, it is understood that inbreeding depression is near ubiquitous in outcrossing species and largely results from the expression of recessive deleterious alleles in genotypes made homozygous as a result of inbreeding (Charlesworth and Willis, 2009). Inbreeding depression features in most models of mating system evolution and its magnitude plays an important role in helping to explain the observed distribution of outcrossing rates in nature, although explaining the occurrence of a significant number of species with a mixture of outcrossing and selfing (mixed mating) has resulted in considerable debate (Goodwillie Dudash, 1990). Novel methods for estimating inbreeding depression in the field using genetic markers (Koelling Campbell et al., 2005). A current challenge is to determine ecologically relevant levels of inbreeding depression under field conditions as it is clear that comparisons conducted under glasshouse conditions significantly underestimate differences between selfed and outcrossed offspring ( et al., 2012) may alleviate this problem, as should recent studies (e.g., et al., 2013) on the extent to which biotic factors (pests and diseases) differentially influence the fitness of selfed and outcrossed offspring.

Inbreeding depression prevents the evolution of high selfing rates in most outcrossing populations. However, selfing evolves often and approximately 10-15% of flowering plants are predominant selfers, indicating that certain conditions can favor the selection of high selfing rates. The two most general mechanisms to explain the transition to selfing are: (1) The 'reproductive assurance hypothesis,' which traces back to Darwin and is the most frequently invoked explanation for why selfing often evolves under conditions of low density. It proposes that selfing is favored whenever pollen vectors and/or compatible mates are absent, limiting seed set by cross-pollination. (2) The 'automatic selection hypothesis,' which is based on R.A. Fisher's idea that a gene for selfing has a 3:2 transmission advantage when it arises in an outcrossing population. Determining the relative importance of these two hypotheses empirically is not straightforward, particularly if both processes operate during the transition to high selfing rates, as it requires determining the modes of self-fertilization in a population, and whether selection of genetic modifiers of the mating system occurs through pollen and/or seed (Busch and Delph, 2012). As yet, this has not been attempted in any species in which the transition to selfing has been investigated.

It has also been suggested that molecular data might be useful for distinguishing the two main hypotheses for the evolution of selfing. The demographic and genetic processes associated with reproductive assurance may result in a different genomic signature than those associated with automatic selection. This is based on the idea that when selfing evolves by reproductive assurance genetic bottlenecks causing the genome-wide loss of diversity should be common (Schoen Barrett et al., 1996). However, recent work demonstrating that linked selection can also reduce genome-wide diversity rapidly following the transition to selfing casts doubt on whether it will be possible to use molecular data to distinguish the two main hypotheses for the evolution of selfing ( et al., 2014). It seems likely that reproductive assurance plays a more important role than automatic selection in initiating the transition to selfing; however, at present the case rests largely on correlative ecological evidence on the geographical distribution of selfing populations and their occurrence under conditions of low density. Experimental evidence on the selective mechanisms driving the transition to selfing is generally lacking, despite a rich theoretical literature on the topic.

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Pathogen Virulence

If host life history characteristics may determine the evolution of immune priming and shape its specificity, those of parasites/pathogens are likely important too. In particular, parasite/pathogen virulence is believed to play a major role. Indeed, level of selection for costly immune priming is expected to be favored when parasites/pathogens are highly damaging and sterilizing to their hosts (Best Tidbury Pinaud Best Best et al., 2013). Accordingly, immune priming is found in the moth, Plodia interpunctella exposed to low doses of its natural baculovirus ( Plodia interpunctella granulosis virus), which, as an obligate killer, requires killing its host for transmission ( et al., 2011). Similarly, immune priming is found in the freshwater snail, Biomphalaria glabrata, following the infection by the human blood fluke, Schistosoma mansoni, which castrates its intermediate host ( et al., 2016). However, exposure to highly virulent parasites/pathogens may lead to divergent evolutionary outcomes at the population level, because it may favor the coexistence of diverse genotypes investing either strongly or weakly to immune priming, maintaining diversity for this trait ( et al., 2013). Finally, virulence should not be too high either, because hosts may not recover to benefit from investment in immune priming ( et al., 2013; Pigeault et al., 2016).

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Phenotypic Plasticity in Life History Traits

Variation in life history traits includes timing of reproduction, size and number of offspring, number of clutches, and individual growth rates. This variation is well known within amphibian and reptile populations and most often appears associated with variation in resource availability. This within-population variation is termed phenotypic plasticity and has been examined in both field and laboratory studies.

Both the number of eggs produced and the frequency of clutch production decline in New Mexico populations of the lizard Urosaurus ornatus as the result of reduced resources. Lower rainfall during one year reduces prey populations, and that in turn limits the lizard's ability to obtain adequate energy for reproduction. Variation in prey availability associated with rainfall also accounts for variation in growth rates in the lizard Sceloporus merriami.

In laboratory studies where energy intake has been precisely controlled, both garter snakes ( Thamnophis marcianus) and ratsnakes ( Pantherophis guttata) respond to increased resource availability by increasing clutch mass and the number of offspring produced. Pantherophis guttata also increases relative clutch mass in response to increased resource availability. In both of these snakes, individual offspring size does not respond to resource levels, indicating that offspring size is optimized within narrow limits in these snakes. Madagascar ground geckos, Paroedura picta, provide a nice example of energy allocation priorities during the course of their life history. Females first allocate energy to growth, such that well-fed females are similar to nutrient-deprived females in their growth trajectories. However, well-fed females invest heavily in reproduction (egg production), producing clutches of larger eggs over shorter time periods. Only well-fed females were able to store energy in the form of fat bodies. In the European frog Rana temporaria, individuals fed controlled high and low food rations for an entire season allocated their resources to both growth and reproduction. As in the two snakes, female frogs with a high feeding rate both grow more and produce more eggs. However, unlike the snakes, frogs with high feeding rates produce larger eggs.

Variation of biophysical regimes within reptile nests can influence the phenotypic variability of offspring. Female skinks ( Bassiana duperreyi) living in a mountainous region of southeastern Australia deposit clutches under logs or rocks at different depths, resulting in nearly identical patterns of temperature fluctuation. Experiments reveal that incubation periods decrease and developmental rates increase with increasing temperature. Hatchlings from clutches incubated at 22°C are larger in snout-vent length, have lower running speeds, and spend less time basking 1 month after birth than hatchlings from clutches incubated at 30°C. High variance associated with fluctuating nest temperatures also influences hatchling phenotypes (morphology, running speed, activity levels, and basking behavior), as does identity of the mother (maternal effects). Similar phenotypic responses to nest conditions occur in pythons ( Liasis fuscus) that facultatively brood their eggs in tropical Australia. Python nest temperatures are influenced by both nest site selection and whether the female broods the eggs.

Until recently, most experimental studies on the influence of temperature or moisture on reptile development and ultimately phenotypes of offspring produced have been performed under constant temperature conditions. Few, if any, amphibian or reptile eggs experience constant temperatures throughout development. Carefully designed field and laboratory experiments that attempt to mimic natural conditions offer the opportunity to determine the phenotypic consequences of variation in the developmental environment of amphibians and reptiles and will contribute considerably to our understanding of the evolution of life histories.

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VI.D. Recombination System and Species Ecology

Attempts to correlate life history traits and the level of recombination to justify theoretical concepts have occasionally encountered serious criticism. The fact that interspecific variation usually includes a broad complex of traits does not allow clear conclusions to be reached when testing theoretical predictions. One may take advantage of intraspecific comparisons, especially when testing hypotheses of recombination maintenance due to its role in adaptation to adverse environments. In particular, it is of great interest to compare, within the same species, the rate of recombination in populations inhabiting stressful versus mild conditions. Recombination rate was evaluated in Lamb D. melanogaster males from the opposing slopes of "Evolution Canyon" on Mount Carmel (Israel) manifesting strong microclimatic differentiation, mainly in temperature and humidity (Korol, 1999). A significant difference in RF was found in chromosome 3 in flies derived from the opposing slopes. In particular, higher (four-fold) RF was manifested by the sub-population of the more stressful south-facing slope compared to the north-facing slope. Parallel patterns have been found in Evolution Canyon in the soil fungus Sordaria fimicola in the rates of mutation, crossing-over, and gene conversion, all being higher on the south-facing slope ( et al., 1998).

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B Senescence and Physiological Trade-offs of Ornaments

Trade-offs between life-history traits are well documented and are thought to have physiological bases; however, the mechanistic underpinnings of these trade-offs are rarely addressed (Barnes and Partridge, 2003; Monaghan et al., 2009; Zera and Harshman, 2001). Physiological trade-offs have been typically considered in terms of energy allocation, where two or more functions compete for the same resources within the same individual (Stearns, 1992). However, these trade-offs can also result when one activity has negative effects on another activity (Alonso-Alvarez et al., 2004; Zera and Harshman, 2001). Oxidative stress is the imbalance between the production of damaging reactive oxygen species (ROS), which are by-products of normal metabolic activities, and antioxidant defenses, which scavenge ROS and limit their toxic effect (Finkel and Holbrook, 2000; Surai, 2002). The role played by oxidative stress in the ageing process has long been recognized (Beckman and Ames, 1998; Harman, 1957). More recently, it has been suggested that oxidative stress represents a cost of reproduction, and may be an important mediator in life history trade-offs (Alonso-Alvarez et al., 2004; Monaghan et al., 2009).

Furthermore, oxidative damage (or the capacity to avoid oxidative stress) might be one of the main proximal causes of the genuine information revealed to prospective females through carotenoid-based male sexual traits (Velando et al., 2008; von Schantz et al., 1999). Animals cannot synthesize carotenoids de novo, and have to ingest them with food. Consequently, it was first thought that carotenoid-based sexual signals were honest indicators of the individual ability to find high-quality food (Endler, 1980; Grether et al., 1999; Hill, 1990, 1991). However, beyond their role in the expression of colored signals, carotenoids have many other physiological functions as antioxidants, recycling other antioxidants and immunostimulants (Hill et al., 1999; Lozano, 1994; MacGraw, 2006; Pérez-Rodríguez, 2008; Surai, 2002; von Schantz et al., 1999). Hence, carotenoid-based ornaments may also function as indicators of the oxidative status of the bearer, with more intensely colored males signaling their enhanced ability to combat oxidative stress (Velando et al., 2008; von Schantz et al., 1999). Although the role of carotenoids as antioxidants is currently under debate, the function of carotenoid-based sexual traits as indicators of oxidative status has received some support from studies in fish and bird species (e.g., Helfestein et al., 2010; Pérez-Rodríguez et al., 2008; Pike et al., 2007, 2010).

The role of oxidative stress is thought to be one of the most important causes of senescence (Beckman and Ames, 1998). Older animals accumulate and are less able to prevent oxidative damage, probably due to a decline of antioxidant defenses and/or repair mechanisms (Beckman and Ames, 1998; Finkel and Holbrook, 2000). Thus, particularly in long-lived animals, females may benefit by preferring ornaments indicative of oxidative status, and probably the rate of senescence of potential mates. We assessed whether sexual attractiveness of blue-footed boobies is affected by immunologically induced oxidative damage (Torres and Velando, 2007). To induce oxidative damage in courting males, we injected a lipopolysaccharide, a bacterial cell wall component that mimics an infection without the negative effects of pathogens (Bonneaud et al., 2003), and is known to increase the formation of reactive oxygen species and lipid peroxidation products (a common type of oxidative damage; e.g., Wiesel et al., 2000; Escames et al., 2003). In this experiment, males of known age were immune-challenged and seven days later we analyzed the changes in the plasma levels of immunoglobulins and lipid peroxidation, along with changes in foot color. As expected, experimental males mounted an immune response, although, no differences were found between middle-aged (younger than ten years) and senescent males (older than ten years; Torres and Velando, 2007). The immune activation induced greater lipid peroxidation in senescent males than in middle-aged males (Torres and Velando, 2007). Interestingly, the immune challenge had no effect on the foot color of middle-aged males, but invoked changes in color expression of senescent males (Torres and Velando, 2007). Senescent males that were challenged immunologically suffered a decrease in foot color, changing to less attractive duller blue feet. Hence, senescent males, but not middle-aged ones, apparently suffered a cost when mounting an immune response in terms of oxidative damage and sexual attractiveness. Conversely, middle-aged males were able to defend against radicals produced by the immune response and did not lose sexual attractiveness. Thus, in the blue-footed booby, foot color of senescent males may mirror their oxidative status. These results also support the idea that oxidative stress is one of the proximal mechanisms of the trade-off between sexual signals and immune competence (von Schantz et al., 1999).

Interestingly, our results suggest that sexual ornamentation may be more honest in ageing animals, as theory predicts (Proulx et al., 2002). In iteroparous organisms, the trade-off between current and future reproduction varies through life. Thus, during the first breeding events high-quality individuals (those with high survival probabilities) may invest less in sexual signals than low-quality individuals due to residual reproductive costs, so that signal strength is not a reliable indicator of condition at this time (Lindström et al., 2009). Older animals should increase their current reproductive investment (including sexual signals) because their future breeding opportunities are relatively fewer (Velando et al., 2006b; Williams, 1966). Consequently, the honesty of sexual signals is expected to increase in old males, and our results support this idea. A recent experimental study of red-legged partridges also found that the reliability of sexual signals increases over the male's lifetime (Alonso-Alvarez et al., 2009).

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Life history traits includes such factors as the number, size and sex ratio of offspring, the timing of reproduction, age and size at maturity and growth pattern, longevity, and so on. All of these are heritable to some degree and thus subject to natural selection. These characteristics are not mutually exclusive but interconnected.

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5. Does natural selection have a goal? Explain why or why not? 

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5. Does natural selection have a goal? Explain why or why not? 

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Special Issue: Transitional Fossils

Evolution: Education and Outreach 2, 156-175 (2009) Cite this article

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Abstract

Natural selection is one of the central mechanisms of evolutionary change and is the process responsible for the evolution of adaptive features. Without a working knowledge of natural selection, it is impossible to understand how or why living things have come to exhibit their diversity and complexity. An understanding of natural selection also is becoming increasingly relevant in practical contexts, including medicine, agriculture, and resource management. Unfortunately, studies indicate that natural selection is generally very poorly understood, even among many individuals with postsecondary biological education. This paper provides an overview of the basic process of natural selection, discusses the extent and possible causes of misunderstandings of the process, and presents a review of the most common misconceptions that must be corrected before a functional understanding of natural selection and adaptive evolution can be achieved.

"There is probably no more original, more complex, and bolder concept in the history of ideas than Darwin's mechanistic explanation of adaptation."

Ernst Mayr ( 1982, p.481)

Introduction

Natural selection is a non-random difference in reproductive output among replicating entities, often due indirectly to differences in survival in a particular environment, leading to an increase in the proportion of beneficial, heritable characteristics within a population from one generation to the next. That this process can be encapsulated within a single (admittedly lengthy) sentence should not diminish the appreciation of its profundity and power. It is one of the core mechanisms of evolutionary change and is the main process responsible for the complexity and adaptive intricacy of the living world. According to philosopher Daniel Dennett ( 1995), this qualifies evolution by natural selection as "the single best idea anyone has ever had."

Natural selection results from the confluence of a small number of basic conditions of ecology and heredity. Often, the circumstances in which those conditions apply are of direct significance to human health and well-being, as in the evolution of antibiotic and pesticide resistance or in the impacts of intense predation by humans (e.g., Palumbi 2001; Jørgensen et al. 2007; Darimont et al. 2009). Understanding this process is therefore of considerable importance in both academic and pragmatic terms. Unfortunately, a growing list of studies indicates that natural selection is, in general, very poorly understood-not only by young students and members of the public but even among those who have had postsecondary instruction in biology.

As is true with many other issues, a lack of understanding of natural selection does not necessarily correlate with a lack of confidence about one's level of comprehension. This could be due in part to the perception, unfortunately reinforced by many biologists, that natural selection is so logically compelling that its implications become self-evident once the basic principles have been conveyed. Thus, many professional biologists may agree that "[evolution] shows how everything from frogs to fleas got here via a few easily grasped biological processes" (Coyne 2006; emphasis added). The unfortunate reality, as noted nearly 20 years ago by Bishop and Anderson ( 1990), is that "the concepts of evolution by natural selection are far more difficult for students to grasp than most biologists imagine." Despite common assumptions to the contrary by both students and instructors, it is evident that misconceptions about natural selection are the rule, whereas a working understanding is the rare exception.

The goal of this paper is to enhance (or, as the case may be, confirm) readers' basic understanding of natural selection. This first involves providing an overview of the basis and (one of the) general outcomes of natural selection as they are understood by evolutionary biologists1. This is followed by a brief discussion of the extent and possible causes of difficulties in fully grasping the concept and consequences of natural selection. Finally, a review of the most widespread misconceptions about natural selection is provided. It must be noted that specific instructional tools capable of creating deeper understanding among students generally have remained elusive, and no new suggestions along these lines are presented here. Rather, this article is aimed at readers who wish to confront and correct any misconceptions that they may harbor and/or to better recognize those held by most students and other non-specialists.

The Basis and Basics of Natural Selection

Though rudimentary forms of the idea had been presented earlier (e.g., Darwin and Wallace 1858 and several others before them), it was in On the Origin of Species by Means of Natural Selection that Darwin ( 1859) provided the first detailed exposition of the process and implications of natural selection2. According to Mayr ( 1982, 2001), Darwin's extensive discussion of natural selection can be distilled to five "facts" (i.e., direct observations) and three associated inferences. These are depicted in Fig. 1.

The basis of natural selection as presented by Darwin ( 1859), based on the summary by Mayr ( 1982)

Some components of the process, most notably the sources of variation and the mechanisms of inheritance, were, due to the limited available information in Darwin's time, either vague or incorrect in his original formulation. Since then, each of the core aspects of the mechanism has been elucidated and well documented, making the modern theory3 of natural selection far more detailed and vigorously supported than when first proposed 150 years ago. This updated understanding of natural selection consists of the elements outlined in the following sections.

Overproduction, Limited Population Growth, and the "Struggle for Existence"

A key observation underlying natural selection is that, in principle, populations have the capacity to increase in numbers exponentially (or "geometrically"). This is a simple function of mathematics: If one organism produces two offspring, and each of them produces two offspring, and so on, then the total number grows at an increasingly rapid rate (1 → 2 → 4 → 8 → 16 → 32 → 64... to 2n after n rounds of reproduction).

The enormity of this potential for exponential growth is difficult to fathom. For example, consider that beginning with a single Escherichia coli bacterium, and assuming that cell division occurs every 30 minutes, it would take less than a week for the descendants of this one cell to exceed the mass of the Earth. Of course, exponential population expansion is not limited to bacteria. As Nobel laureate Jacques Monod once quipped, "What is true for E. coli is also true for the elephant," and indeed, Darwin ( 1859) himself used elephants as an illustration of the principle of rapid population growth, calculating that the number of descendants of a single pair would swell to more than 19,000,000 in only 750 years4. Keown ( 1988) cites the example of oysters, which may produce as many as 114,000,000 eggs in a single spawn. If all these eggs grew into oysters and produced this many eggs of their own that, in turn, survived to reproduce, then within five generations there would be more oysters than the number of electrons in the known universe.

Clearly, the world is not overrun with bacteria, elephants, or oysters. Though these and all other species engage in massive overproduction (or "superfecundity") and therefore could in principle expand exponentially, in practice they do not5. The reason is simple: Most offspring that are produced do not survive to produce offspring of their own. In fact, most population sizes tend to remain relatively stable over the long term. This necessarily means that, on average, each pair of oysters produces only two offspring that go on to reproduce successfully-and that 113,999,998 eggs per female per spawn do not survive (see also Ridley 2004). Many young oysters will be eaten by predators, others will starve, and still others will succumb to infection. As Darwin ( 1859) realized, this massive discrepancy between the number of offspring produced and the number that can be sustained by available resources creates a "struggle for existence" in which often only a tiny fraction of individuals will succeed. As he noted, this can be conceived as a struggle not only against other organisms (especially members of the same species, whose ecological requirements are very similar) but also in a more abstract sense between organisms and their physical environments.

Variation and Inheritance

Variation among individuals is a fundamental requirement for evolutionary change. Given that it was both critical to his theory of natural selection and directly counter to much contemporary thinking, it should not be surprising that Darwin ( 1859) expended considerable effort in attempting to establish that variation is, in fact, ubiquitous. He also emphasized the fact that some organisms-namely relatives, especially parents and their offspring-are more similar to each other than to unrelated members of the population. This, too, he realized is critical for natural selection to operate. As Darwin ( 1859) put it, "Any variation which is not inherited is unimportant for us." However, he could not explain either why variation existed or how specific characteristics were passed from parent to offspring, and therefore was forced to treat both the source of variation and the mechanism of inheritance as a "black box."

The workings of genetics are no longer opaque. Today, it is well understood that inheritance operates through the replication of DNA sequences and that errors in this process (mutations) and the reshuffling of existing variants (recombination) represent the sources of new variation. In particular, mutations are known to be random (or less confusingly, "undirected") with respect to any effects that they may have. Any given mutation is merely a chance error in the genetic system, and as such, its likelihood of occurrence is not influenced by whether it will turn out to be detrimental, beneficial, or (most commonly) neutral.

As Darwin anticipated, extensive variation among individuals has now been well established to exist at the physical, physiological, and behavioral levels. Thanks to the rise of molecular biology and, more recently, of genomics, it also has been possible to document variation at the level of proteins, genes, and even individual DNA nucleotides in humans and many other species.

Non-random Differences in Survival and Reproduction

Darwin saw that overproduction and limited resources create a struggle for existence in which some organisms will succeed and most will not. He also recognized that organisms in populations differ from one another in terms of many traits that tend to be passed on from parent to offspring. Darwin's brilliant insight was to combine these two factors and to realize that success in the struggle for existence would not be determined by chance, but instead would be biased by some of the heritable differences that exist among organisms. Specifically, he noted that some individuals happen to possess traits that make them slightly better suited to a particular environment, meaning that they are more likely to survive than individuals with less well suited traits. As a result, organisms with these traits will, on average, leave more offspring than their competitors.

Whereas the origin of a new genetic variant occurs at random in terms of its effects on the organism, the probability of it being passed on to the next generation is absolutely non-random if it impacts the survival and reproductive capabilities of that organism. The important point is that this is a two-step process: first, the origin of variation by random mutation, and second, the non-random sorting of variation due to its effects on survival and reproduction (Mayr 2001). Though definitions of natural selection have been phrased in many ways (Table 1), it is this non-random difference in survival and reproduction that forms the basis of the process.

Table 1 Glossary definitions of "natural selection" and "fitness" from leading evolutionary textbooks

Darwinian Fitness

The Meaning of Fitness in Evolutionary Biology

In order to study the operation and effects of natural selection, it is important to have a means of describing and quantifying the relationships between genotype (gene complement), phenotype (physical and behavioral features), survival, and reproduction in particular environments. The concept used by evolutionary biologists in this regard is known as "Darwinian fitness," which is defined most simply as a measure of the total (or relative) reproductive output of an organism with a particular genotype (Table 1). In the most basic terms, one can state that the more offspring an individual produces, the higher is its fitness. It must be emphasized that the term "fitness," as used in evolutionary biology, does not refer to physical condition, strength, or stamina and therefore differs markedly from its usage in common language.

"Survival of the Fittest" is Misleading

In the fifth edition of the Origin (published in 1869), Darwin began using the phrase "survival of the fittest", which had been coined a few years earlier by British economist Herbert Spencer, as shorthand for natural selection. This was an unfortunate decision as there are several reasons why "survival of the fittest" is a poor descriptor of natural selection. First, in Darwin's context, "fittest" implied "best suited to a particular environment" rather than "most physically fit," but this crucial distinction is often overlooked in non-technical usage (especially when further distorted to "only the strong survive"). Second, it places undue emphasis on survival: While it is true that dead organisms do not reproduce, survival is only important evolutionarily insofar as it affects the number of offspring produced. Traits that make life longer or less difficult are evolutionarily irrelevant unless they also influence reproductive output. Indeed, traits that enhance net reproduction may increase in frequency over many generations even if they compromise individual longevity. Conversely, differences in fecundity alone can create differences in fitness, even if survival rates are identical among individuals. Third, this implies an excessive focus on organisms, when in fact traits or their underlying genes equally can be identified as more or less fit than alternatives. Lastly, this phrase is often misconstrued as being circular or tautological (Who survives? The fittest. Who are the fittest? Those who survive). However, again, this misinterprets the modern meaning of fitness, which can be both predicted in terms of which traits are expected to be successful in a specific environment and measured in terms of actual reproductive success in that environment.

Which Traits Are the Most Fit?

Directional natural selection can be understood as a process by which fitter traits (or genes) increase in proportion within populations over the course of many generations. It must be understood that the relative fitness of different traits depends on the current environment. Thus, traits that are fit now may become unfit later if the environment changes. Conversely, traits that have now become fit may have been present long before the current environment arose, without having conferred any advantage under previous conditions. Finally, it must be noted that fitness refers to reproductive success relative to alternatives here and now-natural selection cannot increase the proportion of traits solely because they may someday become advantageous. Careful reflection on how natural selection actually works should make it clear why this is so.

Natural Selection and Adaptive Evolution

Natural Selection and the Evolution of Populations

Though each has been tested and shown to be accurate, none of the observations and inferences that underlies natural selection is sufficient individually to provide a mechanism for evolutionary change6. Overproduction alone will have no evolutionary consequences if all individuals are identical. Differences among organisms are not relevant unless they can be inherited. Genetic variation by itself will not result in natural selection unless it exerts some impact on organism survival and reproduction. However, any time all of Darwin's postulates hold simultaneously-as they do in most populations-natural selection will occur. The net result in this case is that certain traits (or, more precisely, genetic variants that specify those traits) will, on average, be passed on from one generation to the next at a higher rate than existing alternatives in the population. Put another way, when one considers who the parents of the current generation were, it will be seen that a disproportionate number of them possessed traits beneficial for survival and reproduction in the particular environment in which they lived.

The important points are that this uneven reproductive success among individuals represents a process that occurs in each generation and that its effects are cumulative over the span of many generations. Over time, beneficial traits will become increasingly prevalent in descendant populations by virtue of the fact that parents with those traits consistently leave more offspring than individuals lacking those traits. If this process happens to occur in a consistent direction-say, the largest individuals in each generation tend to leave more offspring than smaller individuals-then there can be a gradual, generation-by-generation change in the proportion of traits in the population. This change in proportion and not the modification of organisms themselves is what leads to changes in the average value of a particular trait in the population. Organisms do not evolve; populations evolve.

Adaptation

The term "adaptation" derives from ad + aptus, literally meaning "toward + fit". As the name implies, this is the process by which populations of organisms evolve in such a way as to become better suited to their environments as advantageous traits become predominant. On a broader scale, it is also how physical, physiological, and behavioral features that contribute to survival and reproduction ("adaptations") arise over evolutionary time. This latter topic is particularly difficult for many to grasp, though of course a crucial first step is to understand the operation of natural selection on smaller scales of time and consequence. (For a detailed discussion of the evolution of complex organs such as eyes, see Gregory 2008b.)

On first pass, it may be difficult to see how natural selection can ever lead to the evolution of new characteristics if its primary effect is merely to eliminate unfit traits. Indeed, natural selection by itself is incapable of producing new traits, and in fact (as many readers will have surmised), most forms of natural selection deplete genetic variation within populations. How, then, can an eliminative process like natural selection ever lead to creative outcomes?

To answer this question, one must recall that evolution by natural selection is a two-step process. The first step involves the generation of new variation by mutation and recombination, whereas the second step determines which randomly generated variants will persist into the next generation. Most new mutations are neutral with respect to survival and reproduction and therefore are irrelevant in terms of natural selection (but not, it must be pointed out, to evolution more broadly). The majority of mutations that have an impact on survival and reproductive output will do so negatively and, as such, will be less likely than existing alternatives to be passed on to subsequent generations. However, a small percentage of new mutations will turn out to have beneficial effects in a particular environment and will contribute to an elevated rate of reproduction by organisms possessing them. Even a very slight advantage is sufficient to cause new beneficial mutations to increase in proportion over the span of many generations.

Biologists sometimes describe beneficial mutations as "spreading" or "sweeping" through a population, but this shorthand is misleading. Rather, beneficial mutations simply increase in proportion from one generation to the next because, by definition, they happen to contribute to the survival and reproductive success of the organisms carrying them. Eventually, a beneficial mutation may be the only alternative left as all others have ultimately failed to be passed on. At this point, that beneficial genetic variant is said to have become "fixed" in the population.

Again, mutation does not occur in order to improve fitness-it merely represents errors in genetic replication. This means that most mutations do not improve fitness: There are many more ways of making things worse than of making them better. It also means that mutations will continue to occur even after previous beneficial mutations have become fixed. As such, there can be something of a ratcheting effect in which beneficial mutations arise and become fixed by selection, only to be supplemented later by more beneficial mutations which, in turn, become fixed. All the while, neutral and deleterious mutations also occur in the population, the latter being passed on at a lower rate than alternatives and often being lost before reaching any appreciable frequency.

Of course, this is an oversimplification-in species with sexual reproduction, multiple beneficial mutations may be brought together by recombination such that the fixation of beneficial genes need not occur sequentially. Likewise, recombination can juxtapose deleterious mutations, thereby hastening their loss from the population. Nonetheless, it is useful to imagine the process of adaptation as one in which beneficial mutations arise continually (though perhaps very infrequently and with only minor positive impacts) and then accumulate in the population over many generations.

The process of adaptation in a population is depicted in very basic form in Fig. 2. Several important points can be drawn from even such an oversimplified rendition:

1. 1.
Mutations are the source of new variation. Natural selection itself does not create new traits; it only changes the proportion of variation that is already present in the population. The repeated two-step interaction of these processes is what leads to the evolution of novel adaptive features.

2. 2.
Mutation is random with respect to fitness. Natural selection is, by definition, non-random with respect to fitness. This means that, overall, it is a serious misconception to consider adaptation as happening "by chance".

3. 3.
Mutations occur with all three possible outcomes: neutral, deleterious, and beneficial. Beneficial mutations may be rare and deliver only a minor advantage, but these can nonetheless increase in proportion in the population over many generations by natural selection. The occurrence of any particular beneficial mutation may be very improbable, but natural selection is very effective at causing these individually unlikely improvements to accumulate. Natural selection is an improbability concentrator.

4. 4.
No organisms change as the population adapts. Rather, this involves changes in the proportion of beneficial traits across multiple generations.

5. 5.
The direction in which adaptive change occurs is dependent on the environment. A change in environment can make previously beneficial traits neutral or detrimental and vice versa.

6. 6.
Adaptation does not result in optimal characteristics. It is constrained by historical, genetic, and developmental limitations and by trade-offs among features (see Gregory 2008b).

7. 7.
It does not matter what an "ideal" adaptive feature might be-the only relevant factor is that variants that happen to result in greater survival and reproduction relative to alternative variants are passed on more frequently. As Darwin wrote in a letter to Joseph Hooker (11 Sept. 1857), "I have just been writing an audacious little discussion, to show that organic beings are not perfect, only perfect enough to struggle with their competitors."

8. 8.
The process of adaptation by natural selection is not forward-looking, and it cannot produce features on the grounds that they might become beneficial sometime in the future. In fact, adaptations are always to the conditions experienced by generations in the past.

A highly simplified depiction of natural selection ( Correct) and a generalized illustration of various common misconceptions about the mechanism ( Incorrect). Properly understood, natural selection occurs as follows: ( A) A population of organisms exhibits variation in a particular trait that is relevant to survival in a given environment. In this diagram, darker coloration happens to be beneficial, but in another environment, the opposite could be true. As a result of their traits, not all individuals in Generation 1 survive equally well, meaning that only a non-random subsample ultimately will succeed in reproducing and passing on their traits ( B). Note that no individual organisms in Generation 1 change, rather the proportion of individuals with different traits changes in the population. The individuals who survive from Generation 1 reproduce to produce Generation 2. ( C) Because the trait in question is heritable, this second generation will (mostly) resemble the parent generation. However, mutations have also occurred, which are undirected (i.e., they occur at random in terms of the consequences of changing traits), leading to both lighter and darker offspring in Generation 2 as compared to their parents in Generation 1. In this environment, lighter mutants are less successful and darker mutants are more successful than the parental average. Once again, there is non-random survival among individuals in the population, with darker traits becoming disproportionately common due to the death of lighter individuals ( D). This subset of Generation 2 proceeds to reproduce. Again, the traits of the survivors are passed on, but there is also undirected mutation leading to both deleterious and beneficial differences among the offspring ( E). ( F) This process of undirected mutation and natural selection (non-random differences in survival and reproductive success) occurs over many generations, each time leading to a concentration of the most beneficial traits in the next generation. By Generation N, the population is composed almost entirely of very dark individuals. The population can now be said to have become adapted to the environment in which darker traits are the most successful. This contrasts with the intuitive notion of adaptation held by most students and non-biologists. In the most common version, populations are seen as uniform, with variation being at most an anomalous deviation from the norm ( X). It is assumed that all members within a single generation change in response to pressures imposed by the environment ( Y). When these individuals reproduce, they are thought to pass on their acquired traits. Moreover, any changes that do occur due to mutation are imagined to be exclusively in the direction of improvement ( Z). Studies have revealed that it can be very difficult for non-experts to abandon this intuitive interpretation in favor of a scientifically valid understanding of the mechanism. Diagrams based in part on Bishop and Anderson ( 1990)

Natural Selection Is Elegant, Logical, and Notoriously Difficult to Grasp

The Extent of the Problem

In its most basic form, natural selection is an elegant theory that effectively explains the obviously good fit of living things to their environments. As a mechanism, it is remarkably simple in principle yet incredibly powerful in application. However, the fact that it eluded description until 150 years ago suggests that grasping its workings and implications is far more challenging than is usually assumed.

Three decades of research have produced unambiguous data revealing a strikingly high prevalence of misconceptions about natural selection among members of the public and in students at all levels, from elementary school pupils to university science majors (Alters 2005; Bardapurkar 2008; Table 2)7. A finding that less than 10% of those surveyed possess a functional understanding of natural selection is not atypical. It is particularly disconcerting and undoubtedly exacerbating that confusions about natural selection are common even among those responsible for teaching it8. As Nehm and Schonfeld ( 2007) recently concluded, "one cannot assume that biology teachers with extensive backgrounds in biology have an accurate working knowledge of evolution, natural selection, or the nature of science."

Table 2 Summary of studies showing the high degree of misunderstanding of natural selection and adaptation among various groups of subjects

Why is Natural Selection so Difficult to Understand?

Two obvious hypotheses present themselves for why misunderstandings of natural selection are so widespread. The first is that understanding the mechanism of natural selection requires an acceptance of the historical fact of evolution, the latter being rejected by a large fraction of the population. While an improved understanding of the process probably would help to increase overall acceptance of evolution, surveys indicate that rates of acceptance already are much higher than levels of understanding. And, whereas levels of understanding and acceptance may be positively correlated among teachers (Vlaardingerbroek and Roederer 1997; Rutledge and Mitchell 2002; Deniz et al. 2008), the two parameters seem to be at most only very weakly related in students9 (Bishop and Anderson 1990; Demastes et al. 1995; Brem et al. 2003; Sinatra et al. 2003; Ingram and Nelson 2006; Shtulman 2006). Teachers notwithstanding, "it appears that a majority on both sides of the evolution-creation debate do not understand the process of natural selection or its role in evolution" (Bishop and Anderson 1990).

The second intuitive hypothesis is that most people simply lack formal education in biology and have learned incorrect versions of evolutionary mechanisms from non-authoritative sources (e.g., television, movies, parents). Inaccurate portrayals of evolutionary processes in the media, by teachers, and by scientists themselves surely exacerbate the situation (e.g., Jungwirth 1975a, b, 1977; Moore et al. 2002). However, this alone cannot provide a full explanation, because even direct instruction on natural selection tends to produce only modest improvements in students' understanding (e.g., Jensen and Finley 1995; Ferrari and Chi 1998; Nehm and Reilly 2007; Spindler and Doherty 2009). There also is evidence that levels of understanding do not differ greatly between science majors and non-science majors (Sundberg and Dini 1993). In the disquieting words of Ferrari and Chi ( 1998), "misconceptions about even the basic principles of Darwin's theory of evolution are extremely robust, even after years of education in biology."

Misconceptions are well known to be common with many (perhaps most) aspects of science, including much simpler and more commonly encountered phenomena such as the physics of motion (e.g., McCloskey et al. 1980; Halloun and Hestenes 1985; Bloom and Weisberg 2007). The source of this larger problem seems to be a significant disconnect between the nature of the world as reflected in everyday experience and the one revealed by systematic scientific investigation (e.g., Shtulman 2006; Sinatra et al. 2008). Intuitive interpretations of the world, though sufficient for navigating daily life, are usually fundamentally at odds with scientific principles. If common sense were more than superficially accurate, scientific explanations would be less counterintuitive, but they also would be largely unnecessary.

Conceptual Frameworks Versus Spontaneous Constructions

It has been suggested by some authors that young students simply are incapable of understanding natural selection because they have not yet developed the formal reasoning abilities necessary to grasp it (Lawson and Thompson 1988). This could be taken to imply that natural selection should not be taught until later grades; however, those who have studied student understanding directly tend to disagree with any such suggestion (e.g., Clough and Wood-Robinson 1985; Settlage 1994). Overall, the issue does not seem to be a lack of logic (Greene 1990; Settlage 1994), but a combination of incorrect underlying premises about mechanisms and deep-seated cognitive biases that influence interpretations.

Many of the misconceptions that block an understanding of natural selection develop early in childhood as part of "naïve" but practical understandings of how the world is structured. These tend to persist unless replaced with more accurate and equally functional information. In this regard, some experts have argued that the goal of education should be to supplant existing conceptual frameworks with more accurate ones (see Sinatra et al. 2008). Under this view, "Helping people to understand evolution...is not a matter of adding on to their existing knowledge, but helping them to revise their previous models of the world to create an entirely new way of seeing" (Sinatra et al. 2008). Other authors suggest that students do not actually maintain coherent conceptual frameworks relating to complex phenomena, but instead construct explanations spontaneously using intuitions derived from everyday experience (see Southerland et al. 2001). Though less widely accepted, this latter view gains support from the observation that naïve evolutionary explanations given by non-experts may be tentative and inconsistent (Southerland et al. 2001) and may differ depending on the type of organisms being considered (Spiegel et al. 2006). In some cases, students may attempt a more complex explanation but resort to intuitive ideas when they encounter difficulty (Deadman and Kelly 1978). In either case, it is abundantly clear that simply describing the process of natural selection to students is ineffective and that it is imperative that misconceptions be confronted if they are to be corrected (e.g., Greene 1990; Scharmann 1990; Settlage 1994; Ferrari and Chi 1998; Alters and Nelson 2002; Passmore and Stewart 2002; Alters 2005; Nelson 2007).

A Catalog of Common Misconceptions

Whereas the causes of cognitive barriers to understanding remain to be determined, their consequences are well documented. It is clear from many studies that complex but accurate explanations of biological adaptation typically yield to naïve intuitions based on common experience (Fig. 2; Tables 2 and 3). As a result, each of the fundamental components of natural selection may be overlooked or misunderstood when it comes time to consider them in combination, even if individually they appear relatively straightforward. The following sections provide an overview of the various, non-mutually exclusive, and often correlated misconceptions that have been found to be most common. All readers are encouraged to consider these conceptual pitfalls carefully in order that they may be avoided. Teachers, in particular, are urged to familiarize themselves with these errors so that they may identify and address them among their students.

Table 3 Major concepts relating to adaptive evolution by natural selection, summarizing both correct and intuitive (incorrect) interpretations (see also Fig. 2)

Teleology and the "Function Compunction"

Much of the human experience involves overcoming obstacles, achieving goals, and fulfilling needs. Not surprisingly, human psychology includes a powerful bias toward thoughts about the "purpose" or "function" of objects and behaviors-what Kelemen and Rosset ( 2009) dub the "human function compunction." This bias is particularly strong in children, who are apt to see most of the world in terms of purpose; for example, even suggesting that "rocks are pointy to keep animals from sitting on them" (Kelemen 1999a, b; Kelemen and Rosset 2009). This tendency toward explanations based on purpose ("teleology") runs very deep and persists throughout high school (Southerland et al. 2001) and even into postsecondary education (Kelemen and Rosset 2009). In fact, it has been argued that the default mode of teleological thinking is, at best, suppressed rather than supplanted by introductory scientific education. It therefore reappears easily even in those with some basic scientific training; for example, in descriptions of ecological balance ("fungi grow in forests to help decomposition") or species survival ("finches diversified in order to survive"; Kelemen and Rosset 2009).

Teleological explanations for biological features date back to Aristotle and remain very common in naïve interpretations of adaptation (e.g., Tamir and Zohar 1991; Pedersen and Halldén 1992; Southerland et al. 2001; Sinatra et al. 2008; Table 2). On the one hand, teleological reasoning may preclude any consideration of mechanisms altogether if simply identifying a current function for an organ or behavior is taken as sufficient to explain its existence (e.g., Bishop and Anderson 1990). On the other hand, when mechanisms are considered by teleologically oriented thinkers, they are often framed in terms of change occurring in response to a particular need (Table 2). Obviously, this contrasts starkly with a two-step process involving undirected mutations followed by natural selection (see Fig. 2 and Table 3).

Anthropomorphism and Intentionality

A related conceptual bias to teleology is anthropomorphism, in which human-like conscious intent is ascribed either to the objects of natural selection or to the process itself (see below). In this sense, anthropomorphic misconceptions can be characterized as either internal (attributing adaptive change to the intentional actions of organisms) or external (conceiving of natural selection or "Nature" as a conscious agent; e.g., Kampourakis and Zogza 2008; Sinatra et al. 2008).

Internal anthropomorphism or "intentionality" is intimately tied to the misconception that individual organisms evolve in response to challenges imposed by the environment (rather than recognizing evolution as a population-level process). Gould ( 1980) described the obvious appeal of such intuitive notions as follows:

Since the living world is a product of evolution, why not suppose that it arose in the simplest and most direct way? Why not argue that organisms improve themselves by their own efforts and pass these advantages to their offspring in the form of altered genes-a process that has long been called, in technical parlance, the "inheritance of acquired characters." This idea appeals to common sense not only for its simplicity but perhaps even more for its happy implication that evolution travels an inherently progressive path, propelled by the hard work of organisms themselves.

The penchant for seeing conscious intent is often sufficiently strong that it is applied not only to non-human vertebrates (in which consciousness, though certainly not knowledge of genetics and Darwinian fitness, may actually occur), but also to plants and even to single-celled organisms. Thus, adaptations in any taxon may be described as "innovations," "inventions," or "solutions" (sometimes "ingenious" ones, no less). Even the evolution of antibiotic resistance is characterized as a process whereby bacteria "learn" to "outsmart" antibiotics with frustrating regularity. Anthropomorphism with an emphasis on forethought is also behind the common misconception that organisms behave as they do in order to enhance the long-term well-being of their species. Once again, a consideration of the actual mechanics of natural selection should reveal why this is fallacious.

All too often, an anthropomorphic view of evolution is reinforced with sloppy descriptions by trusted authorities (Jungwirth 1975a, b, 1977; Moore et al. 2002). Consider this particularly egregious example from a website maintained by the National Institutes of Health10:

As microbes evolve, they adapt to their environment. If something stops them from growing and spreading-such as an antimicrobial-they evolve new mechanisms to resist the antimicrobials by changing their genetic structure. Changing the genetic structure ensures that the offspring of the resistant microbes are also resistant.

Fundamentally inaccurate descriptions such as this are alarmingly common. As a corrective, it is a useful exercise to translate such faulty characterizations into accurate language11. For example, this could read:

Bacteria that cause disease exist in large populations, and not all individuals are alike. If some individuals happen to possess genetic features that make them resistant to antibiotics, these individuals will survive the treatment while the rest gradually are killed off. As a result of their greater survival, the resistant individuals will leave more offspring than susceptible individuals, such that the proportion of resistant individuals will increase each time a new generation is produced. When only the descendants of the resistant individuals are left, the population of bacteria can be said to have evolved resistance to the antibiotics.

Use and Disuse

Many students who manage to avoid teleological and anthropomorphic pitfalls nonetheless conceive of evolution as involving change due to use or disuse of organs. This view, which was developed explicitly by Jean-Baptiste Lamarck but was also invoked to an extent by Darwin ( 1859), emphasizes changes to individual organisms that occur as they use particular features more or less. For example, Darwin ( 1859) invoked natural selection to explain the loss of sight in some subterranean rodents, but instead favored disuse alone as the explanation for loss of eyes in blind, cave-dwelling animals: "As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse." This sort of intuition remains common in naïve explanations for why unnecessary organs become vestigial or eventually disappear. Modern evolutionary theory recognizes several reasons that may account for the loss of complex features (e.g., Jeffery 2005; Espinasa and Espinasa 2008), some of which involve direct natural selection, but none of which is based simply on disuse.

Soft Inheritance

Evolution involving changes in individual organisms, whether based on conscious choice or use and disuse, would require that characteristics acquired during the lifetime of an individual be passed on to offspring12, a process often termed "soft inheritance." The notion that acquired traits can be transmitted to offspring remained a common assumption among thinkers for more than 2,000 years, including into Darwin's time (Zirkle 1946). As is now understood, inheritance is actually "hard," meaning that physical changes that occur during an organism's lifetime are not passed to offspring. This is because the cells that are involved in reproduction (the germline) are distinct from those that make up the rest of the body (the somatic line); only changes that affect the germline can be passed on. New genetic variants arise through mutation and recombination during replication and will often only exert their effects in offspring and not in the parents in whose reproductive cells they occur (though they could also arise very early in development and appear later in the adult offspring). Correct and incorrect interpretations of inheritance are contrasted in Fig. 3.

A summary of correct ( left) and incorrect ( right) conceptions of heredity as it pertains to adaptive evolutionary change. The panels on the left display the operation of "hard inheritance", whereas those on the right illustrate naïve mechanisms of "soft inheritance". In all diagrams, a set of nine squares represents an individual multicellular organism and each square represents a type of cell of which the organisms are constructed. In the left panels, the organisms include two kinds of cells: those that produce gametes (the germline, black) and those that make up the rest of the body (the somatic line, white). In the top left panel, all cells in a parent organism initially contain a gene that specifies white coloration marked W ( A). A random mutation occurs in the germline, changing the gene from one that specifies white to one that specifies gray marked G ( B). This mutant gene is passed to the egg ( C), which then develops into an offspring exhibiting gray coloration ( D). The mutation in this case occurred in the parent (specifically, in the germline) but its effects did not become apparent until the next generation. In the bottom left panel, a parent once again begins with white coloration and the white gene in all of its cells ( H). During its lifetime, the parent comes to acquire a gray coloration due to exposure to particular environmental conditions ( I). However, because this does not involve any change to the genes in the germline, the original white gene is passed into the egg ( J), and the offspring exhibits none of the gray coloration that was acquired by its parent ( K). In the top right panel, the distinction between germline and somatic line is not understood. In this case, a parent that initially exhibits white coloration ( P) changes during its lifetime to become gray ( Q). Under incorrect views of soft inheritance, this altered coloration is passed on to the egg ( R), and the offspring is born with the gray color acquired by its parent ( S). In the bottom right panel, a more sophisticated but still incorrect view of inheritance is shown. Here, traits are understood to be specified by genes, but no distinction is recognized between the germline and somatic line. In this situation, a parent begins with white coloration and white-specifying genes in all its cells ( W). A mutation occurs in one type of body cells to change those cells to gray ( X). A mixture of white and gray genes is passed on to the egg ( Y), and the offspring develops white coloration in most cells but gray coloration in the cells where gray-inducing mutations arose in the parent ( Z). Intuitive ideas regarding soft inheritance underlie many misconceptions of how adaptive evolution takes place (see Fig. 2)

Studies have indicated that belief in soft inheritance arises early in youth as part of a naïve model of heredity (e.g., Deadman and Kelly 1978; Kargbo et al. 1980; Lawson and Thompson 1988; Wood-Robinson 1994). That it seems intuitive probably explains why the idea of soft inheritance persisted so long among prominent thinkers and why it is so resistant to correction among modern students. Unfortunately, a failure to abandon this belief is fundamentally incompatible with an appreciation of evolution by natural selection as a two-step process in which the origin of new variation and its relevance to survival in a particular environment are independent considerations.

Nature as a Selecting Agent

Thirty years ago, widely respected broadcaster Sir David Attenborough ( 1979) aptly described the challenge of avoiding anthropomorphic shorthand in descriptions of adaptation:

Darwin demonstrated that the driving force of [adaptive] evolution comes from the accumulation, over countless generations, of chance genetical changes sifted by the rigors of natural selection. In describing the consequences of this process it is only too easy to use a form of words that suggests that the animals themselves were striving to bring about change in a purposeful way-that fish wanted to climb onto dry land, and to modify their fins into legs, that reptiles wished to fly, strove to change their scales into feathers and so ultimately became birds.

Unlike many authors, Attenborough ( 1979) admirably endeavored to not use such misleading terminology. However, this quote inadvertently highlights an additional challenge in describing natural selection without loaded language. In it, natural selection is described as a "driving force" that rigorously "sifts" genetic variation, which could be misunderstood to imply that it takes an active role in prompting evolutionary change. Much more seriously, one often encounters descriptions of natural selection as a processes that "chooses" among "preferred" variants or "experiments with" or "explores" different options. Some expressions, such as "favored" and "selected for" are used commonly as shorthand in evolutionary biology and are not meant to impart consciousness to natural selection; however, these too may be misinterpreted in the vernacular sense by non-experts and must be clarified.

Darwin ( 1859) himself could not resist slipping into the language of agency at times:

It may be said that natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.

Perhaps recognizing the ease with which such language can be misconstrued, Darwin ( 1868) later wrote that "The term 'Natural Selection' is in some respects a bad one, as it seems to imply conscious choice; but this will be disregarded after a little familiarity." Unfortunately, more than "a little familiarity" seems necessary to abandon the notion of Nature as an active decision maker.

Being, as it is, the simple outcome of differences in reproductive success due to heritable traits, natural selection cannot have plans, goals, or intentions, nor can it cause changes in response to need. For this reason, Jungwirth ( 1975a, b, 1977) bemoaned the tendency for authors and instructors to invoke teleological and anthropomorphic descriptions of the process and argued that this served to reinforce misconceptions among students (see also Bishop and Anderson 1990; Alters and Nelson 2002; Moore et al. 2002; Sinatra et al. 2008). That said, a study of high school students by Tamir and Zohar ( 1991) suggested that older students can recognize the distinction between an anthropomorphic or teleological formulation (i.e., merely a convenient description) versus an anthropomorphic/teleological explanation (i.e., involving conscious intent or goal-oriented mechanisms as causal factors; see also Bartov 1978, 1981). Moore et al. ( 2002), by contrast, concluded from their study of undergraduates that "students fail to distinguish between the relatively concrete register of genetics and the more figurative language of the specialist shorthand needed to condense the long view of evolutionary processes" (see also Jungwirth 1975a, 1977). Some authors have argued that teleological wording can have some value as shorthand for describing complex phenomena in a simple way precisely because it corresponds to normal thinking patterns, and that contrasting this explicitly with accurate language can be a useful exercise during instruction (Zohar and Ginossar 1998). In any case, biologists and instructors should be cognizant of the risk that linguistic shortcuts may send students off track.

Source Versus Sorting of Variation

Intuitive models of evolution based on soft inheritance are one-step models of adaptation: Traits are modified in one generation and appear in their altered form in the next. This is in conflict with the actual two-step process of adaptation involving the independent processes of mutation and natural selection. Unfortunately, many students who eschew soft inheritance nevertheless fail to distinguish natural selection from the origin of new variation (e.g., Greene 1990; Creedy 1993; Moore et al. 2002). Whereas an accurate understanding recognizes that most new mutations are neutral or harmful in a given environment, such naïve interpretations assume that mutations occur as a response to environmental challenges and therefore are always beneficial (Fig. 2). For example, many students may believe that exposure to antibiotics directly causes bacteria to become resistant, rather than simply changing the relative frequencies of resistant versus non-resistant individuals by killing off the latter13. Again, natural selection itself does not create new variation, it merely influences the proportion of existing variants. Most forms of selection reduce the amount of genetic variation within populations, which may be counteracted by the continual emergence of new variation via undirected mutation and recombination.

Typological, Essentialist, and Transformationist Thinking

Misunderstandings about how variation arises are problematic, but a common failure to recognize that it plays a role at all represents an even a deeper concern. Since Darwin ( 1859), evolutionary theory has been based strongly on "population" thinking that emphasizes differences among individuals. By contrast, many naïve interpretations of evolution remain rooted in the "typological" or "essentialist" thinking that has existed since the ancient Greeks (Mayr 1982, 2001; Sinatra et al. 2008). In this case, species are conceived of as exhibiting a single "type" or a common "essence," with variation among individuals representing anomalous and largely unimportant deviations from the type or essence. As Shtulman ( 2006) notes, "human beings tend to essentialize biological kinds and essentialism is incompatible with natural selection." As with many other conceptual biases, the tendency to essentialize seems to arise early in childhood and remains the default for most individuals (Strevens 2000; Gelman 2004; Evans et al. 2005; Shtulman 2006).

The incorrect belief that species are uniform leads to "transformationist" views of adaptation in which an entire population transforms as a whole as it adapts (Alters 2005; Shtulman 2006; Bardapurkar 2008). This contrasts with the correct, "variational" understanding of natural selection in which it is the proportion of traits within populations that changes (Fig. 2). Not surprisingly, transformationist models of adaptation usually include a tacit assumption of soft inheritance and one-step change in response to challenges. Indeed, Shtulman ( 2006) found that transformationists appeal to "need" as a cause of evolutionary change three times more often than do variationists.

Events and Absolutes Versus Processes and Probabilities

A proper understanding of natural selection recognizes it as a process that occurs within populations over the course of many generations. It does so through cumulative, statistical effects on the proportion of traits differing in their consequences for reproductive success. This contrasts with two major errors that are commonly incorporated into naïve conceptions of the process:

1. 1.
Natural selection is mistakenly seen as an event rather than as a process (Ferrari and Chi 1998; Sinatra et al. 2008). Events generally have a beginning and end, occur in a specific sequential order, consist of distinct actions, and may be goal-oriented. By contrast, natural selection actually occurs continually and simultaneously within entire populations and is not goal-oriented (Ferrari and Chi 1998). Misconstruing selection as an event may contribute to transformationist thinking as adaptive changes are thought to occur in the entire population simultaneously. Viewing natural selection as a single event can also lead to incorrect "saltationist" assumptions in which complex adaptive features are imagined to appear suddenly in a single generation (see Gregory 2008b for an overview of the evolution of complex organs).

2. 2.
Natural selection is incorrectly conceived as being "all or nothing," with all unfit individuals dying and all fit individuals surviving. In actuality, it is a probabilistic process in which some traits make it more likely-but do not guarantee-that organisms possessing them will successfully reproduce. Moreover, the statistical nature of the process is such that even a small difference in reproductive success (say, 1%) is enough to produce a gradual increase in the frequency of a trait over many generations.

Concluding Remarks

Surveys of students at all levels paint a bleak picture regarding the level of understanding of natural selection. Though it is based on well-established and individually straightforward components, a proper grasp of the mechanism and its implications remains very rare among non-specialists. The unavoidable conclusion is that the vast majority of individuals, including most with postsecondary education in science, lack a basic understanding of how adaptive evolution occurs.

While no concrete solutions to this problem have yet been found, it is evident that simply outlining the various components of natural selection rarely imparts an understanding of the process to students. Various alternative teaching strategies and activities have been suggested, and some do help to improve the level of understanding among students (e.g., Bishop and Anderson 1986; Jensen and Finley 1995, 1996; Firenze 1997; Passmore and Stewart 2002; Sundberg 2003; Alters 2005; Scharmann 1990; Wilson 2005; Nelson 2007, 2008; Pennock 2007; Kampourakis and Zogza 2008). Efforts to integrate evolution throughout biology curricula rather than segregating it into a single unit may also prove more effective (Nehm et al. 2009), as may steps taken to make evolution relevant to everyday concerns (e.g., Hillis 2007).

At the very least, it is abundantly clear that teaching and learning natural selection must include efforts to identify, confront, and supplant misconceptions. Most of these derive from deeply held conceptual biases that may have been present since childhood. Natural selection, like most complex scientific theories, runs counter to common experience and therefore competes-usually unsuccessfully-with intuitive ideas about inheritance, variation, function, intentionality, and probability. The tendency, both outside and within academic settings, to use inaccurate language to describe evolutionary phenomena probably serves to reinforce these problems.

Natural selection is a central component of modern evolutionary theory, which in turn is the unifying theme of all biology. Without a grasp of this process and its consequences, it is simply impossible to understand, even in basic terms, how and why life has become so marvelously diverse. The enormous challenge faced by biologists and educators in correcting the widespread misunderstanding of natural selection is matched only by the importance of the task.

Notes

1. For a more advanced treatment, see Bell ( 1997, 2008) or consult any of the major undergraduate-level evolutionary biology or population genetics textbooks.

2. The Origin was, in Darwin's words, an "abstract" of a much larger work he had initially intended to write. Much of the additional material is available in Darwin ( 1868) and Stauffer ( 1975).

3. It cannot be overemphasized that "evolution" and "natural selection" are not interchangeable. This is because not all evolution occurs by natural selection and because not all outcomes of natural selection involve changes in the genetic makeup of populations. A detailed discussion of the different types of selection is beyond the scope of this article, but it can be pointed out that the effect of "stabilizing selection" is to prevent directional change in populations.

4. Instructors interested in assessing their own students' level of understanding may wish to consult tests developed by Bishop and Anderson ( 1986), Anderson et al. ( 2002), Beardsley ( 2004), Shtulman ( 2006), or Kampourakis and Zogza ( 2009).

5. It must be noted that the persistent tendency to label the inheritance of acquired characteristics as "Lamarckian" is false: Soft inheritance was commonly accepted long before Lamarck's time (Zirkle 1946). Likewise, mechanisms involving organisms' conscious desires to change are often incorrectly attributed to Lamarck. For recent critiques of the tendency to describe various misconceptions as Lamarckian, see Geraedts and Boersma ( 2006) and Kampourakis and Zogza ( 2007). It is unfortunate that these mistakenly attributed concepts serve as the primary legacy of Lamarck, who in actuality made several important contributions to biology (a term first used by Lamarck), including greatly advancing the classification of invertebrates (another term he coined) and, of course, developing the first (albeit ultimately incorrect) mechanistic theory of evolution. For discussions of Lamarck's views and contributions to evolutionary biology, see Packard ( 1901), Burkhardt ( 1972, 1995), Corsi ( 1988), Humphreys ( 1995, 1996), and Kampourakis and Zogza ( 2007). Lamarck's works are available online at http://www.lamarck.cnrs.fr/index.php?lang=en.

6. One may wonder how this misconception is reconciled with the common admonition by medical doctors to complete each course of treatment with antibiotics even after symptoms disappear-would this not provide more opportunities for bacteria to "develop" resistance by prolonging exposure?

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Department of Integrative Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada

T. Ryan Gregory

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Correspondence to T. Ryan Gregory.

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Gregory, T.R. Understanding Natural Selection: Essential Concepts and Common Misconceptions. Evo Edu Outreach 2, 156-175 (2009). https://doi.org/10.1007/s12052-009-0128-1
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Understanding Natural Selection: Essential Concepts and Common Misconceptions - Evolution: Education and Outreach

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Natural selection is one of the central mechanisms of evolutionary change and is the process responsible for the evolution of adaptive features. Without a working knowledge of natural selection, it is impossible to understand how or why living things have come to exhibit their diversity and complexity.