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Evolution occurs in and all around us because life evolves

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Evolution occurs in and all around us because life evolves

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Evolution by natural selection occurs when certain genotypes produce more offspring than other genotypes in response to the environment.
There are 4 requirements that Darwin has selected in order for evolution by natural selection.
1.  The trait under selection must be variable in the population, so that the encoding gene has more than one variant, or allele.
2. the trait under selection must be heritable, encoded by a gene or genes.
3. The struggle of existence that many more offspring are born than can survive in the environment.
4. Individuals with different alleles have differential survival and reproduction that is governed by the fit of the organism to its environment.

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Learning Objectives

1. Define and recognize fitness, adaptation, and evolution by natural selection

2. Explain predictions of and evidence for evolution by natural selection

3. Identify, explain, and recognize the consequences of evolution by natural selection in terms of fitness, adaptation, average phenotype, and genetic diversity

4. Differentiate between directional, stabilizing, disruptive, and balancing selection

A Short Primer on Genetics

Charles Darwin and Alfred Russel Wallace articulated the theory of evolution by natural selection without a modern understanding of genetics. But we have the advantage today of being able to discuss evolution with knowledge of genetics, so we'll start with a brief primer on genetics. These basic genetic concepts are good to review to make sure we're all on the same page:

All living things have DNA as their genetic material. DNA is composed of nucleotide bases, commonly abbreviated as A, T, C, and G.

DNA is organized into one or more chromosomes, which are linear or circular structures comprised of DNA and associated proteins

An organism's genome is the complete set of genes or genetic material for that species.

A gene is a hereditary factor that determines (or influences) a particular trait. A gene is comprised of a specific DNA sequence and is located on a specific region of a specific chromosome. Because of its specific location, a gene can also be called a genetic locus.

An allele is a particular variant of a gene, in the same way that chocolate and vanilla are particular variants of ice cream.

An organism's genotype is the particular collection of alleles found in its DNA. An organism with two of the same alleles for a particular gene is homozygous at that locus; an organism with two different alleles for a particular gene is heterozygous at that locus.

An organism's phenotype is its observable traits, which can include physical features and behaviors. Some aspects of phenotype are influenced by genotype, and some are influenced by environment.

A mutation is a change in the DNA sequence (A, T, C, or G). Some mutations are deleterious ('bad'), some have no effect ('neutral'), and some are beneficial ('good'). Mutations create new alleles, so without mutations, there would be no new genetic variation.

Evolution by Natural Selection

Evolution by natural selection occurs when certain genotypes produce more offspring than other genotypes in response to the environment. It is a non-random change in allele frequencies from one generation to the next. In On the Origin of Species by Natural Selection (1859), Charles Darwin described four requirements for evolution by natural selection:

1. the trait under selection must be variable in the population, so that the encoding gene has more than one variant, or allele.

2. the trait under selection must be heritable, encoded by a gene or genes

3. the struggle of existence, that many more offspring are born than can survive in the environment.

4. individuals with different alleles have differential survival and reproduction that is governed by the fit of the organism to its environment

We can apply these postulates to consider whether evolution by NS is occurring in a specific case. Watch this TED Ed video of the evolution of antibiotic resistance in bacteria:

And a followup video that shows live action bacteria evolving antibiotic resistance:

For bacterial evolution in the videos above, here's how Darwin's postulates apply:

1. The population initially contains only antibiotic-sensitive alleles (meaning the antibiotic will kill the cells), but mutations generate antibiotic resistant alleles. Now the trait under selection (antibiotic resistance) is variable in the population, with at least two alleles.

2. The antibiotic-resistant individuals have offspring that are also resistant because they have the same gene mutation for resistance, indicating that the trait is heritable.

3. Both sensitive and resistant bacteria have lots of offspring inside the infected human host (or on the petri dish) and compete for resources inside the host.

4. Because the human host is taking a course of antibiotics (or the petri dish contains increasing dosages of antibiotic), bacteria with the sensitive alleles die more-so than bacteria with the resistant allele. The resistant bacteria are a better fit to the antibiotic-rich environment.

This brings us to the idea of Darwinian fitness, that the organisms that best match their environment will have relatively greater survival and reproduction than those that match less well. Fitness is quantified relative to the average individual in the population; individuals that produce more viable progeny (progeny that can live and reproduce themselves) than average have greater fitness. A trait that is heritable and increases the survival and reproduction odds for those that carry that trait is called an adaptation. If a trait confers 1% greater reproductive advantage, it confers a fitness of 1.01. A trait that confers 10% greater reproductive advantage has a fitness of 1.1. Of all the mechanisms of evolution we'll discuss in this course, only natural selection results in adaptations. Remember, the measure of fitness is production of viable progeny - adaptive traits promote survival of individuals to reproductive age and/or promote reproductive success.

Evolution by natural selection results in individuals that are a better fit to their environment

Evolution by natural selection occurs when the environment exerts a pressure on a population so that only some phenotypes survive and reproduce successfully. The stronger the selective pressure or the selection event the fewer individuals make it through the sieve of natural selection. Those phenotypes that survive a strong selection event, such as a drought, are a better fit for an environment that suffers drought. Another way to say this is that they have higher Darwinian fitness.

The finches on the Galápagos islands have provided a robust study system for observing natural selection in action over the past decades (see the work of Peter and Rosemary Grant and their collaborators). The small finches on the island of Daphna Major have strong beaks to feed on seeds. Smaller beaked birds can only crack open the smallest seeds, while birds with larger beaks prefer larger seeds. In 1977, drought reduced the number of small seeds, so many small-beaked finches starved to death.

In the finch example above, the average phenotype has shifted so most individuals have larger beaks, which is a genetically controlled-trait in the finches. The larger beak size is an adaptation to the seed sizes available during drought conditions. A result of this shift is that small beak phenotypes have become rare or disappeared, so there is reduced phenotypic and therefore reduced genetic diversity in the finch population after selection.

When a population displays a normal distribution for a particular trait, natural selection can drive change in populations in different directions depending on the type of selection. Stabilizing selection results in a narrowing of the normal distribution, because individuals who had the 'average' phenotype, or the phenotype closest to the mean, tend to leave more offspring than those with phenotypes at either extreme. Directional selection results in a shift toward one end of the normal distribution, because individuals who had one extreme of the phenotype tend to leave more offspring than those with the other extreme. Disruptive or diversifying selection results in separation of the normal distribution into two distributions with elimination of the middle of the peak, because individuals with either extreme phenotype tend to have more offspring than those with the intermediate phenotype. Balancing selection occurs when multiple phenotypes (or alleles) are actively maintained in the population (i.e., no single phenotype has a consistent selective advantage over any other). The two most common types of balancing selection are frequency-dependent selection, where fitness depends on how common the phenotype (or allele) is, and heterozygote advantage, where the heterozygote (with the combined phenotype of both alleles) has higher fitness than either homozygote.

The image below illustrates the different effects on a population due to stabilizing, directional, or disruptive (diversifying) selection:

Does evolution of bigger, sexually reproducing organisms happen on time scales faster than geologic time?

Yes! There are lots of great examples of evolution, even in sexually reproducing species, that happen pretty quickly, on the order of years or decades. In fact, the relevant time unit is generations. Rock Pocket mice in the desert southwest are a long-studied example. These small tan mice are hunted by owls, visual predators who spot the mice by their contrasting color against the sand. Most mice are exactly the same color as the sand. This short video explains what happens to a pocket mice population that migrates onto black volcanic rock, with mutation rates and the number of generations until the population shifts from all tan to all black coat color.

Examples of how evolution matters to ordinary people

The example of bacteria evolving resistance to antibiotics is just one example of how evolution affects people's lives. Here are some questions for you to consider in the light of evolution:

How is cancer an evolutionary disease? Cancer arises because individual cells acquire mutations that they pass on to their progeny via mitosis. These mutations allow these cells to escape growth inhibition and hog resources (by creating new blood vessels and ramping up metabolism).

When we use insecticides in hour homes, and farmers spray their fields, how will the targeted insect population evolve?

When fishing regulations limit the catch to larger fish, what consequences might that have?

Evolution occurs in and all around us, because life evolves:

Questions about any of this? Login to add your comments below.

Additional optional readings
http://evolution.berkeley.edu/evosite/relevance/IApathogens.shtml http://evolution.berkeley.edu/evosite/relevance/IA1antibiotics.shtml

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Define and recognize fitness, adaptation, and evolution by natural selection Explain predictions of and evidence for evolution by natural selection Identify, explain, and recognize the consequences of evolution by natural selection in terms of fitness, adaptation, average phenotype, and genetic diversity Differentiate between directional, stabilizing, disruptive, and balancing selection Charles Darwin and Alfred Russel Wallace articulated the theory of evolution by natural selection without a modern understanding of genetics.

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Theory of Evolution: How did Darwin come up with it? - BBC News

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The process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the earth.

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A study on human evolution on 1,900 students, published online in the journal Personality and individual differences in October 2017, found that many people may have trouble finding a mate because of rapidly changing social technological advances that are evolving faster than humans.
"In most cases, these difficulties are not due to something wrong or broken, but due to people living in an environment which is very different from the environment they evolved to function in."

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(Image credit: Copyright NHMU/Credit: Mark Johnston)

The Theory of Evolution by natural selection was first formulated in Charles Darwin's book "On the Origin of Species" published in 1859. In his book, Darwin describes how organisms evolve over generations through the inheritance of physical or behavioral traits, as National Geographic explains. The theory starts with the premise that within a population, there is variation in traits, such as beak shape in one of the finches Darwin studied.

According to the theory, individuals with traits that enable them to adapt to their environments will help them survive and have more offspring, which will inherit those traits. Individuals with less adaptive traits will less frequently survive to pass them on. Over time, the traits that enable species to survive and reproduce will become more frequent in the population and the population will change, or evolve, according to BioMed Central. Through natural selection, Darwin suggested, genetically diverse species could arise from a common ancestor.

Darwin did not know the mechanism by which traits were passed on, according to National Geographic. He did not know about , the mechanism by which genes encode for certain traits and those traits are passed from one generation to the next. He also did not know about genetic mutation, which is the source of natural variation. But future research by geneticists provided the mechanism and additional evidence for evolution by natural selection

What is natural selection?

Darwin chose the term "natural selection" to be in contrast with "artificial selection," in which animal breeders select for particular traits that they deem desirable. In natural selection, it's the natural environment, rather than a human being, that does the selecting.

Put simply, the theory of evolution by means of natural selection can be described as "descent with modification," said Briana Pobiner, an anthropologist and educator at the Smithsonian National Museum of Natural History in Washington, D.C., who specializes in the study of human origins. The theory is sometimes described as "survival of the fittest," but that characterization can be misleading, Pobiner said. Here, "fitness" refers not to an organism's strength or athleticism but rather its ability to survive and reproduce.

Natural selection can alter a species in small ways, causing a population to change color or size over the course of several generations, according to The Natural History Museum. When this process happens over a relatively short period of time and in a species or small group of organisms, scientists call it " microevolution."

(Image credit: Leonello Calvetti/Getty Images)

But when given enough time and accumulated changes, natural selection can create entirely new species, a process known as "macroevolution," according to Derek Turner and Joyce C. havstad in "The Philosophy of Macroevolution." This long-term process is what turned into , amphibious mammals (such as an animal called Indohyus) into whales and a common ancestor of apes and humans into the people, chimps and gorillas we know today.

Darwin also described a form of natural selection that depends on an organism's success at attracting a mate - a process known as sexual selection, according to Nature Education. The colorful plumage of and the antlers of male deer are both examples of traits that evolved under this type of selection.

How did whales evolve?

One of the best examples scientists have of natural selection, is the evolution of whales. By using Darwin's theory as a guide, and understanding how natural selection works, biologists determined that the transition of early whales from land to water occurred in a series of predictable steps.

The evolution of the blowhole, for example, might have started with random genetic changes that resulted in at least one whale having its nostrils farther back on its head, according to Phys.org.

The whales with this adaptation would have been better suited to a marine lifestyle, since they would not have had to completely surface to breathe. Such individuals were more successful and had more offspring. In later generations, more genetic changes occurred, moving the nose farther back on the head.

Other body parts of early whales also changed. Front legs became flippers. Back legs disappeared. Their bodies became more streamlined, and they developed tail flukes to better propel themselves through water, according to the Natural History Museum.

Even though scientists could predict what early whales should look like, for a long time they lacked the fossil evidence to back up their claim. Creationists viewed this absence, not just with regard to whale evolution but more generally, as proof that evolution didn't occur, as pointed out in a Scientific American article.

(Image credit: Illustration by Robert W. Boessenecker)

However, since the early 1990s, scientists have found evidence from paleontology, developmental biology and genetics to support the idea that whales evolved from land mammals. These same lines of evidence support the theory of evolution as a whole.

In the first edition of "On the Origin of Species," Darwin speculated about how natural selection could cause a land mammal to turn into a whale. As a hypothetical example, Darwin used North American black bears ( Ursus americanus), which were known to catch insects by swimming in the water with their mouths open, according to the Darwin Correspondence Project.

"I can see no difficulty in a race of bears being rendered, by natural selection, more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale," he speculated.

The idea didn't go over very well with the public or with other scientists. Darwin was so embarrassed by the ridicule he received that the swimming-bear passage was removed from later editions of the book. Scientists now know that Darwin had the right idea but the wrong animal. Instead of looking at bears, he should have been looking at cows and hippopotamuses.

Other theories of evolution

Darwin wasn't the first or only scientist to develop a theory of evolution. Around the same time as Darwin, British biologist Alfred Russel Wallace independently came up with the theory of evolution by natural selection, according to the Natural History Museum. However this had little impact.

"The concept of evolution as a historical event was a hot topic among biologists and geologists prior to Darwin's book because there was so much evidence accumulating, but I suspect biological evolution hadn't really impinged on people outside of the academic bunker," Dr. P John D. Lambshead, a retired science research leader in marine biodiversity, ecology, and evolution at The Natural History Museum, London, told All About History Magazine. "As long as science knew of no mechanism to explain how evolution happened it could be safely dismissed as a crank idea."

Meanwhile, French biologist Jean-Baptiste Lamarck proposed that an organism could pass on traits to its offspring, though he was wrong about some of the details, according to the University of California's Museum of Paleontology.

Like Darwin, Lamarck believed that organisms adapted to their environments and passed on those adaptations. He thought organisms did this by changing their behavior and, therefore, their bodies - like an athlete working out and getting buff - and that those changes were passed on to offspring.

(Image credit: Anup Shah/Getty Images)

For example, Lamarck thought that giraffes originally had shorter necks but that, as trees around them grew taller, they stretched their necks to reach the tasty leaves and their offspring gradually evolved longer and longer necks. Lamarck also believed that life was somehow driven to evolve through the generations from simple to more complex forms, according to Understanding Evolution, an educational resource from the University of California Museum of Paleontology.

Though Darwin wasn't sure of the mechanism by which traits were passed on, he did not believe that evolution necessarily moved toward greater complexity, according to Understanding Evolution - rather, he believed that complexity arose through natural selection.

A Darwinian view of giraffe evolution, according to Quanta Magazine, would be that giraffes had natural variation in their neck lengths, and that those with longer necks were better able to survive and reproduce in environments full of tall trees, so that subsequent generations had more and more long-necked giraffes.

The main difference between the Lamarckian and Darwinian ideas of giraffe evolution is that there's nothing in the Darwinian explanation about giraffes stretching their necks and passing on an acquired characteristic.

What is modern evolutionary synthesis?

According to Pobiner, Darwin did not know anything about genetics. "He observed the pattern of evolution, but he didn't really know about the mechanism," she said. That came later, with the discovery of how genes encode different biological or behavioral traits, and how are passed down from parents to offspring. The incorporation of genetics into Darwin's theory is known as "modern evolutionary synthesis."

The physical and behavioral changes that make natural selection possible happen at the level of and genes within the gametes, the sperm or egg cells through which parents pass on genetic material to their offspring. Such changes are called . "Mutations are basically the raw material on which evolution acts," Pobiner said.

Mutations can be caused by random errors in DNA replication or repair, or by or radiation damage, according to Nature Education. Usually, mutations are either harmful or neutral, but in rare instances, a mutation might prove beneficial to the organism. If so, it will become more prevalent in the next generation and spread throughout the population.

In this way, natural selection guides the evolutionary process, preserving and adding up the beneficial mutations and rejecting the bad ones. "Mutations are random, but selection for them is not random," Pobiner said.

(Image credit: Shutterstock)

But natural selection isn't the only mechanism by which organisms evolve, she said. For example, genes can be transferred from one population to another when organisms migrate or immigrate - a process known as gene flow. And the frequency of certain genes can also change at random, which is called genetic drift.

The reason Lamarck's theory of evolution is generally wrong is that acquired characteristics don't affect the DNA of sperm and eggs. A giraffe's gametes, for example, aren't affected by whether it stretches its neck; they simply reflect the genes the giraffe inherited from its parents. But as Quanta reported, some aspects of evolution are Lamarckian.

For example, a Swedish study published in 2002 in the European Journal of Human Genetics found that the grandchildren of men who starved as children during a famine passed on better cardiovascular health to their grandchildren. Researchers hypothesize that although experiences such as food deprivation don't change the DNA sequences in the gametes, they may result in external modifications to DNA that turn genes "on" or "off."

Such changes, called epigenetic changes, do not modify the actual DNA sequence itself. For instance, a chemical modification called methylation can affect which genes are turned on or off. Such epigenetic changes can be passed down to offspring. In this way, a person's experiences could affect the DNA he or she passes down, analogous to the way Lamarck thought a giraffe craning its neck would affect the neck length of its offspring.

What is the evidence for evolution?

The Theory of Evolution is one of the best-substantiated theories in the history of science. It is supported by evidence from a wide variety of scientific disciplines, including genetics, which shows that different species have similarities in their DNA.

There is also evidence supporting the Theory of Evolution in paleontology and geology. This is through the fossil record, which shows how that species that existed in the past are different from those present today, according to Bruce S. Lieberman and Roger L. Kaesler in " Prehistoric Life: Evolution and the Fossil Record" (Wiley, 2010).

There is also evidence for Darwin's theory found in developmental biology. It has been discovered that species that seem very different as adults pass through similar stages of embryological development, suggesting a shared evolutionary past, according to the open-access textbook " Concepts of Biology."

Evidence for whale evolution from paleontology

(Image credit: Nobumichi Tamura/Stocktrek Images via Getty Images)

The critical piece of evidence was discovered in 1994, when paleontologists found the fossilized remains of Ambulocetus natans, which means "swimming-walking whale," according to a 2009 review published in the journal Evolution: Education and Outreach. Its forelimbs had fingers and small hooves, but its hind feet were enormous relative to its size. The animal was clearly adapted for swimming, but it was also capable of moving clumsily on land, much like a seal.

When it swam, the ancient creature moved like an , pushing back with its hind feet and undulating its spine and tail.

Modern whales propel themselves through the water with powerful beats of their horizontal tail flukes, but A. natans still had a whip-like tail and had to use its legs to provide most of the propulsive force needed to move through water.

In recent years, more and more of these transitional species, or "missing links," have been discovered, lending further support to Darwin's theory. For example, in 2007, a geologist discovered the fossil of an extinct aquatic mammal, , that was about the size of a cat and had hooves and a long tail.

Scientists think the animal belonged to a group related to cetaceans such as Ambulocetus natans. This creature is considered a "missing link" between artiodactyls - a group of hoofed mammals (even-toed ungulates) that includes hippos, pigs, and cows - and whales, according to the National Science Foundation.

Researchers knew that whales were related to artiodactyls, but until the discovery of this fossil, there were no known artiodactyls that shared physical characteristics with whales. After all, hippos, thought to be cetaceans' closest living relatives, are very different from whales. Indohyus, on the other hand, was an artiodactyl, indicated by the structure of its hooves and ankles, and it also had some similarities to whales, in the structure of its ears, for example.

Evidence for whale evolution from genetics & developmental biology

(Image credit: NOAA)

Genetic evidence also supports the idea that whales evolved from land mammals and provides information about the exact branching of the evolutionary tree. For instance, in 1999, researchers reported in the journal Proceedings of the National Academy of Sciences that according to genetic analysis of "jumping gene" sequences, which copy and paste themselves into genomes, hippos were whales' closest living relatives. Before 1985, researchers thought pigs were more closely related to whales, but this 1999 study overturned that idea, as the Associated Press reported.

In 2019, researchers reported in the journal Science Advances about which genes within the whale genome were inactivated during the process of the creature's evolution from land mammals, as Science Friday reported. The researchers could tell that certain genes, including one involved in making saliva, had been inactivated because there are remnants of them, which the researchers call genomic fossils, in whale genomes. This indicates that whales evolved from a salivating creature.

There's also evidence of cetacean evolution from developmental biology. Developmental biology illustrates the fact that animals that are very different as adults share similarities as embryos because they are evolutionarily related. For example, as embryos, cetaceans started to develop hind limbs, which disappear later in development, while the forelimbs remain and develop into flippers, according to the journal Evolution: Education and Outreach. This suggests that cetaceans evolved from a four-legged ancestor.

Is the theory of evolution controversial?

Despite the wealth of evidence from the fossil record, genetics and other fields of science, some people still question the theory of evolution's validity. Some politicians and religious leaders denounce the theory, invoking a higher being as a designer to explain the complex world of living things, especially humans.

School boards debate whether the theory of evolution should be taught alongside other ideas, such as intelligent design or creationism.

Mainstream scientists see no controversy. "A lot of people have deep religious beliefs and also accept evolution," Pobiner said, adding, "there can be real reconciliation."

Evolution is well supported by many examples of changes in various species leading to the diversity of life seen today. "Natural selection, or to put it another way - variation, heredity, and differential fitness - is the core theory of modern biology," John Lambshead explains. "It is to biology what, say quantum mechanics and special relativity are to physics or the atomic model is to chemistry."
Additional reporting by contributors Alina Bradford, Ashley P. Taylor and Callum McKelvie
Additional resources

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Charles Darwin's theory of evolution and natural selection isn't an idea with holes. It's one of the most solid theories in science. But what exactly is it?

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1. Variation: Organisms (within populations) exhibit individual variation in appearance and behavior. These variations may involve body size, hair color, facial markings, voice properties, or number of offspring.
2. Inheritance: Some traits are consistently passed on from parent to offspring. Such traits are heritable, whereas other traits are strongly influenced by environmental conditions.
3. High rate of population growth: Most populations have more offspring each year than local resources can support, leading to a struggle for resources.
4. Differential survival and reproduction: Individuals possessing traits well suited for the struggle for local resources will contribute more offspring to the next generation.

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Nature encourages no looseness, pardons no errors
- Ralph Waldo Emerson
I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection. - Charles Darwin, The Origin of Species
In this lesson, we wish to ask:

The (R)Evolution of Theory

The theory of evolution is one of the great intellectual revolutions of human history, drastically changing our perception of the world and of our place in it. Charles Darwin put forth a coherent theory of evolution and amassed a great body of evidence in support of this theory. In Darwin's time, most scientists fully believed that each organism and each adaptation was the work of the creator. Linneaus established the system of biological classification that we use today, and did so in the spirit of cataloguing God's creations.

In other words, all of the similarities and dissimilarities among groups of organisms that are the result of the branching process creating the great tree of life ( see Figure 1), were viewed by early 19th century philosophers and scientists as a consequence of omnipotent design.

However, by the 19th Century, a number of natural historians were beginning to think of evolutionary change as an explanation for patterns observed in nature. The following ideas were part of the intellectual climate of Darwin's time.

No one knew how old the earth was, but geologists were beginning to make estimates that the earth was considerably older than explained by biblical creation. Geologists were learning more about strata, or layers formed by successive periods of the deposition of sediments. This suggested a time sequence, with younger strata overlying older strata.

A concept called uniformitarianism, due largely to the influential geologist Charles Lyell, undertook to decipher earth history under the working hypothesis that present conditions and processes are the key to the past, by investigating ongoing, observable processes such as erosion and the deposition of sediments.

"We know that this animal, the tallest of mammals, dwells in the interior of Africa, in places where the soil, almost always arid and without herbage, obliges it to browse on trees and to strain itself continuously to reach them. This habit sustained for long, has had the result in all members of its race that the forelegs have grown longer than the hind legs and that its neck has become so stretched, that the giraffe, without standing on its hind legs, lifts its head to a height of six meters."

In essence, this says that the necks of Giraffes became long as a result of continually stretching to reach high foliage. Larmarck was incorrect in the hypothesized mechanism, of course, but his example makes clear that naturalists were thinking about the possibility of evolutionary change in the early 1800's.

Darwin was influenced by observations made during his youthful voyage as naturalist on the survey ship Beagle. On the Galapagos Islands he noticed the slight variations that made tortoises from different islands recognizably distinct. He also observed a whole array of unique finches, the famous "Darwin's finches," that exhibited slight differences from island to island. In addition, they all appeared to resemble, but differ from, the common finch on the mainland of Ecuador, 600 miles to the east. Patterns in the distribution and similarity of organisms had an important influence of Darwin's thinking. The picture at the top of this page is of Darwin's own sketches of finches in his Journal of Researches.

In 1859, Darwin published his famous ery copy of the book was sold the day that it was released. Members of the religious community, as well as some scientific peers, were outraged by Darwin's ideas and protested. Most scientists, however, recognized the power of Darwin's arguments. Today, school boards still debate the validity and suitability of Darwin's theory in science curricula, and a whole body of debate has grown up around the controversy (see the WWW site On the Origin of Species by Means of Natural Selection, a tome of over 500 pages that marshalled extensive evidence for his theory. Publication of the book caused a furor - evTalk.Origins for an ongoing dialogue). We do not have time to cover all of Darwin's evidence and arguments, but we can examine the core ideas.What does this theory of evolution say?

Darwin's Theory

Darwin's theory of evolution entails the following fundamental ideas. The first three ideas were already under discussion among earlier and contemporaneous naturalists working on the "species problem" as Darwin began his research. Darwin's original contributions were the mechanism of natural selection and copious amounts of evidence for evolutionary change from many sources. He also provided thoughtful explanations of the consequences of evolution for our understanding of the history of life and modern biological diversity.

Species (populations of interbreeding organisms) change over time and space. The representatives of species living today differ from those that lived in the recent past, and populations in different geographic regions today differ slightly in form or behavior. These differences extend into the fossil record, which provides ample support for this claim.

All organisms share common ancestors with other organisms. Over time, populations may divide into different species, which share a common ancestral population. Far enough back in time, any pair of organisms shares a common ancestor. For example, humans shared a common ancestor with chimpanzees about eight million years ago, with whales about 60 million years ago, and with kangaroos over 100 million years ago. Shared ancestry explains the similarities of organisms that are classified together: their similarities reflect the inheritance of traits from a common ancestor.

Evolutionary change is gradual and slow in Darwin's view. This claim was supported by the long episodes of gradual change in organisms in the fossil record and the fact that no naturalist had observed the sudden appearance of a new species in Darwin's time. Since then, biologists and paleontologists have documented a broad spectrum of slow to rapid rates of evolutionary change within lineages.

The Process of Natural Selection
T he primary mechanism of change over time is natural selection, elaborated below. This mechanism causes changes in the properties (traits) of organisms within lineages from generation to generation.

Darwin's process of natural selection has four components.

1. Variation. Organisms (within populations) exhibit individual variation in appearance and behavior. These variations may involve body size, hair color, facial markings, voice properties, or number of offspring. On the other hand, some traits show little to no variation among individuals-for example, number of eyes in vertebrates.

2. Inheritance. Some traits are consistently passed on from parent to offspring. Such traits are heritable, whereas other traits are strongly influenced by environmental conditions and show weak heritability.

3. High rate of population growth. Most populations have more offspring each year than local resources can support leading to a struggle for resources. Each generation experiences substantial mortality.

4. Differential survival and reproduction. Individuals possessing traits well suited for the struggle for local resources will contribute more offspring to the next generation.

From one generation to the next, the struggle for resources (what Darwin called the "struggle for existence") will favor individuals with some variations over others and thereby change the frequency of traits within the population. This process is natural selection. The traits that confer an advantage to those individuals who leave more offspring are called adaptations.

In order for natural selection to operate on a trait, the trait must possess heritable variation and must confer an advantage in the competition for resources. If one of these requirements does not occur, then the trait does not experience natural selection. (We now know that such traits may change by other evolutionary mechanisms that have been discovered since Darwin's time.)

Natural selection operates by comparative advantage, not an absolute standard of design. "... as natural selection acts by competition for resources, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates" (Charles Darwin, On the Origin of Species, 1859).

During the twentieth century, genetics was integrated with Darwin's mechanism, allowing us to evaluate natural selection as the differential survival and reproduction of genotypes, corresponding to particular phenotypes. Natural selection can only work on existing variation within a population. Such variations arise by mutation, a change in some part of the genetic code for a trait. Mutations arise by chance and without foresight for the potential advantage or disadvantage of the mutation. In other words, variations do not arise because they are needed.

Evidence of Natural Selection

Let's look at an example to help make natural selection clear.

Industrial melanism is a phenomenon that affected over 70 species of moths in England. It has been best studied in the peppered moth, Biston betularia. Prior to 1800, the typical moth of the species had a light pattern ( see Figure 2). Dark colored or melanic moths were rare and were therefore collectors' items.

Figure 2. Image of Peppered Moth

During the Industrial Revolution, soot and other industrial wastes darkened tree trunks and killed off lichens. The light-colored morph of the moth became rare and the dark morph became abundant. In 1819, the first melanic morph was seen; by 1886, it was far more common -- illustrating rapid evolutionary change.

Eventually light morphs were common in only a few locales, far from industrial areas. The cause of this change was thought to be selective predation by birds, which favored camouflage coloration in the moth.
Recapture Success
In the 1950's, the biologist Kettlewell did release-recapture experiments using both morphs. A brief summary of his results are shown below. By observing bird predation from blinds, he could confirm that conspicuousness of moth greatly influenced the chance it would be eaten.

Local Adaptation - More Examples

So far in today's lecture we have emphasized that natural selection is the cornerstone of evolutionary theory. It provides the mechanism for adaptive change. Any change in the environment (such as a change in the background color of the tree trunk that you roost on) is likely to lead to local adaptation. Any widespread population is likely to experience different environmental conditions in different parts of its range. As a consequence it will soon consist of a number of sub-populations that differ slightly, or even considerably.

The following are examples that illustrate the adaptation of populations to local conditions.

The rat snake, Elaphe obsoleta, has recognizably different populations in different locales of eastern North America (see Figure 3). Whether these should be called geographic "races" or subspecies is debatable. These populations all comprise one species, because mating can occur between adjacent populations, causing the species to share a common gene pool (see thelecture on speciation).

Figure 3: Subspecies of the rat snake Elaphe obsoleta, which interbreed where their ranges meet.

Galapagos finches are the famous example from Darwin's voyage. Each island of the Galapagos that Darwin visited had its own kind of finch (14 in all), found nowhere else in the world. Some had beaks adapted for eating large seeds, others for small seeds, some had parrot-like beaks for feeding on buds and fruits, and some had slender beaks for feeding on small insects (see Figure 4). One used a thorn to probe for insect larvae in wood, like some woodpeckers do. (Six were ground-dwellers, and eight were tree finches.) (This diversification into different ecological roles, orniches, is thought to be necessary to permit the coexistence of multiple species, a topic we will examined in a later lecture.) To Darwin, it appeared that each was slightly modified from an original colonist, probably the finch on the mainland of South America, some 600 miles to the east. It is probable that adaptive radiation led to the formation of so many species because other birds were few or absent, leaving empty niches to fill; and because the numerous islands of the Galapagos provided ample opportunity for geographic isolation.

Figure 4

Stabilizing, Directional, and Diversifying Selection

Finally, we will look at a statistical way of thinking about selection. Suppose that each population can be portrayed as a frequency distribution for some trait -- beak size, for instance. Note again that variation in a trait is the critical raw material for evolution to occur.

What will the frequency distribution look like in the next generation?

Figures 5a-c

First, the proportion of individuals with each value of the trait (size of beak, or body weight) might be exactly the same. Second, there may be directional change in just one direction. Third (and with such rarity that its existence is debatable), there might be simultaneous change in both directions (e.g. both larger and smaller beaks are favored, at the expense of those of intermediate size). Figures 5a-c capture these three major categories of natural selection.

Figure 6

Under stabilizing selection, extreme varieties from both ends of the frequency distribution are eliminated. The frequency distribution looks exactly as it did in the generation before ( see Figure 5a). Probably this is the most common form of natural selection, and we often mistake it for no selection. A real-life example is that of birth weight of human babies ( see Figure 6).

Under directional selection, individuals at one end of the distribution of beak sizes do especially well, and so the frequency distribution of the trait in the subsequent generation is shifted from where it was in the parental generation ( see Figure 5b). This is what we usually think of as natural selection. Industrial melanism was such an example.

Figure 7

The fossil lineage of the horse provides a remarkable demonstration of directional succession. The full lineage is quite complicated and is not just a simple line from the tiny dawn horse Hyracotherium of the early Eocene, to today's familiar Equus. Overall, though, the horse has evolved from a small-bodied ancestor built for moving through woodlands and thickets to its long- legged descendent built for speed on the open grassland. This evolution has involved well- documented changes in teeth, leg length, and toe structure ( see Figure 7).

Under diversifying (disruptive) selection, both extremes are favored at the expense of intermediate varieties ( see Figure 5c). This is uncommon, but of theoretical interest because it suggests a mechanism for species formation without geographic isolation (see the lecture on speciation).

Summary

Darwin's theory of evolution fundamentally changed the direction of future scientific thought, though it was built on a growing body of thought that began to question prior ideas about the natural world.

The core of Darwin's theory is natural selection, a process that occurs over successive generations and is defined as the differential reproduction of genotypes.

Natural selection requires heritable variation in a given trait, and differential survival and reproduction associated with possession of that trait.

Examples of natural selection are well-documented, both by observation and through the fossil record.

Selection acts on the frequency of traits, and can take the form of stabilizing, directional, or diversifying selection.

Suggested Readings

o Darwin, C. 1959. On the Origin of Species by Means of Natural Selection, or, the Preservation of Favoured Races in the Struggle for Life. London: J. Murray.

o Futuyma, D.J. 1986. Evolutionary Biology. Sunderland, Mass: Sinauer Associates, Inc.

o Dawkins, R. 1989. The Selfish Gene. Oxford: Oxford University Press.

Copyright Regents of the University of Michigan unless noted otherwise.

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Nature encourages no looseness, pardons no errors - Ralph Waldo Emerson I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection.

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Natural selection is the process where organisms better adapted to their environment tend to survive and produce more offspring.

This theory was found by Charles Darwin and now believed to be the main process that brings about evolution.