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Molecular & Genetic Evidence

Molecular and genetic evidence from extant and extinct organisms indicates that all organisms on Earth share a common ancestral

origin of life.

• Scientific evidence includes molecular building blocks that are common to all life forms (carbohydrates, proteins, lipids, amino acids).

•     Scientific evidence includes a common genetic code (DNA and RNA).

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In the 1960s, Sidney Fox synthesized organic polymers such as polypeptides by dripping dilute solutions of organic monomers over hot sand, clay, or rock.
•     This method mimics the condensation of the Miller-Urey model, but with

the idea that rain falling from the early atmosphere or waves washing onto hot substrate would be favorable to the formation of polypeptides and other organic polymers.

• Once these polymers have formed, they can form aggregates, which spontaneously form into proteinoids (protobiont structures similar to living organisms).

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In 1953, Stanley Miller and Harold Urey tested the Oparin- Haldane hypothesis by creating laboratory conditions comparable to those that scientists at the time thought existed on early Earth. Their apparatus/model yielded a variety of amino acids found in organisms today, along with other organic compounds.

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Anaerobic prokaryotes originated, flourished, and then declined as the oxygen content of the atmosphere rose.  Billions of years later, the first tetrapods emerged from the sea, giving rise to amphibians that went on to dominate life on land for 100 mill ions years

– until other tetrapods (dinosaurs and later, mammals) replaced them as the dominant terrestrial vertebrates.

• These and other major changes in life on Earth have been influenced by large-scale processes such as continental drift, mass extinctions, and adaptive radiations.

•     The rise and fall of dominant groups reflect continental drift, mass extinctions, and adaptive radiations.

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The endosymbiotic hypothesis proposes that mitochondria and plastids (chloroplasts) were formerly small prokaryotes that began living within larger cells. Evidence for this hypothesis includes:

•     Both organelles have enzymes and transport systems homologous to those found in the plasma membranes of living prokaryotes.

•     Both replicate by a splitting process similar to prokaryotes.

•     Both contain a single, circular DNA molecule, not associated with histone proteins.

•     Both have their own ribosomes which translate their DNA into proteins.

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(A) endomembrane system of eukaryotes may have evolved from

specialized infoldings of plasma membrane of ancestral prokaryotes (B) chloroplasts are descendants of photosynthetic prokaryotes, probably cyanobacteria

-     proposed ancestors of mitochondria were endosymbiotic

bacteria that were aerobic heterotrophs

- may have first gained entry into larger cells as undigested prey or internal parasites

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According to the RNA World hypothesis, the first genetic material was most likely RNA, not DNA.  RNA molecules called ribozymes have been found to catalyze many different reactions: for example, ribozymes can make complementary copies of short stretches of their own sequence or other short pieces of RNA.

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The necessary conditions for replication and metabolism early in life’s history may have been met by protobionts.  Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure.

• Protobionts may exhibit some properties of life, such as simple reproduction and metabolism, as well as the maintenance of an internal chemical environment different from that of their surroundings.

• Experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds

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In the 1920s, Russian and British chemists Oparin and Haldane hypothesized that Earth’s early atmosphere was a reducing (electron- adding) environment, in which organic compounds could have formed from simple molecules. They suggested that the early oceans were a solution of organic molecules, a “primitive soup” from which life arose.

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It is important to note that speciation can take place  with or without geographic separation. Essentially, speciation can occur in two ways: allopatric speciation or sympatric speciation.

(a)   In allopatric speciation, a population forms a new species while geographically isolated from its parent population
-  In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated

subpopulations. Once separated, individual populations may evolve independently through mutation, natural selection, and/or genetic drift.

(b)  In sympatric speciation, a small population becomes a new

species without geographic separation from its parent population
- In sympatric speciation, divergence can occur when biological barriers prevent gene flow in  overlapping populations as seen in autopolyploidy, allopolyploidy, habitat differentiation, and sexual selection. Note that in sympatric speciation, speciation takes place in geographically overlapping populations

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The key to speciation is reproductive isolation: the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring. Reproductive isolation can be classified by whether factors act before or after fertilization:

Prezygotic Barriers block fertilization from occurring by: impeding different species from attempting to mate or by preventing the successful completion of mating

• Habitat isolation: living in different habitats – not encountering each other

•     Temporal isolation: breeding at different times

•     Behavioral isolation: different behaviors to attract a mate

• Mechanical isolation: two species may attempt to made but physically cannot

• Gametic isolation: gametes from two different species will not fuse to become a zygote

Postzygotic Barriers prevent the hybrid zygote from developing into a viable, fertile adult:

• Reduced Hybrid Viability: genetic incompatibility between the two species may abort the development of the hybrid at some embryonic stage or produce frail offspring.

• Reduced Hybrid Fertility: even if the hybrid offspring are vigorous, they may be infertile and cannot backbreed to either parent.

•     Reduced Hybrid Breakdown: in some cases, first generation hybrids

are viable and fertile, but, when they mate with either parent or with each other, the next generation is feeble or sterile.

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Speciation, the origin of new species, is at the focal point of evolutionary theory because it results in diversity of life forms. Evolutionary theory must explain how new species originate and how populations evolve. Microevolution consists of adaptations that evolve within a population, confined to one gene pool. Macroevolution refers to evolutionary change above the species level.

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Homology  is similarity due to shared ancestry – such as the bones of a whale’s flipper and a tiger’s paw.

Analogy is similarity due to convergent evolution - occurs when similar environmental pressures and natural selection produce similar (analogous) adaptations in organisms from different evolutionary lineages (example: the streamlined bodies of a tuna and a dolphin).

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Phylogeny is the evolutionary history of a species or group of species.
Taxonomy is an ordered division of organisms into categories based on a set of characteristics used to assess similarities and differences . Phylogenetic  trees graphically model evolutionary history and can represent both acquired traits and those lost during evolution.
Phylogenetic trees and cladograms can be constructed from morphological similarities of living or fossil species, and from DNA and protein sequence similarities – by employing computer  programs that have sophisticated ways of measuring and representing relatedness among organisms. Phylogenetic trees and cladograms are dynamic – constantly being revised based on current and emerging knowledge.

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Organisms share many conserved core processes and features that are widely distributed among organisms today. These processes provide evidence  that all organisms (Archaea, Bacteria, and Eukarya, both extant and extinct) are linked  by lines of descent  from common  ancestry.   Elements that are conserved across all domains of life are DNA and RNA a s carriers of genetic information, a universal genetic code, and many metabolic pathways . The existence of these properties in organisms today implies that they were present in a universal  ancestor and that present life evolved from a universal ancestor.
Shared core processses:
- DNA and RNa are carriers of genetic information
- Same bases in DNA and RNa - A C T G U
- Common cell communication processes

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Natural selection and genetic drift are not the only phenomena affecting allele frequencies. Allele frequencies can also change by gene flow, the transfer of alleles into or out of a population. This transfer of alleles is due to the movement of fertile individuals or their gametes (migration).

•     Because alleles are exchanged among populations, gene flow tends to reduce the genetic differences between populations.

.• Gene flow can increase the fitness of a population…insecticides have been used to target mosquitoes that carry West Nile virus and malaria. Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes. The flow of insecticide resistance alleles into a population can cause an increase in fitness

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Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next.  Genetic drift tends to reduce genetic variation through losses of alleles – and this reduction of genetic variation within a given population

can increase the differences between populations of the same species.

∙    The founder effect occurs when a few individuals become isolated from a larger population.

Allele frequencies in the small founder population can be different from those in the larger parent population .  Once

isolated from the larger population, the new gene pools will likely evolve under their own environmental pressures.

∙ The bottleneck effect is a sudden reduction in  population size due to a change in the environment. The resulting gene pool may no longer be reflective of the original population’s gene pool. If the population remains small, it may be further affected by genetic drift.

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Sexual reproduction can shuffle existing alleles into new combinations. In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible. Three mechanisms contribute to the shuffling of alleles during sexual reproduction:

- crossing over
- Independent assorment
- Fertilization-

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Mutations are changes in the nucleotide sequence of DNA. Mutations cause new genes and alleles to arise. Only mutations in cells that produce gametes can be passed to offspring.
-point mutation
-cosomal mutationhrom

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Hardy-Weinberg Theorem – states that allele frequencies tend to remain constant in populations unless something happens OTHER THAN Mendelian segregation and sexual recombination. This situation in which allele frequencies remain constant is called genetic equilibrium.

Assumptions:
Randomly Mating Population

Large Population Size (n>100)/No Genetic Drift

No Immigration or Emigration/Restrict Gene Flow

No Mutations

No Natural Selection

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Natural selection is the major driving mechanism of evolution; the essential features of the mechanism contribute to the change in the genetic makeup of a population over time. Darwin’s theory of natural selection states that inheritable variations occur in individuals in a population. Due to competition for resources that are often limited, individuals with more favorable variations or phenotypes are more likely to survive and produce more offspring, thus passing traits to subsequent generations. Fitness, the number of surviving offspring left to produce the next generation, is a measure of evolutionary success. Individuals do not evolve, but rather, populations evolve. Natural selection acts on phenotypes, not genotypes.