The Principles of Evolution

During the time before the mid-1800's, the prevailing explanation for the diversity of life on earth (at least, in the western world) was that of God's grace and perfection

• All life was created and did not change (how could God improve upon perfection?)
• The outline of creation was clearly explained in Genesis

However, since the Restoration, scientists were noting certain organisms had numerous characteristics which could not be explained:

• Why were there vestigial hips in snakes and whales, etc.
• Why were the bones of unrelated organisms so similar?
• Why did wisdom teeth form poorly or not at all in most people and why was the jaw often too small to accommodate them?
• Just what are these fossils anyways?

Explanations to explain changes in organisms

Lamarck (1744-1829) Environmental changes generate or produce organismal needs Structures become modified through use/disuse These changes are capable of being passed to offspring Heritability of acquired characteristics Example - Larmarck's theory of how giraffes get longer necks

• The giraffe feeds off of the leaves of tall trees. If the environment was favorable for the trees one season, they will grow taller. This places the leaves out of reach from the giraffes. The environment has places a need on the giraffe (the need to grow longer necks to reach the leaves.) Through this need, the giraffe's neck will grow longer so that it may feed on the leaves. The giraffe's offspring will be born with the longer necks.
• Lamark's ideas are now considered false. Unfortunately, he is often ridiculed by textbook authors for being wrong
• However, he brought two very important ideas back into ciruculation:
• The concept that organisms evolve over time
• Organisms evolve toward greater adaptiveness

Charles Darwin and the Theory of Evolution by Natural Selection

Observed numerous cases of diversity and fossil history:

• Finches - he noted that the Galápagos had a diversity of finches, each with a different beak morphology and corresponding food source (insects, seeds, etc.) All were similar to each other and to the mainland species of finch
• Fossils - observed glyptodonts (primitive armadillos) and fossil shellfish which were similar to extant forms, yet extinct.

Observed other natural features

• Animals and plants typically produce more offspring than can possibly survive. Why?
• Form and function correlations
• All populations had variation - again, why?
• Offspring tend to resemble their parents more than other members of the community

Formulated the theory of natural selection

• Descent with modification
• More offspring are produced than can possibly survive. Only those who have the best combination of traits which allow it to survive and reproduce will be able to pass these traits to their offspring
• Note that this is NOT survival of the fittest (only implies survival, not reproduction)
• This selection may cause a slow, gradual shift in morphology of the species. This can lead to speciation
• Published paper in 1859 and book Origin of Species in 1861
• Darwin published his theory hoping that "transitional" animals would be found to strengthen his ideas
• One crucial discovery was made in 1861 - the discovery of Archaeopteryx, the "missing link" between birds and reptiles (dinosaurs)

Alfred Wallace independently came to the same conclusions as Darwin. Wallace published first, but gave credit to Darwin since he had more thoroughly explained the limitations to this theory

Theory of Evolution by Means of Natural Selection

• Natural selection is a major microevolutionary process that results in the differential survival and reproduction of individuals of a population that differ in one or more traits
• Individuals of a population vary in form, function, and behavior
• Much of the variation is heritable
• Some forms of a trait are more adaptive; i.e., they improve the chances of surviving and reproducing
• Natural selection is the difference in survival and reproduction that has occurred among individuals that differ in one or more traits.
• The giraffes revisited
• If the environment changes as before so that the trees become taller, only those giraffes which previously had longer necks will be able to reach the taller branches. The shorter ones will die. Since the taller ones survive, they will breed and most likely produce a generation with longer necks. These giraffes are better suited for their environment (they are better adapted to their changing environment) and will survive and reproduce.

Neodarwinism

Sources of genetic diversity:

• MUTATION - the alteration of an allele to form another allele, is the ultimate source of genetic variation (i.e. it is the only way a completely new allele can be introduced into a species)
• Whether a mutation is harmful, neutral, or beneficial depends on how the altered gene product performs under prevailing conditions

Sources of genetic mixing:

• crossing over: separates linked genes
• independent assortment: increases the diversity of possible gametes
• sexual reproduction: mixed diverse gametes of one parent with those of a second, further increasing the chance of novel combinations
• chromosomal alterations: nothing new is created, but again, they are arranged in novel ways

Other sources of genetic variation

• random fluctuations in allele frequencies over time, due to chance occurance alone
• genetic drift is more rapid in smaller populations

Founder Effect

• A few individuals carrying genes that may or may not be typical of the whole population leave the original population and establish a new population.

Genetic Bottlenecks

• Some stressful situation greatly reduces the size of a population leaving few individuals to reestablish the population. Often, the remaining individuals are atypical of the original population
• Example: Cheetah

Gene Flow

• Genes move with the individual when they move out of, or into a population
• The physical flow (and resultant shuffling) tends to minimize genetic variation between populations
• It decreases the effects of genetic drift, mutation, and natural selection

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Natural selection - an example

• Biston betularia, the peppered moth, has two alleles for skin color

B = dominant allele, black

b = recessive allele, white

• The white morph was initial selected for since white moths could hide more easily on lichen-covered trees
• However, during the mid 1840's, soot from the industrial revolution killed the lichens and covered the trees. The black morph became selected for and rapidly increased in diversity
• As clean-up mechanism were instituted, there was a general shift back to the white morph - however, since there is not a uniform tree color, a more homogenous mix is seen

Speciation

• Defining a species
• A species is one or more populations of individuals who can interbreed under natural conditions and produce fertile offspring
• These individuals are reproductively isolated from other populations
• Divergence
• Divergence is the process whereby local units of a population become reproductively isolated from other units and thus experience changes in gene frequencies between them
• Mutation, natural selection, and genetic drift begin to operate independently in both populations
• When divergence is great enough, interbreeding will stop; speciation has begun
• Speciation Routs
• Geographic isolation - physical barriers restrict gene flow between populations
• Isolation within a population - mutation or, in plants, polyploidy, may yield a new sub-population in which gene flow is reduced or lost

Mechanisms that Prevent Interbreeding

• Isolation of gametes - results from the inability of gametes of one species to fuse with those of another
• Structural isolation - incompatible reproductive apparatus
• Isolation in time - changes in breeding cycle or other mistimings of the reproductive cycle
• Sterile hybrids - even if two separate species can interbreed, if the hybrids are sterile, they are an evolutionary dead-end
• Behavioral isolation - changes in behavior may prevent interbreeding, even if it were physically possible

How could the moths in the above example become speciated?