Patterns of Evolution

Patterns of Evolution: Sections Adaptive Radiation Mechanisms of Speciation Convergent vs. Divergent Evolution Co-evolution Models of change: Gradualism vs. Punctuated Equilibrium The Evolution of Populations Modes of Selection Intra vs. Intersexual Selection

Adaptive Radiation: General adaptive radiation occurs when an ancestral species develops into many different species filling different environmental niches. General adaptation is advantageous for accessing previously unreachable resources (natural selection).

modifies homologous structures Adaptive Radiation - Modifies homologous structures

Adaptive Radiation: Environmental Environmental adaptation occurs when species adapt in response to an environmental change. Examples: Extended period of drought or flooding, geographic isolation

Adaptive Radiation: Archipelagoes Archipelagoes involve the colonization of secluded environments, usually islands, and through isolation, permit rapid variation in species due to a decrease in outside environmental factors.

Mechanisms of Speciation Temporal Isolation Temporal Isolation occurs because species mate at different times Examples: Plants flower at different times throughout the year, various species of fireflies mate at different times of the night.

Mechanisms of Speciation: Geographic Isolation Geographic Isolation occurs when a segment of a population (of the same species) become separated by a physical barrier (body of water, mountains, etc.) and speciation occurs over time.

Mechanisms of Speciation Behavioral Isolation Behavioral isolation mechanisms involve variance in courtship behavior, differences in vocalization, and/or phememones which effectively create and maintain boundaries within closely related species.

Divergent Evolution Divergent evolution is the accumulation of differences between groups (due to different environmental pressures) which eventually leads to speciation (the forming of a new species) **Different examples, types, and modes of divergent evolution will be visited throughout this presentation…

Convergent Evolution Convergent evolution describes the acquisition of similar traits in unrelated lineages* due to comparable environmental niches in different parts of the world. Examples: marsupials, wing development *All species share a common ancestry

Convergent Evolution These animals have evolved similar adaptations for obtaining food because they occupy similar niches.

Co-evolution When 2 interdependent species influence each others morphology, it is referred to as co-evolution

Models of Macroevolution: Gradualism Evolutionary change is both slow and gradual Resulting from the accumulation of many small genetic changes favoured by natural selection Other effects occasionally making small contributions

Models of Macroevolution: Punctuated Equilibrium “instead of a slow, continuous movement, evolution tends to be characterized by long periods of virtual standstill ("equilibrium"), "punctuated" by episodes of very fast development of new forms” -Stephen Jay Gould

Stasis and change: The P.E. Model A species resists evolutionary change A species would rather move to a new area where it can find its habitat than adapt to a new one If a small population of a species should get isolated in an area where its habitat does not exist… …rapid change could take place to bring the population back to equilibrium (stasis) But it is no longer the same species

Time Evolution Species Y Species Z Species X Species W Rapid speciation Stasis Species W Species X Species Y Species Z

The Evolution of Populations: Genetic Drift The “gene pool” refers to the unique set of alleles (within a species) for any given trait in a given population. Genetic Drift is the natural shifting of allele frequency within a population over time. In the absence of outside interference, genetic drift is minimal…

Gene Flow: The Bottleneck Effect When population size is drastically reduced, abnormal allele frequencies may exist in the survivors, resulting in a loss in genetic variation.

Gene Flow: The Founder Effect the founder effect reflects a decrease in genetic variation occurring when a new population is established by a very small number of individuals “transplanted” from a larger population.

Factors Influencing The Evolution of Populations MUTATION GENETIC DRIFT GENE FLOW - MIGRATION

Modes of Natural Selection: Directional Selection For a given population, selection against one extreme can favor a specific phenotype and result in directional selection.

Directional Selection: An Example The first breeders of hound dogs were focused exclusively on speed, they favored and bred the fastest dogs. By continuing this selection for those dogs who ran faster than most of the hound dog population, they gradually produced a dog who could run up to 40 mph!

Modes of Natural Selection: Stabilizing Selection Stabilizing Selection takes place when selection favors the intermediate phenotype over the extreme values.

Stabilizing Selection: An Example A good classic example of this is lion birth weight. Cubs of low weight lose heat more quickly and get ill from infectious disease more easily, whereas cubs of large body weight are more difficult to deliver through the pelvis.

Modes of Natural Selection: Disruptive Selection Disruptive selection favors both extreme phenotypes, while those with the intermediate are selected against.

Disruptive Selection: An Example

Intrasexual Selection Intrasexual Selection is associated with characteristics affecting the outcome of competition the male members of one species for access to the female. Almost always, males among the most genetically “fit” pass their genes on, ensuring the overall fitness of the population and the species as a whole.

Intersexual Selection Intersexual selection involves the females “Choosing” their mates based on characteristics which ‘stand out.’ Unique colors, physiologic features, and elaborate courtship behaviors have evolved among nearly all species in response to intersexual selection

—Charles Darwin from "The Origin of Species" I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection.   —Charles Darwin from "The Origin of Species"

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