Natural Selection & Evolutionary Patterning

Natural Selection Natural Selection acts on VARIATIONS Stabilizing Selection: conditions favor intermediate variants rather than extremes Directional Selection: conditions favor individuals at one extreme Disruptive Selection: conditions favor individuals at both extremes

Which of these modes represents which type of selection? What conditions might account for the observed types of selection among mouse populations?

Sexual Selection: Intrasexual vs. Intersexual Selection

Intrasexual – competition among individual within one of the sexes (same sex competes for mates of the opposite sex) Ex: male-male competition among elk for access to females Intersexual – selection between the sexes (mate choice) Ex: female choice (peahens ) for most vibrant & “showy” males (peacocks)

Evolutionary Patterns Adaptive Radiation: common ancestral species evolves into many different species as populations move into new/ different habitats and have adaptations provide a “best-fit” with environment Example: Darwin’s finches – certain beak types proved more advantageous for procuring food within a particular habitat

Evolutionary Patterns Divergent Evolution: species that were once similar to ancestral species become increasingly more diversified; results in homologous structures

Evolutionary Patterns Convergent Evolution: unrelated species evolve similar traits because they occupy similar environments in different parts of the world Similar environmental pressures result in analogous structures (Ex: aquatic habitat favors streamlined body plan and flipper-like appendages)

Evolutionary Patterns Coevolution – evolution of one species affects the evolution of another species due to a close ecological relationship between the two species Ex: mutualism – many plants have evolved to produce fruits that are consumed by certain organisms so that seeds can be dispersed A parasitic relationship is known as “coevolutionary arms race” – plants evolve a chemical defense against pests and pests in turn evolve to resist the defense

Rates of Speciation Speciation can occur quickly or slowly Gradualism: species originate through a gradual change of adaptations over very long periods of time; many transitional species between ancient ancestor and modern-day species Punctuated Equilibrium: speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in between