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Patterns of Selection and Speciation: Exploring Natural Selection Processes

This topic explores different patterns of selection and speciation, including stabilizing selection, directional selection, disruptive selection, and sexual selection. It also discusses prezygotic and postzygotic barriers, patterns of speciation, and the evolution of organisms through coevolution, convergent evolution, and adaptive radiation.

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Patterns of Selection and Speciation: Exploring Natural Selection Processes

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  1. Topic 3 Patterns of Selection and Speciation

  2. Stabilizing Selection Natural selection acts against the extreme phenotypes in a population and favors the middle ground. Example: birth weight in humans.

  3. Directional selection Natural selection favors one extreme for phenotype over the other, shifting the prevalence of the characteristic from the middle of the range to one extreme. Example: medium ground finch (Galapagos)

  4. Disruptive selection Natural selection favors the extreme phenotype characteristics over the intermediate. Example: African swallowtail butterflies

  5. Pea fowl

  6. Birds of paradise Video 1

  7. Sexual selection • The selection of individuals for secondary sex characteristics eg bird coloration, elk horns • Can be intrasexual or intersexual • Male-male competition • Female choice • Often appears at odds with natural selection

  8. Does natural selection make perfect organisms? • Selection only acts on existing variations • Evolution is limited by historical constraints • Adaptations are compromises • Chance, natural selection, and the environment are all in play

  9. Prezygotic Barriers Temporal isolation Habitat isolation Behavioral isolation Mechanical isolation Gametic isolation

  10. Postzygotic Barriers Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown

  11. Patterns of speciation

  12. The pace of evolution: Punctuated equilibrium vs Gradualism

  13. Evolution is often gradual…

  14. …but stasis and rapid evolution is also common Bryozoan example. From Sepkoski 1989.

  15. Divergent evolution the accumulation of differences between groups which can lead to the formation of new species Via geographic isolation or adaptive radiation

  16. Geographic Isolation …can lead to reproductive isolation, divergence of gene pools and speciation.

  17. Background matching and local adaptation

  18. “…adaptive radiation…refers to more or less simultaneous divergence of numerous lines all from much the same ancestral adaptive type into different, also diverging adaptive zones.” Simpson 1953 Adaptive Radiation Seeds Bark & Crevice Insects Evolution of many species from one common ancestor in response to variations in available environments and resources. Buds Nectar and gleaned insects From Petren et al. 1999

  19. Coevolution Two species evolve in response to each other over time Pollinators and flowering plants Hosts and parasites Predators and prey Hammer orchid

  20. Predation drives evolution of both predators and prey Predators adaptations Locate, sneak up on & subdue prey Prey adaptations elude & defend horns, speed, coloration spines, thorns, toxins

  21. African Rift Lake Cichlids

  22. Model of Cichlid Fish Diversification

  23. Convergent Evolution The evolution of similar characteristics in unrelated (or extremely far removed) species as a result of similar ecological opportunities and environments Analogous structures are due to similar selection pressures

  24. Convergent evolution

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