Transcript Evolution
Topic 3 Patterns of Selection and Speciation Stabilizing Selection Natural selection acts against the extreme phenotypes in a population and favors the middle ground. Example: birth weight in humans. 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) Disruptive selection Natural selection favors the extreme phenotype characteristics over the intermediate. Example: African swallowtail butterflies Pea fowl Birds of paradise Video 1 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 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 Prezygotic Barriers • • • • • Temporal isolation Habitat isolation Behavioral isolation Mechanical isolation Gametic isolation Postzygotic Barriers • Reduced hybrid viability • Reduced hybrid fertility • Hybrid breakdown Patterns of speciation The pace of evolution: Punctuated equilibrium vs Gradualism Evolution is often gradual… …but stasis and rapid evolution is also common Bryozoan example. From Sepkoski 1989. Divergent evolution • the accumulation of differences between groups which can lead to the formation of new species • Via geographic isolation or adaptive radiation Geographic Isolation • …can lead to reproductive isolation, divergence of gene pools and speciation. Background matching and local adaptation Adaptive Radiation “…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 Evolution of many species from one common ancestor in response to variations in available environments and resources. Seeds Bark & Crevice Insects Buds Nectar and gleaned insects From Petren et al. 1999 Coevolution • Two species evolve in response to each other over time – Pollinators and flowering plants – Hosts and parasites – Predators and prey Hammer orchid 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 African Rift Lake Cichlids Model of Cichlid Fish Diversification 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 Convergent evolution