Topic 3 Patterns of Selection and Speciation. Stabilizing Selection Natural selection acts against...
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Transcript of Topic 3 Patterns of Selection and Speciation. Stabilizing Selection Natural selection acts against...
Topic 3
Patterns of Selection and Speciation
Stabilizing SelectionNatural selection acts against the extreme phenotypes in a population and favors the middle ground.
Example: birth weight in humans.
Directional selectionNatural 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…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
From Petren et al. 1999
Adaptive Radiation
Seeds
Bark & Crevice Insects
Buds
Nectar and gleaned insects
Evolution of many species from one common ancestor in response to variations in available environments and resources.
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
spines, thorns, toxins
horns, speed, coloration
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