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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
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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