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