Origin of Species Galapagos Tortoise

Origin of Species patterns of speciation Anagenesis A single population is transformed enough to be designated a new species Cladogenesis Branching evolution. A new species arises from a small population that “Buds” from a parent species. Most species probably evolved this way. The branching evolution is the basis of biological diversity

Two patterns of speciation

Origin of Species Species definitions Biological species = a population whose members have the potential to interbreed with one another and produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of another species. (Reproductive Isolation) The biological species concept cannot be applied to asexually reproducing organisms Morphospecies = species defined by their anatomical features

The biological species concept is based on interfertility rather than physical similarity Eastern Meadowlark Western Meadowlark

Reproductive barriers Any factor that impedes two species from producing fertile hybrids Prevents interbreeding between closely related species Prezygotic barriers (impedes mating or hinders ova fertilization) Postzygotic barriers (after fertilization)

Origin of Species Prezygotic barriers Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation

Habitat Isolation

Temporal Isolation Late-winter mating season Summer mating season

Blue-footed boobies … Courtship ritual as a behavioral barrier between species.

Mechanical Isolation Gametic Isolation “You can’t fit a square peg in a round hole” Gametic Isolation “gamete recognition”

Origin of Species Postzygotic barriers Reduced hybrid viability (vigor) Reduced hybrid fertility Hybrid breakdown

Origin of Species Introgression The transplantation of alleles between species. Alleles occasionally “seep” through all reproductive barriers and pass between gene pools of closely related species. May occur when a fertile hybrid mates successfully with one of the parent species.

Hercules the liger Liger Female Ligers are fertile and can reproduce. If a Female liger mated with a male tiger the offspring would have some lion genes. If that offspring could then mate with a tiger the lion genes would continue on within the Tiger Gene Pool, an example of introgression.

A summary of reproductive barriers between closely related species

Origin of Species Modes of speciation Allopatric speciation A population forms a new species while geographically isolated from its parent population Sympatric speciation Species formation occurs in geographically overlapping populations

Two modes of speciation

Origin of Species Allopatric speciation occurs when the initial block to gene flow is a geographical barrier that physically isolates the population. The genetically isolated group can then follow its own evolutionary course as changes in allele frequencies occur and are undiluted by gene flow from other populations

Has speciation occurred during geographic isolation?

Allopatric speciation of squirrels in the Grand Canyon White-tailed antelope squirrel (north rim) Harris’s antelope squirrel (south rim)

Speciation in Galapagos Finches

Speciation in Cichlids

Origin of Species Allopatric speciation Adaptive Radiation = emergence of numerous species from a common ancestor introduced into an environment. Example: Darwin’s finches on the Galapagos island chain

Adaptive Radiation of Galapagos Finches

A model for adaptive radiation on island chains

Adaptive Radiation in Cichlids (Specialization in feeding)

Origin of Species Sympatric speciation Formation of a new species within the geographical range of the parent population (no geographical isolation) Can occur quickly (in one generation) if a genetic change results in a reproductive barrier between mutants and the parent population

Origin of Species Sympatric speciation Iris Polyploid speciation Many plant species have originated from improper cell division that resulted in polypoidy Autopolyploidy – an organism that has more than two chromosome sets, all derived from the same species Allopolyploidy – a polyploid hybrid resulting from contributions by two different species. More common than autopolyploidy. Hybrids are usually sterile but may be very vigorous and propagate asexually. Daylily

Sympatric speciation by autopolyploidy in plants

One mechanism for allopolyploid speciation in plants

Punctuated Equilibrium vs. Gradualism Punctuated equilibrium model suggests evolution occurring in spurts of relatively rapid change instead of gradual divergence

Two models for the tempo of speciation