Chapter 17:Evolution of PopulationsSection 17-3: The Process of Speciation

Isolating Mechanisms•Natural selection and genetic drift alone

to not create new species•Speciation is the formation of new

species•Reproductive isolation occurs when a

population splits into 2 groups, and the two populations can no longer interbreed

•Different species

Isolating Mechanisms•Behavioral isolation occurs when two

populations that are capable of interbreeding develop differences in courtship rituals or other behaviors

•Geographic isolation occurs when two populations are separated by geographic barriers

•Temporal isolation occurs when two or more species reproduce at different times

Testing Natural Selection in Nature•Darwin proposed that Galapagos finches

descended from common ancestor, with natural selection shaping beaks as they adapted to different food sources

Testing Natural Selection in Nature•The Grants measured and recorded beak

lengths•Documented directional selection as

environmental conditions changed

Speciation in Darwin’s Finches•How founder effect and natural selection

lead to reproductive isolation•Current hypothesis is speciation in

Darwin’s finches occurred by founding of new population geographic isolation, changes in the new population’s gene pool, behavioral isolation, and ecological competition

Founders Arrive• Few finches from South

America arrived in Galapagos

• Founder effect

Geographic Isolation• Island environment

different from mainland

• Island finch population evolved into new species

• Some crossed to new island

• Usually don’t fly over water

Changes in Gene Pools• Populations on each island

adapt to local environments

• Distinct populations form with distinct phenotypes

Behavioral Isolation• If you bring populations

back together, they will probably not breed

• Mating preference

• Now have reproductive isolation – two distinct species

Competition and Continued Evolution• Competition requires

finches to develop different adaptations, different levels of fitness

• More specialized birds have less competition

• New species develop