Formation of Species• A species is a population of organisms that can

successfully interbreed but cannot breed with other groups.

• Existing species are changed versions of older species.

• The many species today diverged from a smaller number of earlier species.

• The process of species formation is called speciation.

• How do species give rise to other, different species?

Isolation and Speciation• Speciation begins with isolation.

• In isolation, two parts of a formerly interbreeding population stop interbreeding.

• Two types of isolation frequently drive speciation:– Geographic isolation– Reproductive isolation

• Geographic Isolation– Geographic isolation results from the separation of

population subgroups by geographic barriers.

– Geographic barriers can be formed by canyons, mountain ranges, bodies of water, deserts, or other geographic features that organisms cannot cross.

– Geographic isolation may lead to allopatric speciation.

– Populations separated by a geographic barrier no longer experience gene flow between them.

– Gene pools of each population may begin to differ due to genetic drift, mutations, and natural selection.

• Reproductive Isolation– Reproductive isolation results from the separation

of population subgroups by barriers to successful breeding.

– Become genetically isolated without being geographically isolated.

– There are two types of reproductive isolation:– Prezygotic isolation– Postzygotic isolation

– Prezygotic isolation – occurs before fertilization– Differences in mating calls or mating times can prevent

two interbreeding species from mating.

– Postzygotic isolation – occurs after fertilization– Offspring of interbreeding species may not develop

completely or die early, or if healthy, they may not be fertile.

– Reproductive isolation within the same geographic area may lead to sympatric speciation.

Rates of Speciation• In the gradual model of speciation (gradualism),

species undergo small changes at a constant rate.

• Under punctuated equilibrium, new species arise abruptly, differ greatly from their ancestors, and then change little over long periods.