1. 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?

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.