Unit 11 - V. Speciation

Disruption of genetic equilibrium may lead to evolution of existing species but

may not result in formation of new species

Formation of new species may occur over

many generations

Formation of new species requires

isolation of subpopulations

1. Geographical isolation

new land/water barriers form

• e.g. change in the course of a river; new highway built across a field

1. allopatric speciation (apart)

species arise in separate settings

2. Reproductive isolation

inability of formerly interbreeding organisms to mate & produce fertile offspring

2. sympatric speciation

species arise in the same setting

2a. Prezygotic (prevents breeding)

species evolve adaptations that prevent mating

• e.g. different breeding times/mating calls

2b Postzygotic (do breed)

though species interbreed, their offspring don’t go on to make other offspring

• mating is unsuccessful

• e.g. tigers & lions produce ligers; ligers are sterile

Various definitions of species exist, generally:

• members of a species share identical traits

• live in the same geographic area

• have the opportunity to mate

• can mate successfully to produce fertile offspring