27.4 Speciation
Learning Goals:
1.
Give examples to illustrate the process of speciation
2.
Explain how adaptive radiation can lead to speciation
3.
Compare and contrast phyletic gradualism with punctuated equilibrium
1.
The Process of Speciation a.
Species: a group of subpopulations that are capable of interbreeding, and are isolated reproductively from other species b.
Reproductive Isolation mechanisms: i.
Prezygotic isolating mechanisms (before fertilization)
1.
Habitat isolation
2.
Temporal isolation
3.
Behavioral isolation
4.
Mechanical isolation ii.
Postzygotic isolating mechanisms (after fertilization)
1.
Gamete isolation
2.
Zygote mortality
3.
Hybrid sterility
4.
F
2
fitness decreased c.
Speciation: i.
Occurs when one species gives rise to two species ii.
Speciation occurs whenever reproductive isolation develops between two formerly interbreeding groups of populations iii.
Types of speciation
1.
Allopatric speciation: occurs when populations become separated by a geographic barrier and gene flow is no longer possible
2.
Sympatric speciation: a single population suddenly divides into two reproductively isolated groups without being geographically isolated
Check Your Understanding p.559 # 1
2.
Adaptive Radiation a.
Defined as the proliferation of a species by adaptation to different ways of life b.
Darwin’s finches on the Galapagos – great example of allopatric speciation c.
Islands were ecologically different enough to have promoted divergent feeding habits i.
Different beaks adapted to food source

3.
The Pace of Speciation a.
Two main hypothesis as to the pace of speciation: phyletic gradualism and punctuated equilibrium b.
Each hypothesis addressed questions as to why so few transitional links are found in the fossil record i.
Phyletic gradualism: States that change is very slow but steady within a lineage before and after a divergence ii.
Punctuated equilibrium: long periods of stasis (no visible change) are followed by rapid periods of speciation
Check Your Progress p.561 # 1-2