Biology 101
Life On Planet Earth
9 March, 1998

Speciation

• Allopatric speciation occurs in populations that are physically separated.

• Sympatric speciation occurs in populations that live in the same area.

• Premating isolating mechanisms prevent mating between species.

• Postmating isolating mechanisms prevent production of vigorous, fertile offspring.

The Genetics of Speciation

• One model of speciation stresses gradual accumulation of many small changes. NOTE: this is Richard Dawkins' point of view!


• A second model stresses the sudden appearance of a few major changes. NOTE: this is Stephen Jay Gould's point of view!


• Which model is correct? NOTE: this is the subject of your 2nd paper (due 20-March-98)

Rates of Speciation

• During adaptive radiation, one species gives rise to many under the differing selection pressures of new habitats.

The Pattern of Evolution

• Gradualism explains speciation as the slow, steady accumulation of small changes over time.


• Punctuated equilibrium explains speciation as occurring relatively rapidly amid long periods of little change.


• Gradualism and punctuated equilibrium models can both be applied to the evolution of the horse.


• Many evolutionary biologists accept a synthesis of gradualism and punctuated equilibrium.