Macroevolution II: Speciation & Extinction

basic elements of speciation

barrier to gene flow —> genetic differentiation —> reproduction isolation

the tempo of speciation

• classic, gradualistic model: slow divergence of isolated populations

• punctuated equilibrium: long periods of stasis followed by sudden episodes of speciation

• earth’s history shows periods of slow evolution punctuated by periods of rapid evolution

  • cichlid fishes in lake victoria: 400 species in 12,000 years

  • horseshoe crabs: phenotypic stasis for 100,000,000 years

what controls the rate of speciation?

• rates of mutations

• rates of selection

  • competition among species

  • available habitat

  • generation time

• rates of environmental change

specific changes leading to rapid speciation

• key genetic processes change phenotype, but maintain function

  • polyploidy

• key developmental processes change phenotype, but maintain function

  • alteration of genes that control organism structure

    • mutations in hox genes

  • hererochrony: changes in developmental timing or sequence

heterochrony (2 types)

• sequence heterochrony: change in the time of appearance of a structure

• allometric heterochrony: change in the rate of development/growth

rates of extinction

• continuous process, always occurring at some background rate due to chance and natural selection

• accelerated by environmental change

• can occur in rapid bursts, known as mass extinction events

mass extinctions

• information from geology about large-scale and/or catastrophic events

• permian extinction: volcanoes? methane gas from deep sea?

• cretaceous extinction: a meteor?

• adaptive radiation of mammals occurred when?