Lecture 3. The origin of species

Updated: 1/15/08

Tuesday, January 22, 2008. Reading: 14

Reliable definitions of a “species” are required both for studies of evolution and taxonomy (the science of naming/classifying life)
- Morphological species concept: classification based on observable and measurable phenotypic traits
- Biological species concept: a species is a population or group thereof whose members potentially interbreed and produce fertile offspring
- Other species concepts (genealogical, ecological) may be more applicable to specific problems or taxa
Reproductive barriers may evolve as populations diverge, and can be classified as:
- Prezygotic: temporal, habitat, behavioral, mechanical, gametic
- Postzygotic: hybrid inviability, hybrid sterility, hybrid breakdown
- You should consult Table 14.3 and understand the definitions of all these mechanisms!
Divergence (reduction of gene flow) can happen through two major groups of mechanisms
- allopatric speciation (geographic isolation): e.g. canyons, islands, etc.
- sympatric speciation: polyploidy, resource isolation, etc.
One current debate concerns the tempo of evolution.
- gradualism: species evolve gradually from the ancestral population as differences accumulate
- punctuated equilibrium: species undergo long periods of evolutionary stasis, and new species arise during short bursts of change

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