1. Defining what is and what isn't a species is a problem. Most people have the idea that they can recognize distinct types of animals. But appearance cannot be the only measure since some animals that look quite different are really members of the same species (consider breeds of dogs). Biologists use a variety of definitions.
2. The biological species concept is based on the idea that a species is a group of interbreeding individuals who are reproductively isolated from other species. Reproductive isolation can be genetic (the hybrids are sterile), behavioural (individuals of the two species are not able to mate because of differences in courtship, for example) or physical (they are separated by geography). This concept is of course not useful for fossils. Species boundaries for extinct species must be created cautiously since this type of data is unavailable.
3. Speciation can happen when the species are separated geographically (allopatric speciation), in the same region geographically (sympatric speciation) or at the borders between two geographic regions (parapatric speciation).
4. The dominant metaphor for speciation is the evolutionary tree. In these trees there are always many fewer species earlier in time, and all of the earlier species then proliferate into many more species with time. This is not accurate since many species arise and go extinct over time, leaving no ancestor species. These species are generally missing from the evolutionary trees.
5. Speciation for Darwin was a gradual event. Populations of individuals would become separated and slowly over time would accumulate differences that resulted in their becoming distinct species. More recently support has been building for the idea that speciation is sporadic and rapid. The idea is referred to as "puncuated equilibria" to give the impression that species can remain stable for long periods, then rapidly change. Evidence is accumulating that the Earth has suffered periodic catastrophes (asteroid collisions?) that have resulted in mass extinctions. These events leave opportunities for massive speciation since many ecological niches (life-styles) are left vacant after such an event.

Pages in this unit:

1. What is a species?
2. Species are created by reproductive isolation.
3. Speciation can occur when populations are geographically isolated, when they are not isolated, or at the margins between isolated populations.
4. The concept of the evolutionary tree is the dominant metaphor in speciation.