Models of speciation

Though the mechanisms listed define populations as members of distinct species, they do not explain how speciation occurs. They are simply the result of processes which cause speciation. We can formally define three types of speciation mechanisms:

• Allopatric speciation: (allopatric means literally "different homelands") in this model two populations which are geographically isolated from each other (eliminating the possibility of gene flow) can gradually diverge to form two distinct species.
• Sympatric speciation: (sympatric means "same homeland") species arise in this model despite being in the same location. Some other effect must isolate a subpopulation from the rest of the population; this is often behavioral, and often occurs very rapidly.
• Parapatric speciation: (parapatric means "near the homeland") species can form at the border between two populations. Often this border mirrors an actual geographic border, and the species formed occurs at this margin.

A "textbook" example of allopatric speciation as a result of separation of populations by the relatively recent uplifting of the Isthmus of Panama (ca. 3 million years ago).

The two bodies of water are very different, in the types of ocean currents present, and especially in water temperature and the stability of that temperature over time (the Pacific side is both colder and more changeable). Reef fish on either side of the Isthmus are highly related, but those on the Pacific side appear to have adapted biochemically to the colder temperatures; enzymes in their bodies are more tolerant of cold temperature as a result of mutational changes. This is an example of the type of speciation that has been caused by the movement of continents over the course of the evolution of the Earth.

An obvious case of sympatric speciation:

Fish (cichlids) in small lakes in West Africa (formed in old volcanic craters) have undergone repeated speciation despite their homogeneous habitat. Up to eleven species of the fish have arisen in a single small lake. Comparison of the DNA in their mitochondria (which diverges much more slowly than the DNA in the nucleus) shows that the species are much more related to each other than they are to fish in a nearby separate lake. It is not clear what mechanism brought about their speciation, though it is very likely to be a behavioral effect.

An example of parapatric speciation involves the production of hybrids between Bullock's oriole and the Baltimore oriole. This occurs at the junction of their territory (Bullock's is a western bird, and the Baltimore oriole is obviously an eastern bird).

The birds formed at the boundary are hybrids. This is perhaps not a good example of the concept since the hybrids are not forming a distinct species, and in fact other effects are probably reinforcing the distinction between the two species.