Models of speciation explain how species are created and how lineages radiate/diverge. In general, there are two steps required for speciation:
- Isolation – subset of an interbreeding population becomes isolated in space, time, OR in terms of selective pressures.
- Genetic divergence – independent evolutionary change in each subset results in populations no longer producing fertile offspring
1) Allopatric speciation requires geographical isolation (usually a vicariant/ dispersal event) in which no gene flow occurs, resulting in speciation.
Pangea is the most famous example of allopatric speciation, where the breaking apart of a supercontinent created geographical isolation between populations of species that resulted in new species.
The following two models in contrast, require selective pressure without geographic isolation:
2) Parapatric speciation occurs when strong selection counteracts gene flow between groups that are geographic neighbors.
Disruptive selection is at play here, where one extreme form has high fitness in one environment, the opposite extreme has high fitness in the other environment and intermediates (the hybrids) has low fitness everywhere.
3) Sympatric speciation occur when 2 or more descendant species form out of a single ancestral species (sister taxa) that are reproductively isolated while occupying overlapping ranges. While gene flow is possible, it does not occur due to ecological barriers or a possibly a strong disruptive selection within the area.
A famous example is the Midas cichlids complex that occurred in the volcanic lakes of Nicaragua. Though the 3 species of fish resided in the same lake, they did not interbreed due to different ecological niches. As a result, over time, they became morphologically and genetically distinct.
Below a drawing to sum up the differences!
Chaves, Johel. “Concepts of Diversity.” CIEE Tropical Diversity Lecture. September 2014.