"When one looks carefully at a biological problem, one can usually discover more than one casual explanation...Indeed, it is quite possible that in biology the majority of phenomena and processes must be explained by a plurality of theories."
Ernst Mayr This is Biology (1997)
One long standing controversy in evolutionary biology is about the respective contributions sympatry and allopatry have made during the origins of the millions of species that now exist.
A new species (or 2 new species) can develop if gene flow between 2 populations ceases or slows down to permit the independent evolution of the 2 populations.
Allopatric (or geographic) speciation takes place when 2 populations are geographically isolated from each other by, for example, a mountain chain, or an ocean. Because the members of the 2 populations cannot mate and exchange genes with each other, the populations may start to diverge genetically and phenotypically and eventually end up being 2 separate species. In extreme cases of allopatry, there will be no gene flow between the diverging populations.
The opposite of this is sympatric speciation, which takes place in 2 populations whose distribution ranges are largely overlapping. Individuals initially belonging to the same species may begin to differentiate from each other when they start eating different foods or living in different habitats. If the slight genetic differences that may exist between such individuals are reinforced by assortive mating1, 2 populations may emerge and begin to diverge genetically and phenotypically to eventually become separate species. New species may originate even when there is some gene flow between the 2 populations.
An intermediate mechanism is parapatric speciation, which takes place in contiguous, but otherwise geographically isolated (allopatric) populations.
Diagrammatic representation of population divergence in allopatric and sympatric speciation. The vertical axis represents time, running from older, below, to younger, above. The circles and crosses represent the different genotypes. Darkening of the symbols symbolize new species. Modified from a drawing in G.G. Simpson. 1983. Fossils and the History of Life. Scientific American Books.
The late Ernst Mayr was a strong proponent of allopatric speciation and for most of his long career, he discounted sympatric speciation as a viable mechanism. However, an increasing number of studies have been demonstrating that sympatric speciation is possible and may have taken place more often than traditionally believed.
A well-written short essay by Chris D. Jiggins2 in the 9 May issue of Current Biology reviews some recent studies and presents a good argument in favor of sympatric speciation, while pointing out that the usual division of speciation events along a strict line as either sympatric or allopatric creates an “artificial dichotomy”. In line with Mayr’s opinion about the necessity to explain biological processes by more than one theory, Jiggins suggests that in most cases of speciation, sympatric and allopatric processes may both have been reponsible.
"…allopatric and sympatric speciation lie at the opposite ends of a continuum, which runs from zero to maximal gene flow between diverging populations. These new studies provide good evidence that fully sympatric speciation can occur, but most examples probably lie somewhere in between these two extremes."
As is usually the case with any genuine scientific controversy, this one will continue to inspire further research and lead to better understandings of the many-faceted speciation events in nature. Jiggins ends his paper by laying out potential paths for future research.
"…we should abandon the common assumption that allopatric speciation is the ‘null hypothesis’ with all the burden of proof lying on the hypothesis of speciation with gene flow. Instead, speciation research should concentrate on the more proximal causes of speciation, rather than intractable questions of geography. Key questions that we can answer include whether speciation results from natural selection and/or genetic drift, and what traits and genetic architectures are causal in divergence."
Also posted at Transitions.
1. The mating of individuals preferentially with others of their own genotype is called assortive mating. The opposite of assortive mating is random mating.
2. Chris D. Jiggins. Sympatric Speciation: Why the Controversy? Current Biology, Vol 16, R333-R334, 09 May 2006.