From Nature News:
Published online 22 July 1999 | Nature | doi:10.1038/news990722-2
Species without frontiers
140 years after the publication of Charles Darwin’s momentous Origin of Species, two groups of theorists seem to be making headway against one of evolutionary biology’s most difficult problems – how the formation of new species, or ‘speciation’, actually happens.
Understanding speciation is still a fundamental problem in biology. It is believed that speciation usually occurs when pre-existing populations are divided by a geographical barrier, such as a river or a range of mountains. Unable to interbreed, the sundered populations go their own way, until, eventually, two new species are found where there was one before. The central part of the argument is the barrier: without one, individuals have the potential to breed freely, mixing up parental traits and keeping the population homogeneous. Theorists have not been able to show convincingly how speciation might occur without physical separation of the founding populations, except in certain rather specialized circumstances.
This difficulty, though, is hard to reconcile with mounting evidence that many species form without physical barriers to interbreeding, so-called ‘sympatric’ speciation. Many lakes in Africa, for example, host ‘flocks’ of dozens or hundreds of closely related species of cichlid fish, all of which seem to have evolved in thge same body of water from a single, ancestral species, often within a few thousand years – an evolutionary eyeblink. Although similar in general anatomy, these fishes display a dazzling array of colours, habits and ecological specializations: some graze algae, whereas others eat other fishes. One species lives by plucking the eyes from other cichlids.
Ecological specialization is the key to driving sympatric speciation, according to a brace of reports in the 22 July issue of Nature. One study is from Alexey S. Kondrashov of the National Institutes of Health, Bethesda, Maryland, and Fyodor A. Kondrashov of Simon’s Rock College, Great Barrington, Massachusetts; the other from Ulf Dieckmann and Michael Doebeli of the Institute of Systems Analysis, Laxenberg, Austria, and the University of Basel, Switzerland.
Sympatric speciation ought to be simple. If a lake, newly colonized by a species of fish, contains two distinct prey items – large beetles and small shrimp, say – then natural selection should favour the evolution of large and small fishes, leaving medium-sized fishes at a disadvantage. Eventually, the population will become divided into large and small fishes, each of which will mate with their own kind.
The problem is sex. Sexual reproduction scrambles the genes in each generation, so that there will always be a proportion of medium-sized fishes in the population. This tendency can only be kept at bay by rigorously finicky fishes, which will only mate with fishes of their own general appearance.
Even then, there is a problem: that is, if the genes that govern mate choice (for attractive coloration, for example) are different from the genes that govern ecological specialization (such as size and shape), there is still the possibility that fishes specialized for one kind of life will prefer to mate with fishes specialized for another.
It is at this point that theorists have fudged, by assuming that genes for mate choice and ecological specialization are either the same, or can effectively be treated as such. But this assumption need not apply to real life. Where next?
This is where the new research comes in. Both sets of researchers model plausibly realistic scenarios, involving several genes for mate choice and ecological specialization. Although their results and methods differ in their technical details, both show how sympatric can be achieved by a steady build-up of genetic associations between genes for mate choice and genes for specialization, until a point comes when the two sub-populations become sexually isolated – the point where speciation can be said to have happened.
The Dieckmann and Doebeli model, in particular, provides a good account for what seems to have happened in some African crater lakes, which become colonized by fishes which then diversify into a range of different forms, despite the small size of each lake. Dieckmann and Doebeli envision a situation in which individuals of a single species compete for a single, abundant kind of food item. In such situations, competition can become so intense that some individuals diversify their tastes, switching to other food items which, although less abundant, are less popular, and so the individuals experience less competition. If choosy mating follows a taste for new food items, speciation soon follows. This process works best in new, empty habitats, such as a newly formed crater lake.