Speciation Genetics is, in a sense, an oxymoron. Genetics is the study of heritable characteristics, but the researchers who study speciation genetics are looking for genes that cause inheritance to fail. They are looking for the genetic incompatibilities that keep species apart.
Speciation is about how a population of similar, interbreeding organisms becomes two or more populations so different from each other that they no longer form a common gene pool. Species' differences can be extremely subtle. In fact, an evolutionary process of speciation means that there must be a point at which the physical differences between two species is hard to discern, as well as a point when two populations aren't quite different species, but well on their way to becoming separate. At some point, when one gene pool splits into two, genetic incompatibilities arise that make cross-breeding between two populations a doomed enterprise.
What kinds of genetic incompatibilities first arise in the process of speciation? What types of genes are involved? A paper in the January 16th issue of Science reports on the discovery of a 'speciation gene' keeping two mouse sub-species from producing viable offspring.
One way for new species to evolve is through hybrid sterility, when organisms from two incompatible populations produce infertile offspring. When you cross a horse and a donkey, you get a sterile mule. Horses and donkeys are obviously two different species, so hybrid sterility is not surprising, but we can also see hybrid sterility in two populations that are well on their way to becoming two different species. For example, there are subspecies of mice that cannot produce fertile offspring. The different subspecies are all mice, but the populations have been separated long enough for genetic incompatibilities to have evolved. They can no longer interbreed; a barrier has been placed between the gene pools of these two populations, and those gene pools can now evolve independently. At some point down the line, these two mouse subspecies will become genuinely separate species (and according to at least one definition of species, these sub-populations are already different species). We're seeing the origin of new species right before our eyes.
But what genetic differences are causing hybrid sterility? In the case of long-diverged species (like the horse and the donkey), there are most likely many DNA differences that cause sterility in the hybrid mules, and researchers would have a hard time identifying which mutations arose first. In the case of the mouse subspecies, not enough time has passed for large numbers of mutations to have arisen, so the researchers can focus in on the first genetic steps of speciation.
A group based at the Czech Academy of Sciences and Cornell University found that one key gene producing infertile hybrids is a gene involved in regulating the larger-order structure of DNA. Somehow this gene, called Prdm9 has changed enough in one subspecies so that it can no longer properly interact with genes in the other subspecies. What's going on in molecular terms is probably like trying to use a metric wrench on an English system bolt - the slight incompatibilities mean the wrench can't grab the bolt.
The researchers were able to definitively prove the role of this speciation gene by putting back a 'fertile' copy of Prdm9. When the fertile copy gene goes into the hybrid mice, they are no longer sterile.
Studies like this bring us a fine-grained view of how species begin to form. Long before there are many obvious differences in external traits in these organisms, the gene pools are beginning to separate, as genetic incompatibilities build up.
Join me tomorrow, here at Adaptive Complexity, for day six of Show Me the Science month. Evolution as a science is alive and well. Each day I will blog about a paper related to evolution published in 2009.