There is a widespread tendency of biologists to overgeneralise from their study group of organisms to the whole of biology. Sometimes this is because the organisms are model organisms, like Drosophila (the “fruit flies” that have been used in genetics since the beginning).Other times it is because specialists tend to overestimate the generality of their results and domain. The recent trend to finding “speciation genes” is an example. For some time now various researchers like Chung-i Wu and his collaborators have sought speciation genes. These are genes that cause speciation, in a general sense, but the slide appears to be made to the conclusion that there are particular genes in many if not most cases of speciation. I want to consider this now.
Phadnis and Orr have found that a particular gene is both a gene causing sterility between hybrids of two Drosphilid subspecies, and a segregation distorter – that is, the gene causes itself to be differentially copied when gametes are formed. A similar process of meiotic distortion occurs in mice as Mihola et al. show. A great result, but how general? In the commentary accompanying the online advance publication, Asher Mullard writes and quotes this:
“There is no question that in this era of whole-genome sequences and genomic data it is much easier to identify speciation genes than it used to be” says Michael Nachman, who studies mouse genetics at the University of Arizona in Tucson.
With more genes should come greater insight into speciation. Some geneticists wonder whether only particular classes of genes are important in speciation ? such as epigenetic genes or segregation distorters ? or whether many sorts of genes help to drive species apart.
“What is surprising about the speciation genes that have been identified [so far] is that there is a whole hodgepodge of different kinds of genes with different functions,” says Nachman. “I don’t think we’re going to see [trends] until dozens of genes are identified, and there’s just a handful now.”
But why think that there should be particular classes of genes that contribute to speciation? Sure, there may be genes that are implicated in Drosophilid speciation, or maybe even in insect speciation, but all that matters in sexual species is that some barrier to reproduction exists. It need not be a particular barrier. Consider this – how many ways are there to impede the flow of traffic? SHould we expect there are only a couple of ways? Witches’ hats and workers’ signs may be common, but there are sinkholes, burning barriers of demonstrations, collapsed cranes, street parties… and the list could be indefinitely extended. I suspect that speciation is like that – it’s a negative property, and oen that can be arrived at in an indefinitely large number of ways. There’s some bad thinking going on here.
In a paper that has been justly ignored, I suggested what we need is the concept of “reproductive reach” – the broad class of things that can interbreed. Speciation occurs when reproductive reach is interrupted.
In another development, a cryptic species of dwarf freshwater crocodile in the Congo Basin has been found, not by using 568 bp of a mitochondrial gene, but by comparison of many nuclear and mitochondrial genes. That’s a good thing.
O. Mihola, Z. Trachtulec, C. Vlcek, J. C. Schimenti, J. Forejt (2008). A Mouse Speciation Gene Encodes a Meiotic Histone H3 Methyltransferase Science DOI: 10.1126/science.1163601
N. Phadnis, H. A. Orr (2008). A Single Gene Causes Both Male Sterility and Segregation Distortion in Drosophila Hybrids Science DOI: 10.1126/science.1163934
John Wilkins (2007). The dimensions, modes and definitions of species and speciation Biology & Philosophy, 22 (2), 247-266 DOI: 10.1007/s10539-006-9043-9