Alan Cowey

theories rigorously. Here are some of my reservations. One generic problem is that Lynch's evidence comes from broad brush comparisons of extremely disparate types of organism. It is true that, on average, bacteria have much large N e values than most eukaryotes for which we currently have data, but they differ in numerous other respects as well, for example lack of regular sexual reproduction. As all good comparative biologists know, it is very difficult to disentangle cause and correlation from wide comparisons. Alternatives to many of Lynch's explanations of the patterns can be envisaged, and his arguments do not seem to rule these out. For example, as he himself describes in Chapter 7, the spread of transposable elements through the genomes of a host population is dependent on some degree of sexual exchange between members of the populations, and the correlations described by Lynch could thus at least partly be explained by lack of such exchange. Furthermore, his insistence on the importance of N e is undermined by the fact that models of the maintenance of transposable elements in intergenic regions (where insertions have little direct effects on fitness) show that there is no difficulty in their establishment in very large populations. In accordance with this, maize and its relatives are chock-a-block full of transposable elements, yet have levels of DNA sequence variability as large or larger than Drosophila species, with their relatively low levels of transposable elements. In relation to the evolution of introns, Lynch's model of their origin looks rather strained in relation to the evidence that introns seem to have been fairly prevalent in ancestral eukaryotes, so that their rarity and small size in many unicellular eukaryotes is a result of secondary loss. It is also undermined by evidence for high levels of DNA sequence diversity in some species of multicellular organisms with introns. Could it be that the invention of regular sexual reproduction made it easier for mobile, initially self-splicing introns to invade the genome in large numbers? This possibility is not explored by Lynch, who resorts (p. 261) to the untestable hypothesis that there was a long period of reduced N e among ancestral eukaryotes. This is getting dangerously close to the adaptationist just-so stories that he ridicules in the final chapter. There are other difficulties worth mentioning. One is that, despite his advocacy of the importance of population genetics, use is made of only …