Brian Charlesworth has reviewed Michael Lynch’s The Origins of Genome Architecture for Current Biology. Charlesworth’s review is generally positive, and he agrees that population size may be an important factor in genome evolution. However, he thinks that Lynch overplays the role relaxed selective constraint in small populations plays in the evolution of genomic complexity.
Charlesworth argues that sexual reproduction may be partly to blame for some of the features found in the bloated genomes of many eukaryotes. For example, the abundance of transposable elements may be the result of sexual reproduction, not relaxed constraint:
It is true that, on average, bacteria have much large Ne 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.
Additionally, introns may be a byproduct of sexual reproduction:
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 Ne among ancestral eukaryotes. This is getting dangerously close to the adaptationist just-so stories that he ridicules in the final chapter.
Is Lynch committing to a frail nearly neutral hypothesis?
So, the book appears a bit biased. Oh, well. It makes some interesting arguments that warrant further inspection. Those arguments, however, should be taken with a grain of salt, as there may be adequate alternatives to the explanations proposed by Lynch.