The online journal Genome Biology and Evolution is now publishing its initial set of articles, so be sure to have a look and watch for some good stuff in the future.
I notice that there’s a paper on mammalian genome sizes planned for the first issue. It looks interesting, though I note the following intriguing quotes:
“The evolutionary patterning of genome architecture by nonadaptive forces is supported by population-genetic theory, estimates of the relative power of the major forces of evolution, and comparative analyses of whole-genome sequences. Nevertheless, some biologists still adhere to the idea that even the most arcane aspects of genome evolution, including expansions of genome size by mobile-element proliferation, are direct products of natural selection (e.g., Gregory 2005; Kirschner and Gerhart 2005; Caporale 2006)”.
“our results challenge the notion that genome size reflects a finely tuned structural determinant of the adaptive phenotypes of organisms (Cavalier-Smith 1978; Hughes and Hughes 1995; Gregory 2005).”
Readers of this blog can probably answer the issue of whether I believe all aspects of genome expansion are adaptive.
I have looked at the article in more detail, and it is indeed interesting. However, as is common with this kind of analysis, there are some questions. Here are the species they examined to claim that no reductions in genome sizes occurred in plants or invertebrates:
Plants — Arabidopsis thaliana (thale cress, genome size 125Mb, one of the smallest in plants), Populus trichocarpa (cottonwood, 500Mb), and Oryza sativa (rice, 420Mb). The average genome size for flowering plants is 6,400Mb based on more than 4,000 species, ranging from around 60Mb to 124,000Mb.
Invertebrates — Strongylocentrotus purporatus (sea urchin, 870Mb), Anopheles gambiae (mosquito, 280Mb), Drosophila melanogaster (vinegar fly, 175Mb), Caenorhabditis elegans (nematode, 100Mb), Ciona intestinalis (tunicate, 195Mb), and Daphnia pulex (water flea, 230Mb). I would not want to suggest a mean genome size for “invertebrates”, but I can say that even in mosquitoes they range at least 8-fold.
If people who work on genome size do not immediately accept the population size explanation for differences among taxa, it’s not because they don’t understand the argument. It’s because the data are based on a tiny and non-random subsample of genomes, and because predictions don’t seem to be upheld when we examine genome sizes. Being miscited doesn’t help.