The Pith: The primary reason for the pervasiveness of sex among complex organisms is to maintain genomic integrity, not to increase genetic variation.
. I just read a very strange article in the journal Evolution, Sex reduces genetic variation. In it the authors argue that contrary to conventional wisdom and evolutionary orthodoxy the rationale for the prevalence of sex amongst eukaryotic organisms is not maintenance of genetic variation, but rather a constraint upon genetic variation! This is a very peculiar view, and as someone not immersed in the literature on sex totally surprising to me. The standard model is simple: sex allows organisms to swap genetic material and generate new combinations. This is at a particular premium for large, complex, and slow-breeding lineages, as is the norm amongst eukaryotes. In contrast, bacteria and their ilk have huge population sizes to draw from, and are quite literally protean in their ability to shift strategies to climb whatever adaptive landscape nature throws at them. Carl has a nice review of a paper in Science which reported just this finding in keeping with expectation. Increase the pathogen pressure, and eukaryotes which exchange genes marginalize those which do not because they can dodge the punches that their evolutionary adversaries throw in their direction. The authors of the Evolution paper don't deny that this is part of the story. Rather, they propose that this is a secondary part of the story. They acknowledge the power of microevolutionary pressure to modulate the basic genomic template on the margins where variation at the level of the gene is the target of selection. But, their contention is that by and large sexual reproduction's primary value, which overrides the two-fold cost of sex, is to
maintain the integrity of the genome of the parent.
In other words, at the scale of the whole genome sex facilitates homogenization and stabilization of the basic ancestral template. There seem two primary issues. Both of which I can not evaluate with any degree of confidence because of my lack of deep familiarity with the literature (no, I don't check every single citation in a paper before reviewing it!). First, there's the empirical issue, where the authors argue that in contrast to expectation asexual lineages are not more homogeneous or less diverse than sexual ones. The assumption has been contradicted, the authors assert, by new genomic techniques, though they grant that more work needs to be done. This very diversity and tendency to radiate in all directions argues against the stability of these asexual lineages. They also suggest that cancer, an asexual process, is a proper analogy for the riotous directional diversity of reproduction without sex (mitosis vs. meiosis in this case). But perhaps more importantly within the paper there are some theoretical considerations. They argue that there is evidence that sex, as defined specifically by the process of meoisis and fusion of gametes, has a role to play in encouraging DNA repair, preventing the replication of chromosomal abnormalities, and dampening epigenetic variation. Let me quote their abstract:
...The common thread among many of these disciplines is that sex acts like a coarse filter, weeding out major changes, such as chromosomal rearrangements (that are almost always deleterious), but letting minor variation, such as changes at the nucleotide or gene level (that are often neutral), flow through the sexual sieve. Sex acts as a constraint on genomic and epigenetic variation, thereby limiting adaptive evolution. The diverse reasons for sex reducing genetic variation (especially at the genome level) and slowing down evolution may provide a sufficient benefit to offset the famed costs of sex.
Since the authors grant that sex does provide benefits in maintaining additive genetic variation at the level of the gene, it stands to reason that the mainstream research program should yield positive results. Rather, the thrust of this paper seems to be that this might be a case of looking for lost keys underneath the lamp-light. Ultimately I think this argument ties back into the unit of selection argument, though they seem to avoid that explosive issue in any explicit sense. I'm somewhat a loss to really tote up the plausibility of this heterodox proposition with any clarity of conscience or confidence. Rather, I am curious as to what readers think? This did get published in Evolution, which from what I recall was created to midwife the Neo-Darwinian Synthesis. But I also know there are people who read this weblog who know quite a bit about DNA repair or epigenetics.... Citation:
Gorelick R, & Heng HH (2011). Sex reduces genetic variation: a multidisciplinary review. Evolution; international journal of organic evolution, 65 (4), 1088-98 PMID: 21091466