by Greg Mayer
And by sex, I mean, of course, “… the union (SYNGAMY) of two genomes, usually carried by gametes, followed some time later by REDUCTION, ordinarily by the process of meiosis and gametogenesis” (Futuyma, 2009:388). Most of the organisms we know and love– oak trees, lobsters, goldfish, cats– reproduce sexually. But a few of our favorite organisms– whiptail lizards prominent among them– reproduce asexually.
At first glance, what the asexual whiptails are doing makes complete evolutionary sense: why bother producing unproductive males, when you can double your reproductive output by having nothing but daughters? If we start a sexual population with one male and one female, and suppose that females on average have four surviving offspring, two of whom will be female, then the population increases from 2 to 4 to 8 to 16 to… etc. If we start with two asexual females, who also average four surviving offspring, all of whom are female, the population increases from 2 to 8 to 32 to 128 to… etc. You can see that asexuals reproduce a lot faster than sexuals. And it wouldn’t matter if the population wasn’t increasing– the asexuals would come to constitute a higher and higher proportion of the total population. This reproductive advantage of asexuality is called the cost of sex (google image that term for an interesting mix of scientific and non-scientific illustrations!).
So if sex has such a high reproductive cost, why are so many organisms sexual? This is where the whiptails are revealing. Tod Reeder, C.J. Cole, and Herb Dessauer, in their 2002 review of Cnemidophorus evolution, found that
the capability of instantly producing parthenogenetic clones through one generation of hybridization has existed for approximately 200 million years, yet the extant unisexual taxa are of very recent origins. Consequently, these lineages must be ephemeral compared to those of bisexual taxa.
Indeed, the asexual whiptails have evolved so recently that the ancestral sexual forms can in most cases be readily identified (see figure 6 in Reeder et. al). That asexual taxa are of recent origin appears to be true for animals in general (with some notable exceptions): asexuality appears to be an evolutionary dead end. This implies that there is some long term advantage to sexuality, so that asexual species do not prosper and diversify, but rather are extinguished. The paucity of asexuals, despite their large reproductive advantage, argues for a short term advantage to sex as well. There have been a number of suggestions, most supposing that sex is advantageous in fluctuating or changing environments, so that sexual lineages would have higher fitness than asexual lineages within a population.
An essay by Matt Ridley posted at the PBS website for their Evolution series of a few years ago considers some of these issues, as does this page by someone at Brown University, and Nature has an open article collection on the subject. Two of the classic introductions to the subject are Sex and Evolution by George C. Williams, and The Evolution of Sex by John Maynard Smith.