Why is sex good?

November 22, 2010 • 5:57 pm

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.

Cnemidophorus inornatus (sexual ancestor), C. neomexicanus (unisexual daughter species) , C. tigris (sexual ancestor). c Alistair J. Cullum. Used with permission.

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.

Neanderthals and sex

March 9, 2009 • 12:10 am

by Greg Mayer

A couple of news items from the past month deserve a quick comment or two. First, at the American Association for the Advancement of Science annual meeting in Chicago last month, Svante Paabo of the Max Planck Institute announced the completion of a draft genome for Neanderthal man, and that it indicated that modern man and Neanderthal man did not interbreed: the Neanderthals are our evolutionary cousins, not members of our own species.  The work by Paabo on the Neanderthal genome, and on ancient DNA in general, is fabulous, but two caveats must be noted: the first draft covers only 63% of the genome; and, most of the DNA comes from one cave in Croatia. So what you can say is in the 63% of the DNA they’ve looked at there’s no sign of interbreeding at this location. But we know secondary contact of differentiated populations in ice age Europe can be very complex; e.g. hooded crows would show evidence of interbreeding with carrion crows if sampled in some places but not others, so the case isn’t closed. And a personal quibble: according to the BBC

The draft genome can give us clues to the genetic regions which make us “uniquely human”, Prof Paabo told BBC News.

Besides the usual need to realize that knowing the genome of “X” doesn’t mean we know what it is that makes “X” so “X-ian”, Paabo implies here that Neanderthals weren’t human.  But by any biologically coherent notion of human they were (hence Homo neanderthalensis). John Hawks (hat tip: Pharyngula) has an excellent discussion of all sorts of issues relating to the Neanderthal genome.

Second, John Long (who has a wonderful book on fish evolution) and two colleagues published a paper in Nature (abstract only) reporting internal fertilization and vivipary in a placoderm, a group of ancient fish. This is a wonderful discovery, showing again that Philip Skell doesn’t know what he’s talking about (Skell, you’ll recall, had said fossils “cannot reveal the details that made these amazing living organisms function”!!!). But the paper got twisted in media reports into these fish inventing sex.  The BBC headline said “Fish fossil clue to origin of sex“, while, even more inexcusably, the British Museum (Natural History) website had “Fish knew first about sex“. Sexual reproduction originated in bacteria, probably billions of years before these fish. These fish may be the earliest vertebrates with copulation with an intromittent organ (a penis or similar structure); such organs have evolved multiple times, including four times in the amniotes (reptiles, birds, mammals).  The price of journalism is eternal vigilance.