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.

25 thoughts on “Why is sex good?

  1. “This reproductive advantage of asexuality is called the cost of sex (google image that term…”

    …thanks, but, I’m eating breakfast ;-))

    Thanks for this! Two more books to hunt down–I’ve recently read Ridley’s The Red Queen and Miller’s The Mating Mind, and it’s like my mind is on fire! I have a *zillion* questions (most of them likely stupid or unanswerable, but…).

    “(…)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.”

    So, does sexual reproduction, as opposed to asexual, make adaptive radiation easier, or more successful? (I apologize if I’m expressing this badly— self-taught:s). What I mean is, does sexual reproduction allow for greater expression of potentially adaptively useful recessive traits to appear (and deleterious traits to disappear)? Am I making sense?

  2. Jerry: do you think that modern evolutionary biology has adequately explained the near ubiquity of sexual reproduction in “higher” organisms? And here’s a related matter: even given sex, why separate sexes? Why wouldn’t you and I be better off as simultaneous or sequential hermaphrodites?

  3. The religious side went completely batshit for me when they started talking about how our society has run rampant with materialism and consumerism and that religion dares to question this. They even brought up the “What about the kids!!!” argument.

  4. I thought (apparently incorrectly) that this was a settled issue with the combinatorically greater variation in sexual reproduction available for selection being the dominant benefit. A population with N individuals that reproduces asexually has the same variation after one generation, but if the species has C chromosomes and reproduces sexually, then you get the possibility of (N choose 2)*4^C unique individuals in the next generation, rather than the original N with copies. That’s A LOT of variation.

    Has anyone ever written fiction about what sexual politics would be like if there were three sexes required for reproduction?

    1. Thank you–nice math. That’s pretty much what I was trying to ask. Surely all that available variation would allow greater radiation.

      (If there were three sexes? Les Liasons Dangéreux? ;-))

      1. Thanks, but love triangles, ménage à trois, and polygamy all give different combinatorics!

        Seriously, I’d also like to clarify the understanding of the difference between male and female, which isn’t very obvious—mammals have XX female and XY male, birds have ZW female and ZZ male, and eusocial insects have some remarkable haplodiploidy.

        As I recall, the best popular explanation of the difference between male and female in sexual species is found in Dawkins’s Selfish Gene, and amounts to the difference in two distinct strategies of producing many gametes that you don’t protect (male) versus producing relative few gametes that you must protect (female).

        1. I don’t have my copy in front of me, but what I recall of Dawkin’s definition was that “female=the sex possessing the larger gamete”. I also recall thinking that his explanation was remarkably clear. It does seem that he left off “protection” and referred only to gamete size, so as not to sow confusion due to the existence of speices (seahorses, beta fish, and others) where the male incubates and “protects” the eggs (seahorses even have a brood pouch and are referred to as “pregnant”). I’m currently reading The Extended Phenotype (finally got ‘hold of it–yay!), so I’ll keep that idea firmly in mind while reading.

          I propose a toast: to clarity!

        2. Males: small, motile gamete, a bare-minimum chromosome-delivery package..
          Females: large, nonmotile gamete supplied with necessary offspring-construction materials.

          It has nothing to do with protection.
          There is an optimum egg-size vs. egg-number, as eggs must be some minimum size to contain enough stuff to build an embryo; often there is some advantage to laying in more than the minimum required. This opens the door to male “cheaters” who don’t invest any building materials but put their resources into making large numbers of swimmers instead.

          Think of finding your friend at Disneyland: it will never work if you both stay put, the chance of running into each other is higher if you both walk around, but the optimal solution is for one of you to stay put (with the snacks) while the other moves around in search.

    2. That’s what I thought too. A successful gene is one that can compete in a wide variety of genetic environments. Sexual species provide that variety; asexual species don’t.

  5. Very interesting.

    So, if adopting asexual reproduction is good for the individual but bad for the long-term survival of the group, is this an example of group selection? (And if not, what is the difference?)

    1. No form of selection cares about or knows the future, so it is only about the here & now. That is why populations boom when resources are plentiful then crash when resources are no longer available.

      1. Well, yes, I know that. But that wasn’t my question.

        Group selection seems to be out of fashion: I’ve seen more than one biologist deny it happens. (I think Dawkins does.) But as this seems to be an example, I was hoping that someone would elucidate.

        1. I’m only an interested amateur, so I feel I’m stepping into some very deep water here, and with no great confidence in my powers as a swimmer. :-s

          I’ve found this confusing as well, and the easiest way for me to understand why this isn’t “group selection”, is to use the word Lineage instead of Group. When I say “group”, I automatically get a mental image of a here-and-now group, herd, pride, and visions of altruistic behavior. “All for one, and one for All”, Natural Selection in a Burning Bush saying “Take off your shoes”, and “Group Hug!”

          Until George C. Williams’ and Robert Trivers’ work, group selection was invoked to explain altruistic behavior. Their work showed that altruistic behavior could be explained as beneficial at the level of the gene, and therefore no group-level explanation was needed. The only reason Group Selection was used prior to 1966 to explain altruistic behavior because, up to that time, there was no way to explain it at a lower level of organisation. George Williams and Robert Trivers showed how it *could* be explained at the level of the gene. Dawkin’s view builds on their work.

          Lineage, as opposed to group, is clearly historical, therefore making it harder to claim that natural selection is acting on its behalf (it would imply that selection is future-directed, or goal-directed, instead of blind). The lineage will, over time, “benefit” by spreading out, giving rise to more daughter species, and so on, but–I think this is the important bit–it can be explained at the level of the gene. You don’t need to invoke group-selection to explain why sexual reproduction is beneficial. The genes in sexually-reproducing bodies ultimately benefit by being in those bodies. The “group” benefits, too, but just downstream, as it were, from the ultimate beneficiaries–the genes.

          Does that make sense? (Real biologists–help me out! And please correct me if I’ve made any dreadful blunders!)

    2. In my Evolution course in college (a couple years ago), the conclusion of our lecture on sex and its two-fold cost was that although we should be very wary of invoking group selection as an explanation for anything in biology, sex is one of the few cases where it seems it could be appropriate.

  6. 35 years ago when I was closely following herpetology, I thought that the Western Whiptail (one of the local lizards here) was largely parthenogenic but that a few specimens were known that were male? Is that now known to be incorrect, or is it possible that whiptails can come up with sexual reproduction in a “crisis”? If so it may not really be an evolutionary dead end.

  7. Surely ID proponents have claimed that sexual reproduction is irreducibly complex. But if something truely could not have evolved *into* an IC form, then there is no way it could evolve *out* of IC. Hence the evolutionary history of these lizards is pretty powerful evidence against ID.

    It’s a point I’ve personally never seen argued before–evidence of evolving out of “IC”. We can get them from two sides at once 😉

  8. I have heard hypotheses that the whiptail hybrid clones may only be a few hundred years old, the parental species having come together as a result of overgrazing-induced habitat change.

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