A Changing Environment As Sex Drive?

Why is there so much sex? This is an evolutionary puzzle. After all, sex is costly. It takes time and energy (find a mate, impress that mate, which incidentally can make you a target for predators, and so on). It halves the amount of genes an organism passes on. And it means producing males. (Ow, sounds harsh, doesn’t it?) But for a female it might be beneficial to produce an all-female offspring, as females reproduce and males do not. So, why invest in males (or, more broadly, sexual reproduction) at all?

Still, there must be some advantage. Otherwise, where did all this sex come from? Interesting question, but how do you study this? Well, interestingly, there are organisms that can reproduce both sexually and asexually. A new study uses such organism (a species of rotifer) to address the riddle of sex.

What was observed was that sexual reproduction was favored during periods of adaptation to a new environment. As the tiny critters got better adapted to their new homes, they engaged less and less in sexual reproduction. A similar pattern was observed in fitness. When the adaptive pressure (here, the need to adapt to a new environment) is strong, the sexual ones were fitter than their asexual cousins. But, as the small animals got used to their environment, the tables turned. Can this explain why there is such a thing as sexual reproduction?

The authors looked at both “short-term” and “long-term” effects. In the short-term, sex breaks down adaptive gene combinations (think of this as genes that “work well together”), resulting in a disadvantage for the sexuals. But, the variation in fitness is higher in sexual than in asexual reproducers, resulting in a long-term advantage (note, though, that the long-term benefit does not imply evolutionary foresight).

The idea is nicely illustrated in following figure from a primer on the study:

Comparison between sexual and asexual reproduction from a genetic point of view.
(Source: Roze, 2012)

The red dots represent bad mutations, the green dots good ones. Sexual reproduction produces a larger array of gene combinations, both worse and better than the asexual ones. So, on average, sexuals have a lower average fitness than their asexual counterparts, but they also produce the fittest individuals (with lots of green dots and/or very little red ones). Since these will do very well, they will contribute most to the next generation, thereby passing on the genes for sexual reproduction.

Or, in the words of the authors:

We show that sex creates a diverse array of genotypes, including many that are quite unfit but also others that are very fit in the new environment. Though the average fitness of these sexually derived offspring is lower than that of asexuals, those well-adapted genotypes generated by sex contribute disproportionately to future generations, causing the genetic propensity for sex to ultimately increase.

Importantly, they also highlight the need for further research as they ask the following question:

Do selective pressures in nature change sufficiently frequently to explain the observed levels of sex? This is an empirical issue requiring data from the field. Lab-based studies such as the one reported here are necessary to directly evaluate the potential of hypotheses and to test their underlying mechanisms. However, such studies alone cannot prove any hypothesis as the explanation for the ubiquity of sex in nature. Attempts to study the evolution of sex in the field will be needed to evaluate the importance of results from theory and lab experimentation.

So, field studies on sex are required…

(Keep in mind that there are still a lot of questions about the evolution of sex, and that various factors are likely to contribute.)

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Becks, L., & Agrawal, A. (2012). The Evolution of Sex Is Favoured During Adaptation to New Environments PLoS Biology, 10 (5) DOI: 10.1371/journal.pbio.1001317

Roze, D. (2012). Disentangling the Benefits of Sex PLoS Biology, 10 (5) DOI: 10.1371/journal.pbio.1001321