A Moment of Science

Sex and Survival

One of many things that evolutionary theory provides is a framework for thinking about and investigating the biological phenomenon of sex.

"Male and female created he them," [Genesis 1:27] is no explanation. Indeed, it begs the question of what "he" means before there was any male and female.

Sex is mixing and matching the genetic material of organisms and there is no doubt of its importance in the living world. Species that lose this ability tend towards extinction. The benefits of it are so great that they outweigh the considerable cost of having, for example in our species and many others, almost half the member organisms being reproductively unproductive males.

There are various explanations. There is the "Red Queen Hypothesis," after the Alice-in-Wonderland character who had to run as fast as she could just to stay in the same place. The idea here is that because sexual reproduction allows faster genetic change, organisms indulging in it can compete better against each other and/or rapidly-changing external forces. An example would be the rapid emergence of antibiotic resistance in bacteria that share their genetic material with each other. Another idea is that sexual reproduction allows faster elimination of harmful mutations as well as better preservation of neutral or less harmful mutations that might come in handy in the future.

PhilodinaBut, then, how to explain the sex-starved members of a whole class of organisms, the bdelloid rotifers? These multicellular organisms — they're each composed of around a thousand cells - live in water, eating things smaller than themselves and being eaten by things larger than themselves. They reproduce completely asexually — a male has never been seen. Based on DNA studies — because each individual bdelloid is reproductively isolated, the degree of genetic separation between them is entirely the result of time — these organisms seem not to have engaged in any sex for many tens or even a hundred million years or more. [Talk about abstinence only!]

Bdelloids are "the exception that proves the rule" when it comes to sex. [BTW, the exception-rule saying is almost certainly a rationalization!]

It turns out that at some time in the past these organisms duplicated their diploid (two copies of each chromosome) genome and then stopped engaging in the exchange of genetic material with each other. They haven't had sex for 85 million years! Over time, some of the genes on the redundant chromosomes were lost, creating a condition known as degenerate tetraploidy.

This condition appears to allow the bdelloids to do something that's very important for their lifestyle, which is to recover from extreme environmental conditions such as being completely dried out with their DNA all beaten up and broken. That is, the redundancy in each organism's genome allows them to repair extreme damage — to "take a licking and keep on ticking." Experiments show that they can withstand doses of ionizing radiation five times higher than their sexually-reproducing fellow rotifers.

Well, all of this has interesting scientific implications for all sorts of things, from sex to evolution to resistance to pathogens to extraterrestrial biology, to mention just a few. It should also suggest to us not just the biological but the accidental nature of our attitudes and feelings about so many things, as well as about sex and gender. Reproduction is, after all, "just" a normal bodily function. We are "only human" after all, and, in addition, "only animals."

But, then, this is only this month's "Moment of Science™" Think of the extraordinary amount of time and energy devoted to sex and everything connected with it: love songs and sonnets, poems, great art, and even great historical events and social transformations fueled by intense and volatile emotions and behaviors, the raptures of romance and the comforts of hearth and home and the devotion of family, as well as the outrage and violence that can accompany the frustration of these needs and desires. What about all of this? Trivial? Extraneous? Or perhaps all of our other cleverness is "only science?"