A pair of mating sacrophagus flies, who, evidently not having read this blog, are blissfully ignorant of the foolishness of their actions.
Sex is a problem for evolutionary biologists, a very big problem. (Let’s be clear: we don’t personally have any more difficulty with it than does anyone else; it’s strictly a professional problem!) And here it is: By all rights, sex shouldn’t exist. Ask most non-biologists what sex is “for,” and they’d probably answer “reproduction,” but they’d be wrong.
In fact, sex is quite simply a terrible way to reproduce.
The reality is that living things can easily make babies without sex, and many do just that. Lots of animals breed asexually, via parthenogenesis (development of an unfertilized egg, without any involvement by males); the list includes many insects, crustaceans, rotifers, flatworms, snails, even some vertebrates including certain species of shark, lizard and the occasional bird. And it’s quite common in plants.
The evolutionary conundrum is that compared to asexual reproduction, breeding sexually poses a daunting number of disadvantages, so many, in fact, that a number of highly regarded evolutionary theorists have concluded rather glumly that sex may actually be a biological liability, something that we—and other species as well—are regrettably stuck with. Here is a brief catalog of its downsides, reasons why sex is such an evolutionary dilemma. (Fear not, however. I’ll conclude, next time, with its likely saving grace.)
First, there are a number of ecological downsides. For starters, every individual who reproduces sexually needs to find a partner, which is often easier said than done. The Indonesian rhinoceros, for example, has become so rare, in part, because male and female literally have difficulty encountering each other, it seems. And we all know people who have their own troubles finding an acceptable mate. How awkward, and how much easier it would be if breeding were simply up to each would-be parent, acting on her own!
According to Aristophanes in Plato’s Symposium, ancestral human beings didn’t have this problem, because they were a bit different from today: Each had two heads, four arms and legs, and two sets of genitals, one male and one female. Every individual was thus a complete, reproductively competent package. But these characters, called Androgynes, grew uppity and self-important, so Zeus decided to diminish them by hurling thunderbolts, as was his wont. Each Androgyne was neatly split in two, and as a result, we have been doomed ever since to search the world for our missing other half. And indeed, people—like Indonesian rhinos—often fail in their quest.
Even success typically comes only after expending huge amounts of time and effort … not just visiting singles’ bars or using Internet dating services, but the far greater expenditures by which people try to make themselves sexually attractive via clothing, the purchase of things from pets and cosmetics to automobiles and houses, not to mention the vast amount of time spent worrying about and trying to enhance one’s “social life.” By contrast, the Androgynes must have saved lots of time and energy; no wonder they became so haughty!
If searching for and attracting a mate weren’t troublesome enough, there is the problem of same-sex competition. Thus, because of the way sexual reproduction works, members of one sex—typically males—are busy (often to the point of obsession) competing with each other for the attention of the opposite sex, competition that not uncommonly results in injury and even death. On top of this, there is the problem that both male and female have to rely on each other’s benevolence, or, failing that, their mutually shared interests.
But the unfortunate reality is that the reproductive interests of male and female don’t necessarily correspond. To be sure, they share an evolutionary interest in the success of their conjoint offspring, but beyond this, each male and female (like any two distinct individuals) is likely to have his and her unique interests, independent of the other. As a result, each can be found, on occasion, actively doing things that works to their own evolutionary benefit, in the process detracting from the other’s fitness. For example, even in a presumably monogamous species, both male and female are often tempted to cheat in several ways: notably, by exaggerating his and her desirability as a reproductive partner, while also being prone to philandering. Infants have their infancy, and adults, their adultery.
Of course, none of this would matter if reproduction were achieved individually, like eating when hungry or sleeping when tired. To mate with another individual, by contrast, is to leave one’s self vulnerable to being exploited.
For all these liabilities, the greatest downside of sex hasn’t yet been mentioned: Its genetic cost. When a rotifer or flatworm or whiptail lizard reproduces asexually, each of its offspring is a genetic clone of the parent. One hundred percent of the parent’s genes are represented in every child. By contrast, when reproduction is sexual, this number is reduced by precisely one-half; there is a 50-percent “cost of meiosis,” named for the process whereby males make sperm and females, eggs. This is because meiosis is a “reduction division,” by which one-half of every mother’s genes are not represented in each of her eggs, just as one-half of the father’s genes don’t make it into any given sperm. And so, every child produced by sexual reproduction represents a 50-percent genetic tariff relative to what an asexually produced offspring would offer.
In the world of evolutionary reality, a very small “fitness differential,” on the order of hundreds or even thousands of a percentage point, can result over time in vast biological changes. In this context, a differential of fully 50 perccent is humongous. Add this unavoidable genetic loss to the various ecological obstacles described above, and the case against sexual reproduction seems staggeringly large.
Yet, sex is very much with us, and the good news is that it appears to have at least one redeeming virtue (although even now, whether this is sufficient to overcome its manifest liabilities remains tantalizingly unclear). I’ll discuss it next time.
(Fornicating flies courtesy of Wikipedia)
