Sep. 07, 2010 - Issue #777: The Sex Issue 2010

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The Science of Sexuality

Evolution is what's making you feel funny

Lewis Kelly / lewis@vueweekly.com

You don't need to be Tiger Woods to know that sex can be kind of expensive. Clothing, chocolates, restaurant meals, bar tabs, hotel rooms, video cameras, legal fees, hush money—it can get downright exorbitant. But there are other, more fundamental costs of sex that all humans pay, even if their spouses never catch them wearing a pair of fuzzy handcuffs in a Las Vegas hotel room. In fact, these biological costs of sex are paid by all organisms that do the dirty, from bonobos to blue whales. Combined, they make sexual reproduction an evolutionary puzzle, or at least something that needs explaining.

The most obvious of sex's drawbacks, first identified by John Maynard Smith in a book called The Evolution of Sex, is the twofold cost of sex. "Organisms that are asexual are able to pass on all their genes to each of their offspring, because they're producing clones of themselves," explains Dr Monika Havelka, a professor of biology at the University of Toronto-Mississauga. A sexual organism, in contrast, needs to mix its genes with those of its partner, so it only gets to pass on half of its genome to its offspring. As Havelka says, sexual organisms "have to reproduce twice as much to pass on the same kind of genetic legacy as an asexually reproducing organism."

The other half of the twofold cost of sex points the finger at the fellas. "Males are essentially useless in that they don't produce their own offspring," says Havelka. "When a female produces a male offspring, it's not that she produced a genetic dead end, because that male will presumably mate, but it doesn't produce it's own offspring. It's just sort of a necessary accessory to the reproductive process." Because males can't bear young, sex is far slower than asexual reproduction—an evolutionary disadvantage.

And there are other costs, too. As Dr Heather Proctor, professor of biology at the University of Alberta, explains, asexual organisms, in addition to reproducing more quickly than sexual ones, avoid a lot of the hassle of sex. Asexual organisms usually produce more offspring than sexual ones, she says, because, "They don't have to bother with courtship."

And courtship can break the bank for some species. Pacific salmon famously die after spawning, while a female praying mantis sometimes bites off her partner's head before copulation.
Or there's Tidarren sisyphoides, a species of spider. Come mating time, males rip off and discard one of their two reproductive organs to better catch flighty females and copulate. Ripping off half of your external genitalia just for the chance to get lucky makes the cost of some new clothes and a few drinks at a bar seem positively trivial.

"Asexual organisms, there's a whole chunk of life they don't even have to bother with," says Havelka. "There's a lot of time and energy and risk devoted to sex that's just absent in asexual organisms."

The costs of sex don't stop there, either. Sex, by definition, breaks up favourable combinations of genes. "When you have sexual reproduction, if you mate with somebody who happens not to have that perfect combination of genes, then you're basically diluting your nice array of genes," says Proctor. "Your offspring would be more poorly adapted than you."

Imagine that you're Superman, and you're looking to give your kids as many advantages in life you can (beyond the obvious ones like getting to hang out in the Fortress of Solitude and saying, "My dad could beat up you dad!" and have it be true beyond doubt). Who do you mate with? Lois Lane? Come on, she's a puny human. Your kids would barely be able to hover, let alone fly. Who else? Since Superman can't clone himself, his kids can't end up Super.

The final cost of sex is also the grossest. Havelka points out that in most sexual species copulation brings moist, warm body parts in contact with one another, creating ideal conditions for parasites, viruses and diseases to jump from one organism to another. Sexually reproducing species "have to have better immune systems in many ways," she says. "The ability for passing on pathogens is much greater when you have sexual contact." Sexual organisms are forced to invest in immune systems to allow them to reproduce, and that investment does not come free.

Yet despite all this, almost all animals reproduce sexually. (Two notable exceptions are whip-tailed lizards and the Amazon molly, a fish that reproduces asexually but must mate with a male from a closely related species to start the process.) Sex "does need explaining," says Havelka. "Why do we see so many sexual organisms? You would think that it would be some sort of weird quirky thing, that it would occur only in a few backwater, weirdo kind of organisms. But yet it's very, very common."

Things get even more confusing when you consider that asexual reproduction predates sexual reproduction by about two billion years. How did sex become so widespread? What are the advantages of sex?

"No one really knows for sure if there is a single, overwhelming advantage," says Proctor, "but there's lots of little advantages."

These advantages usually revolve around greater variety in offspring. For example, while sex does break up favourable genotypes (Superman's dilemma, discussed above), it also can create new ones that might be better adapted to a changing environment.

Sex "creates novel combinations that wouldn't otherwise show up in asexual reproduction," says Proctor. Havelka likens sexual reproduction to banking on a variety of numbers in a lottery instead of just one.

Another big bonus of sex pertains to parasites. Parasites want to become as well-adapted to their hosts as possible. Their hosts want to prevent this, and so a kind of genetic arms-race begins. The random genetic shuffle that sex insists on in each new generation means that hosts born of sex start the race one step ahead of their unwelcome guests. "If the host is changing, then every generation of parasites are sort of a little bit behind, and need to catch up again," says Proctor.

And the bigger an organism, the more parasites can jump on board. "Once you get large-ish bodied, then you can have hundreds or thousands of other things trying to take advantage of you, because you can be lived in and on," says Proctor. Thus, large-bodied organisms exploit the parasite-fighting powers of sex more than small-bodied ones. The vast majority of large-bodied animals reproduce sexually.

So sex persists. We may not totally understand how it came to be, but we can infer that it pays for its costs by observing how widespread it is. "Sex is pretty much the norm for animals," says Proctor, "and most plant lineages at least occasionally have sex."

Still, it's tough not to speculate about what humans would be like if they reproduced asexually.
"We'd all be female, because every member of the population would be able to give birth," says Havelka. "I guess we'd all be active lesbians." V