Sex, it turns out, is a risky affair. Besides infidelity, sexually transmitted diseases or pregnancy, it can also perpetuate harmful genes. Also, compared to asexual reproduction, it’s very inefficient—bacteria, strawberries and even some amphibians don’t have to look for a mate. So, why have sex—sexual reproduction, that is?
The answer actually is a little controversial. But a common thread among all the theories behind sexual reproduction is—it reduces the risk of extinction in the face of evolutionary change.
Asexual reproduction is simple; a cell simply divides, or an organism creates a bud off itself. Because the only genes involved are those of the parent, there’s not a lot of genetic variation taking place. But if you look at meiosis, the cellular process of taking half the chromosomes of the father and half of the mother and combining them in the offspring, you create a multitude of reproductive possibilities. So, 100 mutations that could benefit (or harm) a species in the future can result in genetic combinations that border the number of elementary particles in the universe, versus about a hundred combinations if the same mutations were passed on through asexual reproduction.
So, where’s the controversy? The problem lies in the mutations; most are harmful. What’s the point of passing on traits, no matter how diverse, if they’re just going to kill your offspring, or your offspring’s offspring?
There are two key hypotheses for the existence of sexual reproduction:
• Sex can get rid of harmful mutations. This theory compares asexual reproduction, in which an organism with a bad mutation dies with that mutation, with sexual reproduction, in which creatures with the most bad mutations die, leaving creatures with the least bad mutations to carry on. Thus, the mutation could still crop up again in an asexual creature, but sexual reproduction does a better job of making sure that mutation is gone for good.
• Sex fights disease. Many diseases involve a struggle by pathogens to figure out the defenses the body puts up against them. These defenses are encoded by genes, and many parasites and other bugs try to figure out how to bypass the various proteins that are designed to fight them off. The more variation that exists, the harder it is for pathogens to break these genetic codes.
Both hypotheses have their pluses and minuses. But in a Nature paper a few years ago, Sara Otto points out that sexual reproduction’s success is due to its imperfection. She points out that sex is a response to changing conditions, and different forces of natural selection. So, a perfect system now won’t be later, or vice versa. Sexual reproduction is the only kind that keeps variation in reserve.
Of course, there is one more explanation: sex is fun. But try telling that to the first organisms that reproduced sexually, about 2 billion years ago. Good luck; these protists and proto-bacteria didn’t have nervous systems that detected pleasure, much less language.
Otto, S. (2008). Sexual reproduction and the evolution of sex Nature Education, 1 (1)