Why did Sex Evolve?
In order for populations of any organism to survive, they have to be compatible with the selective forces of their environment. But environments are not static- the study of geology is partially the study of changing environments. Over millions of years, continents move around, seas and lakes dry up and fill again, forests turn to grasslands and back again, and tropics can turn into frozen wastelands. So to be able to adapt to an environment constantly in a state of flux, populations have to change their makeup, and that means changing their genes.
Let’s look at an example of how populations change without sex. Bacteria, as I mentioned before, reproduce asexually- one becomes two, becomes four, becomes, eight, becomes sixteen, etc. The only mechanism for genetic change in bacteria is simple mutation. Mutations occur randomly in a population, and only affect one bacterium at a time, which means that unless some overwhelming environmental pressure is present, that mutation might appear and disappear just as quickly when that bacterium dies. If, however, some change in the environment makes that mutation extremely advantageous, so much so that death is the alternative, then the entire population crashes down to a handful of bacterium which have that mutation. So the life of a bacterial culture is very chaotic- it can be fine an dandy one day, and then the next the entire population is reduced to one or two lucky cells, which just happen to have protective mutations. Now, obviously, this strategy has worked out reasonably well for bacteria- as I mentioned, they’re the most populous type of organism on the planet. But it’s only efficient for them because they’re so small, they don’t need large sources of energy to survive, and they reproduce very, very quickly. In the evolutionary past, organisms which began to expand larger than bacteria found that asexual reproduction wasn’t as efficient or effective anymore.
First of all, sex didn’t evolve out of nothing. At its very essence, the purpose of sex is the horizontal exchange of genetic material between members of a population. Now, although bacteria are technically asexual, they have been observed to exchange bits of DNA with each other. This is a very rudimentary kind of genetic exchange, and it’s not considered sex, but I mention it only to let you know that it’s the same kind of exchange that characterizes what we would truly consider as sex. Also, sex is not all or nothing. That is, an organism doesn’t have to choose between only sexual or only asexual reproduction. Take yeast, for example- ordinary baker’s yeast. Yeast is actually both asexual and sexual- depending on the environment. If the environment is favorable, then yeast are happy to reproduce just like bacteria. But if the environment becomes difficult, then yeast undergo sexual reproduction. And how in the world does a single-celled organism like yeast have sex? Well, remember, the purpose of sex is exchange of genes. So this is basically all that’s happening- genes are being exchanged. But to grasp how this is accomplished, I’ll have to explain another concept: chromosomes.
Chromosomes are long chains of DNA that operate, more or less, as discrete units of an organism’s genome. If an organism’s genome is like an encyclopedia, then you can think of them as the individual volumes. You don’t find chromosomes in bacteria- their genomes are just one long strand of DNA. But in all the organisms from yeast all the way to humans, chromosomes are used. Chromosomes help to organize an organism’s genome for the purpose of a process essential to sexual reproduction- called meiosis. Meiosis is a lot like the binary fission that bacteria use- the cell simply splits in half. But instead of reproducing the entire genome and passing it on, meiosis begins with two copies of the genome, and produces cells which only have one. But why start out with two copies? Well, having two copies of a gene is like having two copies of a book. If something happens to one copy, you have the second as a backup. In the case of genes, mutations are the primary threat, and so having another copy in the cell allows for it to have a pretty good chance at fixing whatever mutations crop up. It also allows for efficient gene shuffling. For bacteria, the genome is one long strand of DNA, so it can be tricky to figure out where to add or subtract DNA in the even of a horizontal transfer. But if you split up the genome into discrete units, as in chromosomes, and then you make sure to have two copies of each, all you have to do is swap chromosomes back and forth to get a pretty efficient shuffling of the genes. Just think of shuffling playing cards- each suit represents a different organism’s genome, and each card is a different chromosome. If you shuffle the cards together, there are many possible groups of cards that could result. For bacteria, it’s like the cards are taped together in a long chain- not so easy to shuffle.
And that’s essentially what sexual reproduction does, all the way from yeast to humans. A single copy of each chromosome from both parental cells is combined to make a new cell that has two copies of each chromosome, one from each parent. The upshot of this is that mutations have a higher penetrance in the population, but without much risk, because if they’re unhelpful, then the second copy of the gene usually makes up for it, and if they’re helpful, they tend to increase in the population. This effect of gene shuffling allows for greater adaptability, as compared to asexual reproduction where all members of a population are essentially clones of each other. If the environment becomes unfavorable for one organism of an asexual population, then it’s unfavorable for all the organisms, because they’re essentially identical clones. But for a sexual population, gene shuffling makes more variation among the population itself, meaning that on average, a greater percentage of the population will be able to adapt to a changing environment that becomes unfavorable for many of the population.
One specific explanation of this advantage of sex is called the “Red Queen Hypothesis.” The name of this hypothesis comes from the character of the “Red Queen” in Lewis Carroll’s story, “Through the Looking Glass,” which is a sequel of sorts to the more widely-known “Alice in Wonderland.” In the story, Alice meets characters from a chess game playing a very abstract game of chess, and the character of the Red Queen tells her that in order to stay even with the other players, it’s necessary to run as fast as you can. This idea, of strenuous competition simply to maintain the status quo, is at the heart of the Red Queen hypothesis. Now, organisms, and even populations, don’t exist in a vacuum- they interact with all manner of other species all the time. So if two species are inter-dependent- let’s say, wolves which prey on rabbits- then a mutation in one species affects the environment of the other, in that the environment also includes all organism populations. So let’s say that a mutation appears in rabbits which makes them twice as fast. This mutation will be highly selected for in the rabbit population, and so within a short amount of time, the population of rabbits will be able to outrun all the wolves. Now, obviously, the selective pressure is now on the wolf population. All the slowest wolves will be unable to catch any of the faster rabbits, and will be selected against. Only the very fastest wolves, perhaps those with mutations that make them faster than the rest of the population, will be able to catch rabbits and pass on their own genes.
In addition to predator-prey relationships, there are also parasite-host relationships. Parasites tend to have shorter lifespans than their hosts, and thus reproduce much more quickly. So the potential for mutational change is much greater in a parasite, which means that for a host organism to successfully resist any particular parasite, it has to have the right combination of genes. The most effective and efficient way to maintain a population with the right combination of genes, while at the same time maintaining genes which are not necessary now but may be necessary in the future, is sexual reproduction.
To review, sex is the horizontal exchange of genetic information between members of the same population. The purpose is to increase the amount of genetic variability within a population, especially for those organisms which have a slow reproductive rate, such as vertebrates. The evolutionary benefit of this increased genetic variability is the enhanced ability to adapt to changing environments, which include interdependent organisms, as well as avoiding dependent organisms such as parasites. I know this hasn’t been quite as titillating as some of you might have hoped, but there’s much more evolution that goes into the naughty aspects of sex, and we’ll get to those eventually.