The evolution of sex is one of the hardest problems in evolutionary biology. I'm defining here sex as the combination of genes between two individuals producing a new individual or set of individuals that is genetically novel from the parents. There are two pieces of it and I'm only going to talk about one of them. These are:
- How did sex originate?
- Why has it remained?
You would expect asexual reproduction to to win hands down at evolutionary roulette. After all, there's a fair cost to sexual reproduction. Regardless of how it's done, whether there are two organisms that find each other against all, subsequently losing their reproductive opportunity when their host ship strikes an iceberg or vast clouds of paired cell types mixing together in the restless sea, two different genetic patterns have to discover each other, meld and produce a new organism.
Cells reproduce non-sexually by means of mitosis: duplication of the DNA into two new cells. Reproductive cell division is different in that the result is a cells with half the original complement of DNA. Even worse, for the DNA to be meaningful it has to be a meaningful half-- a complete working copy of the mechanism. Which means each cell of a sexually reproducing organism has to have double the amount of necessary DNA for its operation. Otherwise, when it produced sex cells they would not have the necessary completeness for the next generation. More expense.
We have the expense of sexual delivery and the expense of duplicated material. In addition to that, given the increase of raw material there is the increase of the total amount of mutations in that raw material.
These are the basic costs of sex itself. But there is a second category of problems with it. The reproducing organism only gets to pass on 50% of its genes.
With all of this, nearly all of the multicellular organisms use sex. Multicellular organisms who reproduce exclusively asexually are often found to be evolutionarily recent and die out quickly. All this despite its cost and inefficiency.
You can't win. You can't break even. You can't get out of the game.
There are a lot of theories about sexual advantages, it being one of the central problems.
The oldest idea, proposed by August Weismann back in the nineteenth century, was that sex introduced dissimilarity in offspring thereby enabling variation. Variation is the necessary precondition to natural selection. A variable population is better able to respond to a changing environment. Asexually produced offspring and genetically identical to the parent. Whatever variation has to derive from beneficial mutations-- a rarity.
Another approach to variation tackles the issue of variation propagation. In an asexual species a beneficial mutation is limited to the bloodline of the parent. In effect, all bloodlines within a species are competing directly with each other as if each bloodline were a separate species. There is no mechanism to share possibilities with one another. In a sexual species a beneficial variation can spread through the species, sometimes very quickly. Such a propagation is called a selective sweep.
One of the more widely held concepts (and one that I particularly like) suggests that a variable population is better able to resist parasites and disease. Let's say you have a genetically identical population of animals. Like any other population, they have parasites that prey on them. If a parasite develops a novel and effective way of attacking the population it can strike them down to the last individual since they are all genetically identical. (This theory, by the way, was derived prior to the discovery of shifts in traits across generations by non-genetic means. As far as I know there is no modulation of the hypothesis from this recent discovery.) A genetically diverse population, and one that continues to propagate its diversity, presents a moving target to parasites.
This is something we've seen in both plant and animal husbandry in recent decades. Several plant disease, such as the potato blight, had as a precondition use of a monoculture of agricultural organisms.
It's also been seen in the wild recently. Potamopyrgus antipodarum is a common fresh water snail in New Zealand. There are sexual versions of this snail and asexual versions. The population dynamics of the two versions were studied over the ten years. (See here.) Initially, the clones had the numeric advantage. But over time they degraded and some bloodlines disappeared altogether. The sexual populations remained largely stable.
There is strength in diversity.