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Everyone likes sex. But why do so many species of animals reproduce sexually? An interesting question I though, and a widely debated subject in biology... I thought I'd pull together a few of the main theories, and see whether they can explain the appeal of sex...

Asexual reproduction, seen in bacteria, some plants, and a few small animals is much more advantageous. It's quick, easy, means you get to pass twice as much genetic material to the next generation. The existence of sexual reproduction can seem like an evolutionary puzzle - it has many costs associated with it; animals have to find and compete for mates, run the risk of catching STDs, do not get to contribute such as high amount of genetic material to the offsrping as asexual individuals, and also run the risk of breaking up good combinations of genes (no matter how handsome you are - if your partner has some 'ugly' genes somewhere - your offspring might inherit them!)

However, there are many theories as to why sex might exist, mostly focusing on the indirect benefits. Although sex can break up good genetic combinations, it can also break up bad mutations that are combined with good ones, i.e. freeing the good ones. The Fischer-Muller hypothesis focuses on this, stating that in large (but not too large) populations, multiple beneficial mutations will occur simultaneously, and sex brings these together. The Hill Robertson hypothesis furthers this by stating that in small populations, ,sexual reproduction will be beneficial if genetic drift (i.e. chance) results in beneficial mutations being associated with bad genetic backgrounds.
The Mutualistic determinstic hypothesis works on a slightly different theory, which assumes that both parents will have good and bad mutations, and that by sexually reproducing, they can dump all their bad mutations in some offspring, have some that are a mix of good and bad, and have some that get all the good mutations. There is little evidence to support it however, despite explicit tests.

Muller's ratchet is a hypothesis that explains how functionally important genes can be lost when transmitted solely by vertical transfer (e.g. bacterial endosymbionts in insects, which are only transmitted from mother to daughter.) Here, if a beneficial mutation occurs alongside a negative one, it is trapped next to it, and offspring do not have a chance to get a good copy of the gene. The deleterious mutations have the action of a ratchet, in that the organism can never go back. In contrast, sexual reproduction means that an organism can get two good copies of a gene via genetic mixing, so two parents with advantageous mutations can pass both on to their offspring. Horizontal gene transfer allows a similar situation, so it is generally only vertically transmitted genes and single copy ones such as the human Y chromosome that suffer from this. Muller's ratchet is an important theory, and there is some evidence to back it up (Chao 1992, work on bacteriophages) but it work better in small populations than larger ones.

Generally considered superior is the Red Queen Hypothesis, championed by Van Valen, and described in detail by Ridley in pop-science book of the same name. The theory treats each offspring as an experimental mix of the parents genes, in a rapidly changing environement, where new genes combinations may allow it to just hold on to the niche (i.e. the ecological space) it has obtained. The name comes from Lewis Carroll's 'Through the looking Glass' when Alice complains to the Queen that she is having to run very fast to stay in the same place - to which the Queen replies "Here you see, it takes all the running you can do to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!"

The theory describes a cyclic arms race that all organisms are running with their parasites, in which the role of sexual evolution is to create new combinations, and preserve genes taht are not useful in one generation, but may be useful in the next. Parasites evolve quickly because of their short life spans, and can evolve new ways to attack hosts, meaning that two consecutive generations of organisms can fact vastly different selection pressures. If this change is rapid enough, it might explain the prevalance of sex.

There is some evidence to back this up, from snails and trematodes in New Zealand lakes (1998) where genotypes (genetic characteristics) cycle through time, and rare genotypes are relatively less infected. Sexual snails are more common where there is high parasite prevalence. There are some problems with this however, all the data is correlational - and it is unclear whether the observed fluctuations are enough to select for sex. Coevolution with parasites may select for diversity, rather than sex.

It turns out there is not good support for any one theory, it may be that it has different causes from system to system. In fact we probably need to rely on a pluralistic approach. Coevolution with parasites increases the likelihood of Muller's ratchet through reductions in population size, parsites may increase the cost of mutations - reducing how many turns of Muller's ratchet are needed to give sex an advantage.