Evolution Update

Evolution Update

The Downside of Sex

Dan Bowes September 25, 2015

When sexual selection works against natural selection.

The appearance of sexual reproduction is a key step in the evolution of complex organisms. The advantage is pretty clear, it allows an organism to have access to a much greater variety of genes; by combining the genes of two individuals an organism can ensure the survival of its offspring even if it does not possess the essential genes itself. This is because half of the organisms offspring's genetic information come from another source, so may contain the genes necessary for survival.

There is another advantage to sexual reproduction: more advanced animals can deduce information about their potential mates, make judgements about their quality of genes, and select the best mate accordingly. However this advantage comes with a potential downside, it relies on the individual's ability to judge what traits in a potential mate would lead to successful offspring.

This results in an alternate mechanism of evolution: sexual selection. Sexual selection differs from natural selection in that the characteristics introduced by sexual selection are a consequence of the individuals choosing others to mate with, whereas the characteristics introduced by natural selection are a consequence of unfit individuals being unable to survive long enough to reproduce.

Sexual selection can often lead to a downward spiral of evolution: individuals within a species may start to recognise certain features as indicating good health and hence good genes, illustrious feathers for example, or large horns. Individuals with these traits will be selected for mates and their genes will proliferate.

To start with this helps the species, it allows for quick selection of the best mates and the best genes. But after time it starts to get out of hand, sexual selection promotes the evolution of larger and larger horns and more fabulous feather displays. This trend can continue until the species has reached a point where its survival may actually be hindered by these weird and unusual adaptations.

Another disadvantage exists. In many cases, being identified as an individual with strong genes may result in excessive and unwanted attention from the other gender (which I’m sure many attractive humans can understand), to the point where it can actually reduce their survival prospects.

This exact effect was shown in the model species Drosophila serrata (the fruit fly) by biologists at the University of Queensland. By separating the flies into groups and comparing large females to small females they showed that where males had no choice of mate, i.e. they were presented with either a large female or a small female, the large females were both more fecund (higher reproductivity) and had higher survival rates than the smaller females.

However, when the males were presented with both small and large females, and could choose between them, the males would preferentially attempt to mate with the larger females. Consequently this increased attention reduced the survival rates and fecundity of the larger females to the point that there was no difference between the large and small females.
In essence, any advantage a more fit female had was completely negated by the males who wanted better genes for their own offspring, even when in this case the larger females didn’t actually have a genetic advantage as the size difference was caused by the environment they were raised in.

The biologists were also able to identify genomic changes in the fly populations as a result of this experiment and one pattern was particularly common. Mutations that were increased in frequency by natural selection alone were also increased, but to a lesser extent, when sexual selection alone occurred. Even more interestingly, when sexual and natural selection were in effect the frequency of these mutations was further reduced.

The implication of this pattern is pretty clear: genetic mutations that are advantageous to survival (which we know because under natural selection only they increased in frequency), are repressed by sexual selection. Sexual selection therefore is slowing this species ability to adapt to changes in its environment.

Photo by Sue Salisbury

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