Saturday, 23 January 2016

Unintelligently Designed War Between Sexes

(a) Histogram of pigmentation score on the last two abdominal segments in ten females in three melanogaster subgroup species grown under similar conditions. Only D. erecta (red) shows a bimodal distribution, whereas both D. orena (green) and D. melanogaster (blue) show a continuous, unimodal distribution.
(b) Photomicrographs of male and female D. erecta showing the dark (male-like) and light female morphs. Scale bar, 1 mm.
Ancient balancing selection at tan underlies female colour dimorphism in Drosophila erecta | Nature Communications

Imagine a designer coming up with something so bizarre as sexual reproduction in which the female gets so badly injured by the male that she has to be redesigned to avoid him because the designer keeps redesigning the male to do her more damage! Would this be a sign of intelligence or a sign of either malevolent sadism or stupidity?

Yet, if we fall for the Intelligent Design hoax, this is what we are required to believe this 'intelligent (sic) designer' has done.

Yes, it's our old friend the arms race, only this time it isn't a war between hunter and prey or parasite and host; it's a war between sexes, this time in a species of fruit fly, Drosophila erecta. You might think that sexual reproduction would be a cooperative thing where both sexes have the same interests - getting their respective gametes fused as efficiently as possible. The problem is that males produce lots of gametes and there are lots of males all trying to fuse at least one of their own gametes or sperms with the small number of female gametes, or eggs. The reason for this is that it takes a lot more resource to make an egg than to make a sperm.

What this difference means is that the different sexes have different strategies. Males want to mate with lots of different females, even females which have already mated, while females only need to mate once.

In the species of fruit fly in this study, researchers have found that this has led to some interesting evolutionary features. Males have a rough penis with which to forcibly mate with a reluctant female; so rough that it causes considerable damage to the female with no benefit to her from this damage, and the more times she is mated the more damaged she becomes. However, the damage isn't enough to prevent her eggs from being fertilised or to prevent her laying them, so there is no evolutionary pressure on the males to evolve a less damaging penis and everything to be gained by him retaining it.

Females, on the other hand, have a great deal to be gained from two strategies: evolving ways to minimise the damage and evolving ways to deter matings while not eliminating them all together. So, there is a balancing act for the females to perform here between not mating at all and mating too many times.

Dimorphic traits are ubiquitous in nature, but the evolutionary factors leading to dimorphism are largely unclear. We investigate a potential case of sexual mimicry in Drosophila erecta, in which females show contrasting resemblance to males. We map the genetic basis of this sex-limited colour dimorphism to a region containing the gene tan. We find a striking signal of ancient balancing selection at the ‘male-specific enhancer’ of tan, with exceptionally high sequence divergence between light and dark alleles, suggesting that this dimorphism has been adaptively maintained for millions of years. Using transgenic reporter assays, we confirm that these enhancer alleles encode expression differences that are predicted to generate this pigmentation dimorphism. These results are compatible with the theoretical prediction that divergent phenotypes maintained by selection can evolve simple genetic architectures.

Copyright © 2016, Rights Managed by Nature Publishing Group. Reprinted under terms of Creative Commons (CC-BY) license.

What this paper shows is that females of this species of fruit fly have not only evolved a form of armour plating to protect their reproductive tract but they have also evolved a strategy for overcoming the genetic sexual dimorphism that evolved earlier in the species history. Sexual dimorphism normally evolves for all sorts of reasons - one being so that time isn't wasted trying to mate with the same sex. Other reasons might be to camouflage the female by cryptic mimicry or to enable her to find the more food she needs to supply the resources of her eggs or young.

In the ancestors of Drosophila erecta it appears to have been to make the males more recognisable to other males both to avoid wasted mating attempts but also to protect males themselves from damage from the penis of a would-be suitor. Males, unlike females, have nothing to gain in reproductive success terms from being forcibly mated by another male and, also unlike females, everything to gain by preventing it.

So, the other strategy used by females, in addition to the armour plated reproductive tract, is to mimic males to reduce the number of matings. Now, you might think this would be disastrous for the species, especially if taken to extremes. However, evolution occurs in the entire genepool, not in individuals, so there is also a counter evolution pressure to retain the original dimorphism - another balancing act. This has been achieved because there is evolutionary pressure away from either allele for male coloration or female coloration in females to become too dominant in the gene pool. The researchers have shown that both forms have been retained for millions of years, with neither being eliminated. Male colouration is produced by a pigment produced by the gene 'tan'. Evolution has operated in the mechanism for regulating how much tan is activated.

Now, there is no mystery here for genetic evolution to explain; it is simply another fascinating phenomenon not requiring magic and another example of the power of evolution by natural selection to come up with these endlessly fascinating examples in nature. However, can anyone see any intelligent reason why a designer would have got to this stage of absurdity where it is in the interests of females to avoid being mated and yet, to avoid extinction, some females have to suffer the damage of repeated matings with the penis this same designer apparently designed to inflict on her?

Me neither.

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