163 years later, new research challenges Darwin’s most famous theory about sex

sex is a Biologically, it’s an expensive business. Finding a suitable partner takes time and energy. Offspring are also a huge investment of resources. But sex offers a rewarding opportunity: children who, thanks to new and “better” gene combinations, are fitter than their parents. Darwin recognized that many animal species therefore choose their partners carefully.

However, there is an innate biological inequality. Eggs are relatively few – a large and costly investment – while sperm are small and far more common. And embryos often need further investment inside or outside the body. Since the larger investment tends to fall on women, they are often the more selective sex (while men compete to be selected).

But according to a new paper published in scienceCharles Darwin’s patriarchal worldview led him to reject female agency and human mate choice.

He also downplayed the role of female variation in other animal species, assuming they were fairly uniform, and always made similar choices. And he thought there were huge differences between men competing for female attention by flaunting stunning skill and beauty.

This maintained the focus on the dynamics of male dominance hierarchies, sexual ornamentation, and variation as drivers of sexual selection, even if women sometimes made the choice.

But do Darwin’s ideas on sexual selection hold up today?

complex decisions

Male ring needles carry the eggs until they hatch.Shutterstock

When animals choose a mate, their appearance, sounds, and smell can provide accurate clues as to the potential mate’s survivability. For example, large antlers in deer are a good indicator of fighting ability, dominance, and general fitness.

But many other traits can be chosen because they are otherwise striking and attractive but can be a poor indicator of overall genetic quality or even misleading.

Females can evolve to choose mates with whom their offspring are less likely to survive, provided there are more such offspring as a compromise. For example, in some species of poecilids, male attractiveness is linked to genes that can affect their survival.

Females therefore face a dilemma: mate with a more attractive male and sire some highly attractive but otherwise less vigorous sons, or mate with a less attractive male to maximize the survival of those sons. Which strategy produces the most grandchildren?

Females can therefore select traits in males that do not appear to otherwise affect their ability to survive. The peacock’s tail is a handicap in most other aspects of its life — an obstacle to flight and avoidance of predators — aside from its appeal to a female. However, it may also be true that a male’s ability to handle such a strain is itself an indicator of overall genetic quality and severity.

It’s not always women who vote. With pipefish, the males invest heavily in carrying the fertilized eggs until they hatch, and it is the females who compete to secure the males’ attention.

The optimal choice of partner is not the same for all individuals or at every point in their development. For example, younger Silky Bowerbirds are afraid of the strongest males, while older females usually find these the most attractive. And many fish are sequential hermaphrodites, changing sex—and thus mate choice—as they age.

Research since Darwin therefore shows that mate choice is a far more complex process than he might have thought, and is driven by differences in both sexes.

Was Darwin a sexist?

So, is the charge of sexism leveled against Darwin really valid, and has this clouded his science? There is certainly some evidence that Darwin underestimated the importance of variation, strategy, and even promiscuity in most females.

For example, perhaps as a result of the prevailing prudery, Darwin placed little emphasis on working mechanisms of sexual selection after Pairing. Female birds and mammals may choose to mate with multiple males, and their sperm may compete to fertilize one or more eggs in the reproductive tract.

Cats, dogs, and other animals can have litters with multiple fathers (the glorious name of “heteropaternal super-fertilization” — although it sounds pretty cruel really!). There is even evidence that the human penis – which is thicker than our closest primate relatives – is an adaptation to physically displace the sperm of competing males. Such earthy speculations were anathema to Darwin’s sensibilities.

Female blue tits often mate with multiple males to ensure their protection and support – a somewhat manipulative strategy when paternity is uncertain for the future fathers. All of this challenges Darwin’s assumption that women are relatively passive and non-strategic.

Where men make a larger investment, they become more active in choosing a partner. Male (rather than female) poison dart frogs (Dendrobates auratus) protect the young and therefore attract multiple females who compete to lay eggs so they can be fertilized. Many bird species are cared for by both parents and therefore have a greater variety of mating systems.

Darwin’s worldview was inevitably shaped by the culture of his time, and his personal writings make it difficult to construct a particularly robust defense. In a letter of 1882 he wrote: “I do think that women, although generally superior to men [sic] moral qualities are intellectually inferior; & there seems to me a great difficulty with the laws of heredity…in that they become the intellectual equals of man”.

He also consulted on the relative merits of marriage, famously observing: “Home and someone to look after the house – charms of music and feminine banter. — These things are good for health. – but terrible waste of time.”

Not surprisingly, Darwin did not fully understand much. Like Albert Einstein, HG Wells and Edgar Allan Poe, Darwin married his first cousin, Emma Wedgwood. Ironically, he knew nothing about genetics and the mechanisms by which close relatives are more likely to have offspring with certain genetic diseases. Intriguingly, our closest relatives in the tree of life, the chimpanzees, naturally circumvent this problem as females choose mates more distantly related to them than the average male in the available pool.

Despite his omissions, however, Darwin’s understanding was radically more advanced than anything that had preceded it. Combined with the later understanding of genetics and heredity, Darwin’s writings are still the bedrock of all modern evolutionary biology.

This article was originally published on The conversation by Mark Maslin and Carmen Nab at UCL. Read the original article here.

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