June 18, 2021 | David F. Coppedge

Darwin Was Wrong on Sexual Selection

Even if you believe sexual selection is a valid theory,
don’t believe what Darwin said about it. He was 180° off.



Leave it to Charles Darwin to invent an explanation that sounds plausible and intuitive, but is wrong. 150 years after he promoted “sexual selection” theory in The Descent of Man, his disciples among biologists still can’t figure out what it means or whether it works or not. Some discount it all together, and complain that it led to sexism in human populations. Some accept it uncritically. Others try to salvage it with modifications. The latter option was chosen by Tamas Szekely at the University of Bath, who tried to wash sexual selection by turning it around. It’s hard for evolutionists to criticize their Bearded Buddha. Writing in The Conversation, Szekely began with obligatory homage:

Charles Darwin was a careful scientist. In the middle of the 19th century, while he was collecting evidence for his theory that species evolve by natural selection, he noticed it didn’t explain the fancy tails of male peacocks, the antlers paraded by male deer, or why some the males of some species are far larger then their female counterparts.

For these quirks, Darwin proposed a secondary theory: the sexual selection of traits that increase an animal’s chance of securing a mate and reproducing.

Actually, Darwin’s only degree was in theology; he was a ‘citizen scientist’ by today’s lingo, but that’s OK. Some citizen scientists do good work, and having a PhD in biology does not guarantee someone is a careful scientist. But alas, Szekely has to admit, “Darwin got sexual selection backwards,” his headline trumpets. Darwin thought that a lower male-to-female sex ratio forced males to compete for females in the struggle to win the “evolutionary lottery.” Wrong:

In a new study, my colleagues and I have confirmed a link between sexual selection and sex ratios, as Darwin suspected. But surprisingly, our findings suggest Darwin got things the wrong way round. We found that sexual selection is most pronounced not when potential mates are scarce, but when they’re abundant – and this means looking again at the selection pressures at play in animal populations that feature uneven sex ratios.

What kind of theory deserves a name when it explains opposite patterns with equal ease? Either Darwinian sexual selection was false, or Szekelian sexual selection is false. They cannot be simultaneously true. But then, Szekely finds that both notions are true – one version for birds, but the opposite version for mammals. What kind of theory is this? What kind of understanding does it provide to the biologist?

So why do many birds species have more males, while mammals often have more females? The short answer is that we don’t know.

Szekely uses the power of suggestion to imagine things that might explain the welter of divergent observations that contribute to sex ratios in different animals. It might be:

  • Lifespan differences
  • Predator preferences
  • Differences between sexes in susceptibility to parasites or disease
  • Other variables not considered

After so many years or idolizing Darwin, it is surprising for Szekely to admit, “Darwin’s insight linking sex ratios with sexual selection has received little attention from scientists.” What have they been doing for 150 years? Has Darwin’s notion made them lazy, simply taking his word for it without doing the rigorous study necessary to prove a hypothesis correct?

Sexual differences are widespread in animals, but no single rule explains them. Exceptions abound. Some males and females are indistinguishable in birds and mammals; others are dramatically different. So are sex ratios.

Every specific bird or mammal mentioned in the article seems an exception to any rule. Szekely thinks he has clarified the doctrine of sexual selection, but has he? His team apparently found a link between sexual size dimorphism and sex ratios in mammals (i.e., fewer but larger males), but birds don’t fit the pattern, if there is one. As usual, the answer is out there in the land of futureware:

We need further studies to help us understand how males and females seek out new partners in male-skewed and female-skewed populations, and in what circumstances ornaments, armaments and sheer size are particularly useful. Such studies could provide us with unprecedented new insights into how nature works, building on Darwin’s original theory of sexual selection.

Keep the funding coming, in short. And whatever you do as a biologist, keep King Charles smiling on his throne:

This in no way invalidates Darwin’s theories of natural selection and sexual selection. Our finding simply shows that a different mechanism to the one Darwin proposed is driving mating competition for animals living in sex-skewed populations.

In actual practice, sexual selection theory acts as a stand-in every time natural selection fails. Both mistakenly personify “selection” as a magical agent responsible for change. Both mistakenly locate the force for change in the environment. If, instead, the organism possesses internal mechanisms for adaptation, that would destroy Darwinian naturalism, because it would imply foresight by an intelligent Designer.

Interacting Myths

For example, a paper published June 8 in Nature Communications claims evidence for sexual selection – in flour beetles. The authors assume that growing larger horns requires a “masculinized” body (larger thorax) that would make females more susceptible to predators. The press release from the University of Exeter assumes that neither sex gets an optimal body, because the horns are beneficial to males (an idea that assumes sexual selection works), while the female gets immunity from predation. Is this not anthropomorphizing human values onto bugs? Yes; the authors apply their myth to humans, because women have wider hips for childbirth but narrow hips, they say, are ideal for men:

“For example, humans share the genes for hips – which males need for walking, and females need for both walking and childbirth.

Optimal hips for women would be broad enough to allow childbirth, while optimal hip width for men is narrower.

Humans reach a sort of evolutionary compromise, in which neither males nor females get the body shape that would be optimal for them.

This is nonsensical, because both sexes need to walk. It implies a sexist view of women; are males only hunters and females only birthing people? One shouldn’t say that in today’s political environment. Plus, there is no way to measure it. Let them try to predict that ‘For every centimeter of increase in female hip width, the human race will gain fitness by 3.416 percent.’ It cannot be done. The range in hip width of all childbearing women overlaps the range of hip width in men, when all people groups are considered; compare Hobbit males with Neanderthal females. And why not examine trends in all mammals?

Needless to say, if the female beetles had bigger horns and were attacked more by predators, the evolutionists would use the same theory – sexual selection – to explain it. They embarrass their colleagues in evolutionary biology by saying nobody ever checked this in 150 years.

Theory shows how sexual selection can exaggerate male traits beyond naturally selected optima and also how natural selection can ultimately halt trait elaboration. Empirical evidence supports this theory, but to our knowledge, there have been no experimental evolution studies directly testing this logic, and little examination of possible associated effects on female fitness.

The authors err from the get-go by confusing experimentally-manipulated bugs (artificial selection) with those in the wild. The paper uses the word fitness over 40 times and selection 53 times, but equates fitness with reproductive success, which is the tautologous formulation. Consequently, these ideas reduce to just-so stories, not scientific explanations. By pitting one mythoid (natural selection) against another mythoid (sexual selection), furthermore, they present a picture malleable enough for confabulating any possible story for any observation. Whatever exists, it was ‘selected’ by some mystical, undefinable, anthropomorphized Selector.

Finally, the authors committed bluffing and card stacking by omitting any mention of the controversies surrounding the notion of sexual selection, which have been many over the years. Their readers deserve to know that other evolutionary biologists disagree, not be told that “This finding is consistent with a vast body of fundamental theory.” Scholarship demands answering the objections to a theory as well as presenting evidence for it.

Much of what we do at CEH is informing our readers about controversies in evolutionary biology that they would never know about in the secular press. All the public usually gets is Darwinian triumphalism – but Emperor Charlie has no clothes.

For more on the mess that is sexual selection, see our previous entries:

Exercise: Try your hand at debugging New Scientist‘s June 11th article, “Many female animals are evolving to look more attractive to mates.”

Cartoon by Brett Miller. Used by permission.


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