Yet another letter has appeared in the Guardian about Stephen Buranyi’s misleading “long read” on the site, “Do we need a new theory of evolution?” (Buranyi says “yes”). I’ve mentioned the problems with Buranyi’s article before, and three of us even wrote a letter about the article’s flaws that the Guardian published.
Apparently, though, the fracas hasn’t died down, because another one just appeared, this time on sexual selection. The letter is by anthropologist Heather Remoff, who wrote a book on sexual selection mentioned at the bottom of her letter.
Here’s the letter (click to go to the Guardian site), and my take on it is below:
There’s a lot to “unpack” here, and I’ll try to be brief.
First, the letter doesn’t address Burayni’s claims, which was that the modern theory of evolution was incomplete and perhaps obsolete. He was not referring to Darwin’s theory of evolution but to Darwin’s theory as it has been updated and expanded in light of modern research. Darwin’s failure to understand everything does not mean that the modern theory of evolution is woefully lacking, for we’ve had more than a century and a half of work on evolution since The Origin.
Ergo, showing that Darwin’s theory of sexual selection was incomplete—and yes, it was his theory, rejected even by A. R. Wallace (except in humans!)—does not support Buranyi’s thesis. That theory was published in 1871, and now it’s 150 years on. Modern evolutionary biology has added tons of knowledge and theory about sexual selection. There are entire books on the topic (here’s one) that go far beyond Darwin’s ideas. But showing that “Darwin’s theory was incomplete” doesn’t say anything about the modern theory of evolution, which is what this whole controversy is about.
Darwin actually had two theories of sexual selection, one involving male-male combat for females, and the other involving female preference for “beauty”. The former theory, which Darwin called the “law of combat”, explains the evolution of weapons like antlers in male deer—weapons far less developed in females because they’re not used. Darwin’s second theory is that females have an aesthetic sense that males appeal to with ornaments, striking colors, extreme behaviors, or lovely calls. This causes female-imposed natural selection on males, which, thought Darwin, explains sexual dimorphism in appearance, behavior, calls, and so on.
Note first that, contra Remoff, female preference was already a crucial part of Darwin’s theory, for without that preference we wouldn’t have the striking sexual dimorphism we see in many animals. Even though male-male competition remains an important explanation for male-limited weapons or competitive behaviors, Darwin had already diagnosed a large portion of the sexually dimorphic world using the lens of female preference.
But Darwin’s theory was incomplete in a way Remoff fails to mention. Exactly why do males compete for females? Darwin had no answer, and you don’t find an answer simply by viewing the issue through the female lens. In general, biologists agree that sexual selection results from this: female investment in offspring is often much larger than that of males. When females have to do the hard work of gestation and rearing of offspring, as well as contributing metabolically expensive large gametes (eggs), while male investment is often limited only to a small amount of tiny sperm, an asymmetry in the interests of the sexes arises. Evolutionarily, males can leave more of their genes by copulating with any female they can, while it pays for females to be choosy about her mates, since once she mates, she’s made a huge investment that has to be tended. A good choice by a female often means her offspring have a better change of surviving, ergo it pays to be picky. A male fly can mate with 20 females in a few days and have 20 batches of offspring, but a female fly who mates with 20 males within a few days doesn’t have many more offspring than if she copulated only once. It thus pays the males to be profligate and the females to be choosy.
I often use this example in evolution class to show the asymmetry (see this page for the records). This is what your body is capable of producing if you’re a woman or a man:
Record number of children produced by one mother: 69 (many twins and triplets birthed by a Russian woman)
Record number of children produced by one father: 1000-2000 by Genghis Khan (estimated) or, in more modern times, over 868 fathered by Moulay Ismail Ibn Sharif, a Sultan of Morocco, in the 18th century.
A male can have more than ten times as many offspring over his life than can a woman! Of course the average number of offspring has to be the same for men and women (after all, each child has one mother and one father), but the variation is such that while women produce relatively comparable numbers of offspring, a lot of males produce just a few and a few males produce many. That is, males have a much higher variance in offspring number. And that is the basis for sexual selection. (This difference in variance is seen in humans as well as in many other species.)
The asymmetry between the sexes, then, rests on the best way to choose. For males it’s not evolutionarily “wise” to be choosy (I am generalizing here, for of course there are cases in which males should also be choosy), while for female it pays to make sure you choose well, as you don’t have as many shots as being a parent. As I said, this is a short explanation for sexual selection that has exceptions, but it’s the going explanation for why, when the sexes differ in ornamentation, behavior, or calls, it is males who show elaborated traits.
This asymmetry is critical in understanding the whole process of sexual selection, and it rests not on seeing it through a female lens, but seeing it through a lens that looks at what both sexes have to gain from behaving in various ways. In the end, it’s largely based on gamete size. That was what Darwin missed, but we understand it now and can test it.
Further, since Darwin’s time we have new theories of sexual selection that have been mathematically elaborated: the runaway model (Richard Prum has used this to update Darwin’s “beauty” hypothesis), the “honest signalling” model, the “sexy son” hypothesis, and so on. Some of these models overlap. All of them consider female preference.
Now I’ve said in the past that, in my view, one of the contributions of the “female view” of biology has been an increased emphasis on female choice in sexual selection, for the process involves an interaction between males and females. Some women (but not solely women) helped direct research by emphasizing female preference. And that’s understandable; you don’t want your sex and its importance in evolution to be overlooked.
That said, though, both men and women have made important contributions to the modern theory of sexual selection; it was not incomplete because the “female lens” was totally overlooked by patriarchal male biologists. And, as I said, female behavior—aesthetic preference—was absolutely critical for the “beauty” aspect of Darwin’s original theory.
As for the “genetic breakthoughs” that have led to a new understanding of sexual selection, particularly when viewed through that female lens, I am stymied. I don’t know what breakthroughs Remoff is talking about. Perhaps she’s referring to this:
The evolutionary moonshot that enabled Homo sapiens to go where other species have failed to follow has its roots in a reproductive mutation – concealed ovulation and continuous sexual receptivity – that dramatically increased the strategic agency employed by females.
Concealed ovulation and continuous sexual receptivity (the latter is possessed by many animals) are not “mutations”; they are traits, likely ones that arose via many mutations of small effect. And yes, these traits have obviously changed the playing field for sexual selection. But whether they have been “moonshot” that has enabled us to go where other species have not, well, other species have had their own “moonshots”, like hypodermic insemination in some invertebrates, the “pseudopenis” of the female hyena,”and the male pouches of pipefish and seahorses.
The last trait gives male seahorses most of the investment in offspring (males, in effect, get “pregnant”, and females, who can produce lots of eggs, must compete for limited male pouch space). The result that in this group it’s most often females rather than males who are ornamented. This reversal of investment, coupled with a reversal of the sexual dimorphism, is striking support for the “differential investment” theory of sexual selection.
Sexual selection operates in different ways in different species, and, truth be told, we don’t understand the details that have led to the evolution of most sexually dimorphic traits. not involved in male-male competition. We know the basis for the evolutionary process—differential investment in offspring—but we don’t know why particular traits are chosen and whether they are indicators of fitness or of something else. If you ask me why the peacock has a long tail instead of a big head crest, and what information that elaborate tail conveys to females, I wouldn’t be able to tell you. We do know that the more spots a male peacock has on his tail, the more likely he is to be chosen as a mate, but we don’t know the advantage accruing to females that have such a preference.
And no, sexual selection does not “establish the origins of everything that defines human exceptionalism”. Semantic language? Bipedality and manual dexterity? Our remarkably complex brain? Did all those traits rest on sexual selection? I think not.
Remoff ends with a paragraph that is pure hyperbole:
Why does all this matter? Because humans are facing an environmental disaster of our own making. Only by developing an accurate understanding of the factors that shaped human species-specific behaviour will we be able to avert the rapidly approaching climate apocalypse. Sexual selection may have shaped us, but our failure to take an unbiased look at ourselves could be handing natural selection the power to eliminate us.
Will understanding sexual selection, or human evolution in general, help us stave off climate change? Again I think not. Only by limiting carbon emissions will we be able to avert climate change. And that does not depend on understanding human evolution, much less sexual selection.
In the end, Remoff is tilting at two windmills that have already fallen. Her attack on Darwin is wrongheaded since Darwin’s correctness is not the issue in Buranyi’s piece and because female preference was already a crucial part of Darwin’s theory. And her claim that it was only the “female lens”, used recently, that helped us understand sexual selection, is also misleading. Female preference has been considered by evolutionists since 1871.