Evolution: survival of the fittest? Maybe not, acording to female birds.

I’ve been watching ducks recently.  Mallard ducks show the remarkable difference beteen the fairly drab female and the quite glamorous male.  Is this just another case of an unfair asymmetry in nature?

Maybe not. An opposing proposal is that what we are seeing is the effect of female avian aesthetics.  Females are generally those who select the partner in a pair.  What we see is the effect of their taste, shared to some extent by females of at least one other species. Namely, human beings.

Another interesting fact is that female aesthetic preference does not always pick out the fitter male bird, fitter, that is, in birdly things like flight.  In fact, it very much looks as though the song of one species is improved by wing configurations that, when increased, create a bird less capable of good flying.

Yale Professor Richard O. Prum argues, as it is put, that sexual selection, unlike natural selection, is not always selection of the fittest. In fact, he thinks it is based on female avian sexual aesthetics.

To grasp his view, a little bit of history is in order. Darwin famously proposed the idea of evolution by natural selection, what is often called survival of the fittest. To put it simply, living things vary in their inherited traits, from speed to color to sense of smell. The traits of the individuals who survive longer and have the most offspring become more common. So, over time, the faster antelope have more young, the fastest of them have more offspring, and antelope end up very speedy.

But reproduction isn’t just about surviving and staying healthy long enough to mate. You have to find a mate. And in many species, your mate must choose you. This process is sexual selection. Female birds are often the ones choosing. And their choices can produce male birds that are incredibly colorful, and some that are elaborate dancers or designers of striking boudoirs — like the bower birds. If, for example, females like males with long tails, then long-tailed males have more offspring, and the longest-tailed of those offspring reproduce more. In the end, that species becomes known for its long tails.

Whether he is generally right, there do seem to be cases of sexual selection that can degrade the species. What does this say about the survival of the fittest?

Crudely natural selection is supposed to select for those who meet the challenges in their environment in a way that enhances reproductive success. But if we include getting selected as a mate, then overall fitness need not be enhanced in the species. Or so two articles in the NY Times seem to suggest. See here and here. To be accurate, though, I don’t think anyone writing sees any problem in distinguishing natural from sexual selection. Justifying the distinction is another matter, or so Professor Prum’s reference to free will for female birds could indicate. See the end of the second article:


Once organisms evolve the capacity for subjective evaluation, and the freedom of choice, then animals become agents in their own evolution. One of the hallmarks of autonomy, of course, is the freedom to mess up




10 thoughts on “Evolution: survival of the fittest? Maybe not, acording to female birds.

  1. I’ve not read Prum’s book; however, from the NY Times article some of it sounds pretty speculative (e.g., his theory of how same-sex attraction evolved in humans).
    But: the idea that selection does not always make a population better off is old news (among biologists). Selection often works at the level of the individual, favouring the fitter individual. Lewontin has an old example of a population that is at its “carrying capacity” (the environment can’t support a greater number of individuals.) Still, selection can favour a trait of producing twice as many eggs – this trait might go to 100% but at the end of the selection process the overall census is the same.

    Also: if there is a population of individuals, some of whom are “polluters’ and some of whom aren’t, the polluters can evolve by selection if the pollution harms non-polluters more than it harms the polluters (suppose there is an extra cost to not polluting, but that otherwise the pollution harms everyone.) The population can be driven to extinction if the pollution is bad enough. This process can be driven by selection. It doesn’t require a conflict with sexual selection.

    And maybe this is just the circles I’m in, but I would have thought lots of biologists were alive to the possibility of conflicts between sexual selection and natural selection, and would be skeptical of the idea that all cases of the evolution of beauty and aesthetic preference are honest signalling.

  2. Chris, I’ve been trying to think about how the notion of functioning well features in cognitive neuroscience. After reading philosophers on functions and evolution, and discussing the related issues with some, I prepared for anything to be attributed to biologists. I’m relieved they’ve noticed that attractiveness needn’t be tied to, e.g., fitness.
    Do you know how natural and sexual selection are usually distinguished?

  3. Sexual selection was a theory that Darwin himself proposed, based on the two forms: female choice based on aesthetic preference with respect to males or male-male combat. He also acknowledged occasional “exceptions”: male choice based on aesthetic preference with respect to females and female-female combat. There is nothing new truly here, just a new defense of Darwin. No overturning of natural selection or paradigm shifts.

    Darwin thought that natural selection and sexual selection were different. Today, most, but not all, biologists consider sexual selection to be a type of natural selection, but that is largely mathematical – they are distinct as causal processes. I have a paper that touches on this along with other issues, part of a roundtable on Joan Roughgarden’s book, which defends an alternative to sexual selection that she calls social selection. The roundtable can be found here: http://www.rlm.net/papers/SexAndSensibility.pdf

  4. Anne, sure, happy to. And I echo Trevor’s recommendation for Erika Milam’s book – she was a member of the roundtable that I linked to and I think very highly of her work.

  5. Milam’s book is excellent, but I think the last word (for now) in the history is Evelleen Richards’ brand new “Darwin and the Making of Sexual Selection”

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