Natural selection and unneeded traits

I wanted to be moving on to writing about Pollan’s hunting trip today, but yesterday my mind wandered back to a line from the section about animal happiness that I decided I just couldn’t leave without comment. In his discussion of domestication Pollan writes,

Both parties were transformed by the new relationship: The animals grew tame and lost their ability to fend for themselves in the wild (natural selection tends to dispense with unneeded traits) and the humans traded their hunter-gatherer ways for the settled lives of agriculturists. (320)

My interest here is in that parenthetical comment, “natural selection tends to dispense with unneeded traits.” It’s a statement that anybody who has ever had appendicitis might take issue with.

In saying that natural selection eliminates traits merely because they are not needed, Pollan misrepresents the concept of natural selection. By its definition, natural selection is the process by which traits that increase an organism’s chance of reproducing become more common from generation to generation. That means that for a trait to be eliminated by natural selection, it has to somehow decrease an organism’s chance of passing on its genes. It’s not enough for the trait to simply be “unneeded.”

Traits that are neither useful nor harmful can become less prevalent in a population over time, but that’s not natural selection. That can happen by a process called genetic drift. Removal of a trait from a population by genetic drift is something that can happen by chance, but it’s not something that “tends” to happen. There’s no reason expect genetic drift to “dispense with” a particular unneeded trait.

So what happened in the case of domesticated animals? Their transformation, as Pollan suggests, was primarily due to natural selection, though it might be termed artificial selection to reflect the human influence on the selection process. The traits that allowed these animals to defend themselves weren’t just unneeded. When humans domesticated these animals, these traits came to hurt the animals’ chance of reproducing. Humans prefer a tame animal to a ferocious animal, a bird that can’t fly away to one that can, and a meatier animal to one that can flee from predators. Since humans control the breeding on a farm, many traits that are useful in the wild are selected against in the domesticated setting.



  1. Anna said

    Their transformation, as Pollan suggests, was primarily due to natural selection, though it might be termed artificial selection to reflect the human influence on the selection process.

    It not only “might” be termed artificial selection — it is termed artificial selection! I remember that from Biology 101. 😉

    Thanks for this blog — I’m having fun looking through it. I like Pollan’s writing style and think he brings up a lot of important points, but so much of it rubs me the wrong way. Nice to see that articulated outside of my own head!

    • Adam Merberg said

      Thanks for stopping by and commenting.

      I say “might” because there are those who argue that the distinction between artificial selection and natural selection is itself artificial because humans are part of the natural world. I prefer not to get into that debate here as it isn’t really relevant to the point at hand, which is why I tried to be open to both terms.

      • Anna said

        Thanks for clarifying your choice of wording. I see where you’re coming from now — though I’ve heard that argument before, I’d pretty much forgotten about it. I see what they’re saying, but I think “artificial selection” as a term offers a useful distinction.

  2. Pollan’s claim here (“natural selection tends to dispense with unneeded traits”) is the premise behind the “irreducible complexity” argument used by “Intelligent Design” proponents. Basically, the argument says that certain biological systems — e.g. a bacterial flagellum — couldn’t have evolved gradually, part by part, because the removal of any part of the system would render the whole system inoperative, so that it wouldn’t benefit the organism. Clearly, this argument rests on the premise that natural selection would remove non-beneficial traits.

    Evolutionary biologists have responded to the argument by pointing to co-option, the way in which a trait that originally served one function can come to serve a different function. The system that now serves as a flagellum might have once served a different function, back before it had all the parts that it needed to work as a flagellum.

    Anyway, my point here isn’t to tar Pollan by associating him with a discredited idea like Intelligent Design. I guess I’m looking for some clarification. To your knowledge, if natural selection doesn’t tend to eliminate non-beneficial traits, then why do evolutionary biologists appeal to co-option in responding to the irreducible complexity argument? Couldn’t they just point out that natural selection doesn’t automatically remove non-beneficial traits? Do they have good evidence that co-option has been involved in the evolution of most biological systems, or is there some other reason?

    • Sorry, I should have said that the appeal to co-option is *one* of the responses that evolutionary biologists have given. There have been others. However, in my (admittedly cursory) research, I haven’t been able to find anyone who simply points out that natural selection doesn’t necessarily eliminate non-beneficial traits.

    • Adam Merberg said

      I’m pretty sure that the irreducible complexity argument actually depends on a weaker statement than the one Pollan makes here. For instance, look at the eye, which is a favorite example of the proponents of intelligent design. An eye that isn’t useful for seeing can actually be an evolutionary disadvantage because such an eye is prone to serious injury if it gets poked (which is more likely for an individual who can’t see). And generally, I would expect that most traits related to development of the nervous system are energy-intensive, so if they don’t provide some benefit, they’re actually harmful.

      I would argue that Pollan’s statement doesn’t even make logical, let alone evolutionary, sense. The problem is that the absence of a trait is another trait. Suppose we have two neutral traits, say brown fur and gray fur. By assumption, both of these traits are unneeded. By Pollan’s argument, natural selection should eliminate both colors of fur because both are unneeded, even though either color may be better than having no fur at all. (I guess I veered a little bit toward an evolutionary argument in that last sentence.)

      • Okay, thanks for clarifying your problem with Pollan’s statment. The fur example is definitely helpful.

        Now that I think about it, I’m not sure what assumption underlies the irreducible complexity argument. AFAIK, intelligent design proponents don’t explicitly say that natural selection eliminates non-beneficial traits, so my interpretation is perhaps wrong. On the other hand, at least in the more concise formulations of the argument, no reference is made to the evolutionary disadvantage of, e.g., a non-functional eye.

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