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.