I mentioned offhand the other day that the notion of “selection pressures” in evolutionary biology is a metaphor, not dissimilar to the metaphor of a “selfish gene,” but Matthew convinced me that this idea was more profound than I thought (LOL!), so I’ll write a bit more about it.
A quick refresher: the other day Andrew Brown wrote one of his usual muddled columns in the Guardian, claiming, as he often does, that Richard Dawkins is a malign influence on evolutionary biology. His argument was the usual blather that gene aren’t really “selfish,” and that this metaphor has led to deep confusion. In the comments after his piece, Brown repeats this claim in response to a criticism (h/t to moleatthecounter for finding this):
If you can’t read the above because the print’s too small, Brown says that “. . . Dawkins got confused by his own title. This confusion was intermittent but it’s absurd to pretend it didn’t happen.” (He also give props to Mary Midgley, who deeply misunderstood the book, attacked Dawkins in a quite vitriolic way, and got a strong response from Richard. Although the original papers don’t seem to be on the Internet, you can read about that controversy here. [Note: reader pacopicopedira has found Dawkins’s reply online; he/she gives the link in the comments but you can read it here.] If you think that Richard’s responses to Midgley were “disgusting”, try reading what she wrote in the first place.)
At any rate, Brown, fulminating about a metaphor that supposedly misled not only Dawkins’s readers, but Dawkins himself, said the following in his Guardian piece:
In particular, the ascription of agency to genes led him and his followers into endless confusion. The point is not merely whether genes can be selfish or generous, but whether they can be said to have any activity at all in the world. This is a point which he freely concedes and then forgets – his manner of dealing with most criticism. If a gene is defined, as he defines it, as a piece of chromosomal material subject to the pressures of selection, it is the pressures of selection which are the active and changing parts of the picture, and the DNA sequence is entirely passive.
Sadly, here Brown is hoist with his own petard. What he doesn’t realize is that there is no such thing as “the pressures of selection”—it is a metaphor, a descriptor of what happens when different genes (i.e. “alleles”, or forms of a single type of gene) leave different number of copies. That differential reproduction of genes is what constitutes natural selection,, and it is a process of gene sorting.
There are no “pressures” of selection imposed on the organism from the outside. What happens, as everyone knows who learns introductory evolution, is that, in a given environment, some genes leave more copies than others, usually because they increase the reproductive output of their possessor. Take, for example, a population of brown bears that somehow find themselves in a white-colored environment, like the Arctic. There are genes affecting coat color, and imagine that a given gene comes in several forms, one of which makes the bear lighter in color than do the alternative forms. This being a population of bears, there will be variation among their genes due to mutation. Those bears carrying the “light” forms of genes might do better than their browner confrères because they’re more camouflaged in the snow, and thus better at sneaking up on seals and killing them. “Light-gene” bears will be better fed, and thus have better survival and (crucially) more offspring. (If the color change affects survival but not reproductive output, no natural selection ensues.) In the next generation, the proportion of color genes having the light form will be higher than before. And the average color of the bear population will be a bit lighter.
If this continues over many generations, and other mutations occur that yield even lighter coats, natural selection will move the bears from brown to white. Presumably this is what happened in polar bears, whose ancestors were probably brown. And it’s happened in many Arctic animals whose ancestors were brown but evolved white color (either pemanently or seasonally) via natural selection. Such animals include the Arctic fox, the Arctic hare, the ptarmigan, the snowy owl, the harp seal, and so on (see a list here).
Note that the environment isn’t exerting any “pressure” here. It is simply providing a milieu in which one gene has an advantage over another. The environment cannot see the genes and their constituent DNA. We speak of “selective pressure to become light-colored” as simple shorthand for the process I’ve described above.
Now I suppose people like Brown could say, “But that’s confusing! Saying that the environment exerts selective pressures could mislead people into thinking that the environment is in someway animate, and can exert a force on animals to mold them one way or another. It could lead to vitalism!” But it’s not really confusing. In fact, it’s so not confusing that Brown used the metaphor himself, without realizing it. When I teach my students introductory evolution, I’m always careful to note that “selection pressures” is a shorthand term for something a bit more complicated.
But if Brown can easily recognize this metaphor, so can everybody else, and the notion of “selection pressures” has not been deeply confusing. Likewise, “selfish gene” is not confusing if you have sufficient neurons to see that Richard what Dawkins meant: the differential sorting of genes that is natural selection involves genes behaving as if they were selfish: trying to outcompete their mates and get into the next generation of bodies. Apparently Midgley had such a problem. But Dawkins certainly did not, despite what Brown claims.
Metaphors are useful if as they enlighten rather than confuse. I claim that both “selection pressures” and “selfish genes” are enlightening, and, judging by the sales of Dawkins’s book, so do most readers.
Let me add, since I’m writing about physics for my book now (kudos to Sean Carroll, my Official Physics Tutor™), that the term “laws of physics” is also a metaphor, or a shorthand descriptor. There are no “laws of physics,” but just regularities in the universe that appear to be ubiquitously “followed”. People like Brown might argue that the term “laws” implies “lawgiver” and hence God—a confusing misconception unwittingly promulgated by atheistic physicists. But again, this metaphor is not confusing—except, perhaps, to theists who want to see those laws having been decreed by God.
As one reader pointed out yesterday, metaphors, often involving the use of anthropomorphizing, are ubiquitous in all of science, not just evolutionary biology. “Selfish gene” is just one of these. I would argue that anyone who can’t understand, after a minute’s explanation, that selfish genes really aren’t conscious and malevolent entities, is lacking some crucial rationality. Likewise, it’s not hard to see how “selfish genes” can lead to cooperative behavior. That, too, is easily explained, and has been done many times by Dawkins and others. Nevertheless, people like Midgley and many theologians argue that the notion of selfish genes simply can’t explain unselfish behavior. They’re ignorant—often willfully so.