Gene Expression

Wow. Well, Jerry Coyne has never been one for weak words. A few days ago evolutionary biologist & journalist Olivia Judson posted The Monster is Back, and It's Hopeful on her blog The Wild Side. Jerry Coyne, a prominent evolutionary biologist at the University of Chicago and co-author of the magisterial Speciation, has posted a strongly worded response over at Carl Zimmer's weblog:

Judson commits two errors of reasoning when arguing a la Goldschmidt (or Gould). The first is what I call the "macromutationist fallacy," for this error is so common that it deserves a name. It is this: we see some single mutations within species that make big changes in a trait, and then infer that differences between species in that same trait are also due to mutations in that same gene, or to other mutations of large effect. Judson makes this mistake when discussing the naked head of the vulture, presumably an adaptation for sticking its noggin into rotten meat. She thinks that the loss of vulture head feathers may well be due to a single mutation because there are occasional mutations in domestic chickens that give them bare heads and necks.

But you can't blithely extrapolate from rare large-effect mutations within species, especially domesticated ones, to mutations causing large evolutionary changes between species. These macromutations, like those producing bald chickens, almost always have highly deleterious side effects that make them unlikely to form the basis for evolution in nature. In fact, many cultivated species would never survive, much less take over, in the wild. Domestic corn is good to eat, but would never thrive in nature because the seeds don't disperse. Or, take single mutations having drastic effects on body size. One of these, achondroplastic dwarfism in humans, has severe negative effects on health and reproduction. Saying that the bald-chicken mutation implies that vultures lost their top feathers courtesy of a single mutation is like saying that because there is dwarfism in humans, the size difference between humans and chimps must have the same genetic basis as achrondroplasty. Most big-effect mutations that occur in the laboratory, greenhouse, or henhouse could never survive in nature, and have to be coddled by humans to survive.

Read the whole thing! (as they say) Interesting that Coyne, a serious contributor to evolutionary scholarship, has gotten his hands dirty and reached out to the public so directly to correct what he perceives is journalistic malpractice. Hey Dick Lewontin, I'd be willing to set up a blog for you if you want now that you're emeritus....

Note: Coyne is a traditionalist who doesn't suffer fools; his recent critiques of evo-devo make this sort of sally understandable. The ideas resurrected by Judson are fundamentally more radical than anything Sean Carroll has proposed. But, I do want to offer that I remember listening to a young zebrafish researcher explaining how shocked he was a lot of the extant variation they saw was controlled by only a few genes. There is a real story to be told about the exposure via genomics that many quantitative traits are controlled by a few loci of large effect as opposed to architectures hypothesized by the infinitesimal models....