Selecting Corn Oil Genes Produces More Corn Oil, but What Else?
Breeders have been trying to squeeze more corn oil out of corn for over a century, one of the longest-running scientific experiments ever. They have made pretty dramatic gains in yield, from 5% to 20%, in 100 generations, says William G. Hill in Science.1 Now also, geneticists have the tools to look for which genes are evolving. The story is not all that clear, however, both for corn and for similar studies trying to beef up poultry (if you’ll pardon the expression).
Do lots of small genetic changes add up (the “infinitesimal model” suggested by Darwin), or do single nucleotide polymorphisms (SNP) produce dramatic changes? Maybe both, maybe one or the other, or maybe something else. It’s not clear how to define a small genetic change, for one thing, or how to isolate one change from effects on other genes.
The continuing responses to selection, therefore, are not likely to be due mainly to continuing tiny changes in gene frequency predicted by the infinitesimal model; instead they may be due to the fixation of genes, including those arising by mutation after selection started, which have appreciable effects while segregating. The biological processes leading to oil concentration or chicken growth are obviously highly interactive, but genes that contribute to selection response must differ in effect when averaged over all other segregating genes. (Emphasis added in all quotes.)
The experiments to which Hill refers did not provide unambiguous evidence that small genetic mutations (quantitative trait loci, or QTLs) added up, even in this clear-cut case of artificial selection. He leaves it as an exercise: “We have yet to discover how such QTLs work, but several of the SNPs associated with oil concentration were at candidate loci, so there are opportunities to find out.” So this is an ongoing challenge for geneticists: “identify the genes and the molecular changes in them that cause these many small but important differences in quantitative traits.” Nevertheless, he is sure that this century-long experiment will illuminate how evolution works: “It is these small differences that generate variability in populations, providing fuel for change through the action of natural and artificial selection.”
1William G. Hill, “A Century of Corn Selection,” Science, Vol 307, Issue 5710, 683-684, 4 February 2005, [DOI: 10.1126/science.1105459].
Excuse me, but after a hundred years of forcing selection to go one way, which nature does not do, are you saying that corn still only produces corn oil, and not something else? Are you hinting that breeders have hit a plateau at 20%? That with all the tools of genetics to look for a known effect, they are not sure which genes are producing it?
Corn with more oil is still corn, and chickens with more meat are still chickens, but natural selection is very different from artificial selection. You can’t claim that these experiments are going to fuel “natural” selection. Let’s put these hyper-specialized breeds out into the wild to compete, and see if they keep getting oilier and meatier. That would be like expecting a poodle to become more poodly in the dog-eat-dog world of the Yellowstone wolf-pack ecosystem. Stop equivocating with the word selection. Artificial selection is intelligent design.