A note from a real working geneticist, a guy who’s forgotten more about
biology than I shall ever know:
[Quoting me]: “the real conundrum is how species-change proceeds. Is it a
fairly steady process, or does it come in bursts?”
[He says]: “There is starting to be some decent analysis and simulation of
this, not yet what you’d call a definitive theory: there was a nice review
of it last week in Nature Reviews Genetics . The answer is bursts. When
the environment changes, populations start adapting to the new conditions,
and increasingly it looks as if the first genetic changes tend to be ones
that have big effects, while later ones tend to have smaller and smaller
effects. This has practical significance We have no trouble find alleles
in domesticated plants and animals that explain a lot of the variance in
traits of interest. there’s a single myostatin mutant that greatly
influences muscle mass in some breed of beef cattle, there are simple
mutations that cause twinning in sheep, etc. Changes in just a few genes
(seven) explain most of the difference between maize and its wild ancestor
teosinte.
“But you don’t usually see single genetic variants that explain a lot of the
variance in traits that have been subject to ( more or less) the same
selective pressures over a long period of time. You see this in the
transient – and of course plants and animals haven’t been domesticated for
all that long a time, moreover we change the selective pressures every now
and then.”