Natural selection is a fact of life. As Steven Jay Gould put it, it’s an “inescapable conclusion” arising from the “undeniable facts” that (1) populations of living things have inheritable variation in many traits; and (2) produce a surplus of offspring. But populations often experience selection from multiple sources, and in conflicting directions. The cover article for this month’s issue of Evolution suggests that bears may be creating ongoing selection in wild salmon populations, but the strength, and outcome, of that selection varies from stream to stream [$-a].
Salmon are famously anadromous — they hatch in freshwater streams and swim out to sea, only to return to the stream of their birth to spawn before they die. Male salmon are generally better off if they’re bigger, both to maximize stored energy for the return to their spawning site, and to better compete for mates when they arrive. Natural selection for larger bodies, however, is checked by bears, who preferentially target large, fatty fish. Yet bear predation varies from stream to stream: in narrower streams, where salmon are easier to catch, bears can fill up on big, newly-arrived fish; but in wide streams, bigger fish can more easily evade bears, so bears tend to target older, weaker fish instead.