Big Guys Finish First, Except in Drought
Nigel Williams tried to explain in Current Biology1 why “size matters” among marine iguanas in the Galapagos Islands: the vectors of natural and sexual selection don’t always line up. Females appear to like the big males when times are good, but when drought comes, the smaller dudes do better.
There’s a difficulty with such investigations. Even though this habitat was a “rich source of information for Charles Darwin when developing his theory of evolution,” the article admits that “Factors influencing the evolution of complex traits such as body size are notoriously difficult to study but a new review of work on marine iguanas in the Galapagos islands suggests an answer may lie in the interplay of natural and sexual selection” (emphasis added).
1Nigel Williams, “Size matters,” Current Biology, Volume 15, Issue 18, 20 September 2005, Page R742.
Why should Darwin be mentioned in this article, except as a historical embarrassment? There is no evolution here. Heap big iguana is still iguana as much as peewee. Size is not a “complex trait” in the sense of evolving wings or some new organs; it is just a modification of parts already present. There is no long-term evolutionary trend here, but rather only oscillations around a mean that reflect climate conditions – otherwise we should see iguanas the size of Godzilla by now. If natural and sexual selection work against each other, then stasis rules, not evolution. Charlie won’t get anywhere with slippage on the treadmill (see 03/17/2003 entry).