Adaptive Radiation: A Darwinian Mechanism Inherits the Wind
Another Darwinian assumption needs to be re-examined. Adaptive radiation, the belief that a species isolated on an island will diverge into many species, has been hit by a hurricane.
Calsbeek and Smith, writing in the Dec. 4 issue of Nature1, studied lizards on the Bahamas after Hurricane Floyd devastated the islands. “Islands are considered to be natural laboratories in which to examine evolution because of the implicit assumption that limited gene flow allows tests of evolutionary processes in isolated replicates,” they begin: “Here we show that this well-accepted idea requires re-examination.” Why? Gene flow is not limited after all. Apparently, ocean currents and hurricanes are very successful at spreading the critters around from island to island (gene flow, this is called). And high gene flow counteracts adaptive radiation by homogenizing the gene pool: “After severe storms, islands may be recolonized by over-water dispersal of lizards from neighbouring islands. High levels of gene flow may homogenize genes responsible for divergence, and are widely viewed as a constraining force on evolution.”
These islands have been a textbook case for adaptive radiation theories, because the number of Anolis lizard species is high: up to 140 species. The authors write, “The adaptive radiation of Caribbean anoles is believed to be driven by ecologically based natural selection arising from variation in habitat use.” Some of these lizards climb the broad trunks of trees and have long legs, whereas some perch on twigs, with short legs. These microevolutionary changes appear to be adaptive, because they would seem to help the critters run faster after food or avoid prey, or keep their balance in their preferred habitat.
The scientists found that the gene flow correlated with prevailing ocean currents. Moreover, the repopulation of the islands was very rapid: “Although no islands were reported to have received immigrants as a result of hurricane transport, subsequent recolonization of islands over the next 17 months was rapid and indicated over-water dispersal of adult lizards from neighbouring islands,“ they write with a bit of surprise. Although they have found a constraint on adaptive radiation in this classic case, they are confident that island studies are good for evolutionary theory. They conclude:
Studies on islands have revealed many of the fundamental mechanisms of evolution, particularly the paramount influence of geographical isolation to diversification. Here, we add an important caveat to these studies, showing that prevailing ocean currents may influence gene flow and adaptive divergence in a terrestrial vertebrate. The adaptive radiation of anoles in the Caribbean is thought to have arisen by ecologically based natural selection related to habitat use. However, the level of gene flow between populations will impose an upper limit on the ability of natural selection to drive adaptive divergence. We have provided evidence that weather-related abiotic phenomena might have important effects on the evolution and adaptive radiation of lizard populations. (Emphasis added in all quotes.)
1Ryan Calsbeek and Thomas B. Smith, “Ocean currents mediate evolution in island lizards,” Nature 426, 552 – 555 (04 December 2003); doi:10.1038/nature02143.
You better believe it might have important effects on evolution. It stops it.
First of all, notice that this is another tale about microevolution, so it doesn’t discriminate between creationists and evolutionists. But is there anything in this story that props up Charlie? Lizard populations in the Bahamas and surrounding islands were supposed to be a textbook case for adaptive radiation theory, and now look. They found that the islands were rapidly repopulated – within months – after Hurricane Floyd swept through, with the same species that existed before. But then, how can they rule out the possibility that some survived the storm? Did they check under every rock and in the tops of every tree? This looks like a bad science paper every way you cut it. That’s why we call evolutionists lazy, and accuse them of appearing to do science while vacationing in the Bahamas.
Adaptive radiation is only supposed to work if the gene pool gets cut off from the surroundings. They might be able to cling to that hope, but one of their best examples has just suffered “an important caveat” which, being interpreted, means, “Yeah, BUT…” It’s the unwelcome lab assistant tapping the evolutionist lecturer on the arm during his praise-for-Darwin speech, whispering in his ear, “Professor, we found a flaw in your data….” He stumbles for a moment, but continues unabated, “Moreover, ladies and gentlemen, evolution is a fact, supported by countless examples of thorough scientific research.”