Distinguishing natural selection from Darwinian evolution (the latter combines natural selection with the idea that all life has an ancestor in common) is one of the primary challenges modern creationists face in the origins debate. To many laypersons and science journalists—and to some scientists, apparently—the concepts are interchangeable, which means that experimental confirmation of natural selection is interpreted as proof of Darwin’s theory.
Experimental confirmation of natural selection is interpreted as proof of Darwin’s theory.
This week, we encounter the problem in news of a natural experiment run by Dartmouth biologists in the Bahamas. Ryan Calsbeek and Robert Cox not only studied competition in lizard populations on several Bahamian islands; they actively manipulated the environments: for example, adding nets on some islands to stymie birds that prey on the lizards, while introducing predatory snakes on other islands to threaten the lizards further.
The surprising result is that the intensity of predation didn’t matter much in how natural selection affected the lizard populations. “We found repeated evidence that death by predators occurred at random with respect to traits like body size and running ability,” Cox said. What mattered far more was competition between the lizards themselves. “We also found that increasing the density of lizards on an island consistently created strong natural selection favoring larger size and better running ability.” Calsbeek added, “Intense natural selection can also arise through competition. Sometimes, death by competitor can be more important than death by predator.”
Additionally, the scientists claim the experiment is a novel confirmation of Darwin’s theory. “Many people are skeptical of evolutionary biology because they perceive it as a purely historical science that can’t be tested experimentally,” Calsbeek said. “Here, we’re providing a real experimental test of natural selection as it happens in the wild. That’s an exciting way for us to advance the public’s perception of evolution.” But what seems a cautiously worded criticism of creationism misses the mark. First, the scientists’ experiment still doesn’t verify the historical claim that all life had a common ancestor, and, second, Calsbeek conflates natural selection and evolution. That lizards compete with one another and that only some survive does nothing to show that lizards and man are distant kin.
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