Natural selection and the limits of strict constructionism

Darwin said natural selection was the main force behind evolution, not the only force. Yet the strict constructionists claim it explains every biological feature and every species' success or failure. They're taking a stronger position than the person they're supposedly following.

That word every gives us something to test. One kind of evolutionary change that natural selection cannot explain would be enough to challenge their claim. We don't need to show that natural selection never works. Keep that distinction in mind: the author is arguing against an exclusive explanation, not rejecting evolution.

The second paragraph explains what the strict constructionists have committed themselves to. Natural selection generally produces adaptations: features that help organisms reproduce in the environment they face. If selection explains everything, then every persistent feature and every species' survival must be explained by such an advantage. The author says nature supplies counterexamples. Now we need to see why those examples fall outside that account.

First comes genetics. Some mutations help reproduction, others hurt it, and most do neither. Natural selection can explain why helpful changes are favored and harmful ones eliminated. But what about changes that persist without helping reproduction?

Neutral doesn't mean nothing about the organism changes. The third paragraph says these mutations often change a species' attributes. Imagine a change in appearance that makes no difference to reproductive success. There's a change to explain, but no reproductive advantage to explain its persistence. That's why this example matters: the author identifies evolutionary changes that the strict constructionists' account leaves out.

The dinosaur example raises a different problem. An extraterrestrial impact probably caused catastrophic climate changes. Small species were better able to survive, and the mammals of that era happened to be small. Their size reflected the earlier world, where they couldn't compete on the scale of large dinosaurs.

Being small turned out to help. But they hadn't become small in preparation for a future impact. Adaptations, as described here, arise in response to conditions already present. These mammals had a useful characteristic before the new conditions existed. Their survival therefore involved historical luck, which the author calls “dumb luck.”

It's tempting to respond that the changed environment still favored small animals. That doesn't erase the distinction we need to understand: possessing a feature that helps under new conditions isn't the same as having developed that feature in response to those conditions. The author uses that gap to challenge the claim that every survival outcome follows from adaptation to the environment in which the species succeeds.

The two examples do different jobs. Neutral mutations challenge the explanation of persistent attributes. Mass extinction challenges the explanation of species' success and failure. Both target the same sweeping claim: natural selection accounts for everything.

Our main-point prediction should preserve that scope. The author thinks evidence from genetics and paleontology undermines the strict constructionists' exclusive account. Natural selection can still be a major force; it just isn't the whole explanation.