Why Evolution Is True

by Jerry A. Coyne

Life on earth evolved gradually beginning with one primitive species—perhaps a self-replicating molecule—that lived more than 3.5 billion years ago; it then branched out over time, throwing off many new and diverse

species; and the mechanism for most (but not all) of evolutionary change is natural

The probable answer—and it’s a good one—involves recognizing that as one species evolves into another, the descendant inherits the developmental program of its ancestor: that is, all the genes that form ancestral structures. And development is a very conservative process. Many structures that form later in development require biochemical “cues” from features that appear earlier. If, for example, you try to tinker with the circulatory system by remodeling it from the very onset of

development, you might produce all sorts of adverse side effects in the formation of other structures, like bones, that mustn’t be changed. To avoid these deleterious side effects, it’s usually easier to simply tack some less drastic changes onto what is already a robust and basic developmental plan. It is best for things that evolved later to be programmed to develop later in the embryo.

Human fetuses, for example, never resemble adult fish or reptiles, but in certain ways they do resemble embryonic fish and reptiles.

The captain obliged, and Selkirk was voluntarily marooned, taking ashore only clothes, bedding, some tools, a flintlock, tobacco, a kettle, and a Bible. Thus began four and a half years of solitude.

“For years and years people thought marsupials had to be there. This ties together all the suppositions made about Antarctica. The things we found are what you’d expect we would have.”

When biologists say that selection is acting “on” a trait, they’re merely using shorthand to say that the trait is undergoing the process. In the same sense, species don’t try to adapt to their environment. There is no will involved, no conscious striving. Adaptation to the environment is inevitable if a species has the right kind of genetic variation.

Richard Dawkins provided the most concise definition of natural selection: it is “the non-random survival of random variants.”

Here’s another requirement. An adaptation must evolve by increasing the reproductive output of its possessor. For it is reproduction, not survival, that determines which genes make it to the next generation and cause evolution. Of course, passing on a gene requires that you first survive to the age at which you can have offspring. On the other hand, a gene that knocks you off after reproductive age incurs no evolutionary disadvantage. It will remain in the gene pool.

When a group of male lions displaces the resident males of a pride, this is often followed by a gruesome slaughter of the unweaned cubs. This behavior is bad for the species since it reduces the total number of lions, increasing their likelihood of extinction. But it’s good for the invading lions, as they can quickly fertilize the females (who come back into estrus when they’re not nursing) and replace the slaughtered cubs with their own offspring. It is easy—though unsettling—to see how a gene causing infanticide would spread at the expense of “nicer” genes, which would have the invading males simply babysit the unrelated cubs. As evolution predicts, we never see adaptations that benefit the species at the expense of the individual—something that we might have expected if organisms were designed by a beneficent creator.