Chapter 15.3, Problem 2CSC

What Good Are Wisdom Teeth and Ostrich Wings?

Just as anatomical homology can lead to vestigial structures such as human wisdom teeth and the wings of flightless birds, genetic homology can lead to vestigial DMA sequences. For example, most mammal species produce an enzyme. L-gulonolactone oxidase, that catalyzes the last step in the production of vitamin C. The species that produce the enzyme are able to do so because they all inherited the gene that encodes it from a common ancestor. Humans, however, do not produce L-gulonolactone oxidase, so we can’t produce vitamin C ourselves and must consume it in our diets. But even though we don’t produce the enzyme, our cells do contain a stretch of DMA with a sequence very similar to that of the enzyme-producing gene present in rats and most other mammals. The human version. though, does not encode the enzyme (or any protein). We inherited this stretch of DNA from an ancestor that we share with other mammal species, but in us, the sequence has undergone a change that rendered it nonfunctional. (The change probably did not confer a strong disadvantage, because our ancestors got sufficient vitamin C in their diets.) The nonfunctional sequence remains as a vestigial trait, evidence of our shared ancestry.

Vestigial traits are evidence of both shared ancestry and change in traits over time. What kinds of observations and experiments show that natural selection contributes to evolutionary change?