Few phrases in science are as powerful or as widely misunderstood as "survival of the fittest." Virtually everyone associates it with Charles Darwin, who was born 200 years ago this week.

The phrase was actually coined by Darwin's 19th century contemporary Herbert Spencer, and it is perfectly ambiguous. The phrase could mean that "of all the possible creatures that one might imagine, only the fittest possible survive." Or it could mean something considerably less lavish -- that "only the fittest creatures that happened to...