Expert: Dana Krempels, Ph.D. - 5/13/2007

Question
Could you explain the prmise "evolution does not act on a part; it acts on the body as a whole"?
Would that imply that it is wrong to discuss the evolution of the eye or the human brain separately?

Answer
Dear Josh,

The unit of evolution is the population, and selection acts at the level of the individual organism, not its individual parts.  For example, a wild monkey might have eyesight not as keen as that of his rivals, but perhaps for some reason he is irresistible to the females for some other characteristic.  If he leaves more offspring to the next generation because of his "sexiness" trait, then any genes that contributed to his poor eyesight might also hitch along for the ride.  Hence, an "inferior" gene is not removed from the gene pool because the whole individual that carried that "inferior" gene had a selective advantage for some other reason.

Still, that doesn't mean it's wrong to discuss the evolution of the eye or the evolution of the brain.  Those things *did* evolve, and in species in which eyes and brains are important to survival and reproduction, the genes governing their shape and function were no doubt very subject to the forces of natural selection, even though they were "packaged" in whole organisms carrying many other genes--some adaptive, some potentially maladaptive.

Hope some of that made sense!  Please write back if I need to clear up any of that!  :)

Dana