8.13: Natural selection does not lead to perfect organisms.

In Lewis Carroll’s Through the Looking-Glass, the Red Queen tells Alice that “in this place it takes all the running you can do, to keep in the same place.” She might have been speaking about the process of evolution by natural selection. After all, if the least fit individuals are continuously weeded out of the population, we might logically conclude that, eventually, fitness will reach a maximum, and all organisms in all populations will be perfectly adapted to their environment. But this never happens. That is where the Red Queen’s wisdom comes in.

Consider one of the many clearly documented cases of evolution in nature: the beak size of Galápagos finches. Over the course of a multidecade study, biologists Rosemary Grant and Peter Grant closely monitored the average size of the finches’ beaks. They found that the average beak size within a population fluctuated according to the food supply. During dry years—when the finches had to eat large, hard seeds—birds with bigger, stronger beaks were more successful and multiplied. During wet years, smaller-beaked birds were more successful, because there was a surplus of small, soft seeds.

The ever-changing “average” finch beak illustrates that adaptation does not simply march toward some optimal endpoint (FIGURE 8-24). Evolution in general, and natural selection specifically, does not guide organisms toward “betterness” or perfection. Natural selection is simply a process by which, in each generation, the alleles that cause organisms to have the traits that make them most fit in that environment tend to increase in frequency. If the environment changes, the alleles that are most favored may change, too.

In the next to last paragraph of The Origin of Species, Darwin wrote: “as natural selection works solely by and for the good of each being, all corporeal [physical] and mental endowments will tend to progress towards perfection.” In this passage, he overlooks several factors that prevent populations from progressing inevitably toward perfection:

• 1. Environments change quickly. Natural selection may be too slow to adapt the organisms in a population to such a constantly moving target.
• 2. Variation is needed as the raw material of selection—remember, it is the first necessary condition for natural selection to occur. If a mutation creating a new, “perfect” version of a gene never arises, the individuals within a population will never be perfectly adapted.
  
• 3. There may be different alleles for a trait, each causing individuals to have the same fitness. In this case, each allele represents an equally fit “solution” to the environmental challenges.