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.

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Question 8.4

Why doesn’t natural selection lead to perfect organisms?

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.

Figure 8.24: What’s fit in one time and place may not be fit in another. Beak size in Galápagos finches changes along with the average rainfall.

“I was taught that the human brain was the crowning glory of evolution so far, but I think it’s a very poor scheme for survival.”

— KURT VONNEGUT, American writer

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:

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TAKE-HOME MESSAGE 8.13

Natural selection does not lead to organisms perfectly adapted to their environment, because (1) environments can change more quickly than natural selection can adapt organisms; (2) mutation does not produce all possible alleles; and (3) there is not always a single, optimum adaptation for a given environment.

Is there a “perfect” beak size for a seed-eating finch in the Galápagos Islands? Why or why not?