8.11–8.17: Through natural selection, populations of organisms can become adapted to their environments.

A harp seal pup’s white coat helps it hide from polar bears.
8.11: Traits causing some individuals to have more offspring than others become more prevalent in the population.

“Survival of the fittest.” This is perhaps one of the most famous but most misunderstood phrases. Taken literally, it seems to be circular: those organisms that survive must be the fittest. But this is true only if being fit is defined as the ability to survive. As we’ll see, in evolution, the word fitness is not defined by an organism’s ability to survive or its physical strength or its health. Rather, fitness has everything to do with an organism’s reproductive success.

Here’s an interesting side note: the phrase “survival of the fittest” was coined not by Darwin but by Herbert Spencer, an influential sociologist and philosopher. Moreover, the phrase did not appear in The Origin of Species when Darwin first published it. It wasn’t until the fifth edition, 10 years later, that he used the phrase in describing natural selection.

Before we see how fitness affects natural selection and a population’s adaptation to its environment, let’s define fitness. Fitness is a measure of the relative amount of reproduction of an individual with a particular phenotype compared with the reproductive output of individuals of the same species with alternative phenotypes.

Suppose there are two fruit flies. One fly carries the genes for a version of a trait that allows it to survive a long time without food. The other has the genes for a different version of the trait that allows it to survive only a short while without food. Which fly has the greater fitness? If the environment is one in which there are long periods of time without food, such as in the experiment described at the beginning of the chapter, the fly that can live a long time without food is likely to produce more offspring than the other fly, and so over the course of its life it has greater fitness. The alleles carried by an individual with high fitness will increase their proportion in a population over time, and the population will evolve.

There are three important elements to an organism’s fitness.

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Q

Question 8.3

“Survival of the fittest” is a misnomer. Why?

“Survival of the fittest” is a misleading phrase because it is the individuals with the greatest reproductive output that are the most fit in any population. It becomes a more meaningful phrase if we consider it as a description of the fact that those alleles that increase an individual’s fitness will “survive” in a population more than those that decrease an individual’s fitness.

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TAKE-HOME MESSAGE 8.11

Fitness is a measure of the relative amount of reproduction by an individual with a particular phenotype, compared with the reproductive output of individuals with alternative phenotypes. An individual’s fitness can vary, depending on the environment in which it lives.

Who has greater fitness: a world-class bodybuilder with a single child, or an overweight, middle-aged accountant with four children?