The biological concept of "fitness" is critical to an understanding of natural selection and of evolution in general. While philosophers continue to argue about the best way to define "fitness," there are some generally accepted aspects of biological definitions of fitness.
First, it is a term applied to an allele or genotype, a particular form of the genome. Thus, it is a property not of a single individual, but of a collection of individuals who share certain heritable traits. Fitness then is the contribution of the average individual with that trait to the next generation.
Explore Evolution misdefines this key biological concept, stating that fitness is a property of an individual, and is measured by that individual's contribution to future generations. By shifting from a focus on heritable traits to individuals, EE obscures the way in which biologists employ the concept of fitness, and mislead students.
The central theme of this chapter is a common creationist assertion that natural selection is invalid because it is circularly defined. If natural selection means "survival of the fittest," and the fittest are those that survive, then (they insist) the definitions are circular. Natural selection is the survival of those that survive. Of course, this is not the definition of natural selection, and it is not the definition of fitness. Fitness is defined in terms of the genotypes represented among future generations, not of the current generation, and natural selection is defined in terms of differential reproduction, not as "survival of the fittest." The entire chapter is based on a pair of flawed definitions and trivial wordgames. Philosophers dismissed this creationist argument long ago.
Tautology: Creationists have relied on this flawed argument for over a century, but it is no more valid today than it was when first introduced.