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Survival of the Fittest

Does ‘survival of the fittest’ simply mean ‘survival of those best able to survive’? If so, is the theory of evolution just an empty statement of the obvious? Fred Leavitt unravels a logical challenge to Darwinism.

The phrase ‘survival of the fittest’ seems to play a key role in the theory of evolution. Yet both creationists and some prominent philosophers and biologists have argued that the phrase is used tautologously; as a result, they assert, the whole theory is invalidated. I shall try to show that their reasoning is incorrect.

The theory of evolution by natural selection rests on three principles:

1. Individuals vary.
2. Some variations affect fitness and, therefore, survival (reproductive success is a more accurate term than survival, but either will do).
3. At least some of the variations that affect fitness are transmitted to offspring.

The critics claim that the second principle is meaningless. They assert that ‘fitness’ is used synonymously with ‘survival’ and therefore cannot affect survival. Thus the whole theory is a tautology. For example, Karl Popper wrote:

“The trouble about evolutionary theory is its tautological, or almost tautological character; there does not seem to be much difference, if any, between the assertion ‘those that survive are the fittest’ and the tautology ‘those that survive are those that survive.’ For we have no other criterion of fitness than survival.” (Objective Knowledge, revised. ed., 1979, pp.69)

In the same vein, C.H. Waddington claimed that:

“Natural selection turns out on closer inspection to be a tautology, a statement of an inevitable although previously unrecognized relation. It states that the fittest individuals in a population will leave most offspring. Once the statement is made, its truth is apparent.” (in Evolution After Darwin, ed. Sol Tax, 1960, p.385)

Stephen Gould (Ever Since Darwin: Reflections in Natural History, 1977, p.42) sought to defuse these criticisms by suggesting that certain designs are superior to others “by an engineer’s criterion of good design.” But listing independent criteria for fitness does not avoid the tautology. An engineer might say that long legs facilitate speed and therefore confer fitness advantage to cheetahs, but if long-legged cheetahs raised fewer offspring than normal-legged ones, evolutionists would be unperturbed. They would conclude that other factors were relevant, such as increased fragility of long legs, impaired immune systems in long-legged cheetahs, preferences of potential mates for cheetahs with average-sized legs, or unknown factors. More generally, evolutionary biologists assume that animals that raise more offspring than their competitors have the best overall design; and successful animals with apparently poor designs must have unknown compensatory features. Thus, the statement that the fittest individuals in a population will leave the most offspring seems protected from falsification and cannot be part of science.

But the concept of fitness is not needed. The theory can be rephrased as follows:

1. Individuals vary in many ways. (This is empirical.)
2. Individuals survive and reproduce with varying degrees of success. (This is empirical.)
3. Some of the variations in characteristics that affect survival and reproductive success are transmitted to offspring. (This is empirical.)
4. Populations will come to resemble successful individuals more than unsuccessful ones. (This is both empirical and a logical deduction from the preceding.)

Omitted from the new formulation is any discussion of the particular qualities that enhance or impair reproductive success. These differ from species to species and in different environments, and they may change over time. For example, Bernard Kettlewell’s classic book (The Evolution of Melanism, 1977) documents how dark-colored moths evolved from lightcolored ones in industrial areas of England. Whereas lightcolored moths had been better camouflaged in unpolluted areas, dark-colored ones are less conspicuous when resting on tree trunks blackened by pollution. Kettlewell’s work notwithstanding, evolutionists are frequently uncertain about the adaptive value of specific characteristics. Still, all living organisms are descendants of, and thus inherited characteristics from, ancestors who were fit enough to mature and reproduce. So evolutionists are justified in presuming that the vast majority of present-day organisms are fit.

It would be tautologous to argue that certain cheetahs outreproduce others because they are more fit. However, to suggest that the success of cheetahs depends partially on their leg size is not tautologous; it is a falsifiable scientific hypothesis. Scientists can use their previous experiences (observations that fast cheetahs are more successful than slow ones and that long legs facilitate speed) plus the principle of induction to predict future reproductive outcomes based on leg size. Alternatively, they can use their prior observations plus the inductive principle to hypothesize that present day cheetahs have long legs because ancient cheetahs with long legs left more descendants.

Fred Leavitt is a Professor of Psychology at California State University