<P> However, the expression "survival of the fittest" (taken on its own and out of context) gives a very incomplete account of the mechanism of natural selection . The reason is that it does not mention a key requirement for natural selection, namely the requirement of heritability . It is true that the phrase "survival of the fittest", in and by itself, is a tautology if fitness is defined by survival and reproduction . Natural selection is the portion of variation in reproductive success that is caused by heritable characters (see the article on natural selection). </P> <P> If certain heritable characters increase or decrease the chances of survival and reproduction of their bearers, then it follows mechanically (by definition of "heritable") that those characters that improve survival and reproduction will increase in frequency over generations . This is precisely what is called "evolution by natural selection ." On the other hand, if the characters which lead to differential reproductive success are not heritable, then no meaningful evolution will occur, "survival of the fittest" or not: if improvement in reproductive success is caused by traits that are not heritable, then there is no reason why these traits should increase in frequency over generations . In other words, natural selection does not simply state that "survivors survive" or "reproducers reproduce"; rather, it states that "survivors survive, reproduce and therefore propagate any heritable characters which have affected their survival and reproductive success". This statement is not tautological: it hinges on the testable hypothesis that such fitness - impacting heritable variations actually exist (a hypothesis that has been amply confirmed .) </P> <P> Momme von Sydow suggested further definitions of' survival of the fittest' that may yield a testable meaning in biology and also in other areas where Darwinian processes have been influential . However, much care would be needed to disentangle tautological from testable aspects . Moreover, an "implicit shifting between a testable and an untestable interpretation can be an illicit tactic to immunize natural selection (...) while conveying the impression that one is concerned with testable hypotheses ." </P> <P> Skeptic Society founder and Skeptic magazine publisher Michael Shermer addresses the tautology problem in his 1997 book, Why People Believe Weird Things, in which he points out that although tautologies are sometimes the beginning of science, they are never the end, and that scientific principles like natural selection are testable and falsifiable by virtue of their predictive power . Shermer points out, as an example, that population genetics accurately demonstrate when natural selection will and will not effect change on a population . Shermer hypothesizes that if hominid fossils were found in the same geological strata as trilobites, it would be evidence against natural selection . </P>

Theory of natural selection and survival of the fittest