Expected time to extinction and the concept of fundamental fitness

Abstract

The concept of “fitness” is of undisputed importance to evolutionary biology, but there is as yet little unanimity among evolutionists as to how it should be defined formally. In consequence a variety of mathematical fitness measures are in current use, and it is unclear which, if any of them, should be regarded as fundamental. In this paper a number of popular fitness measures are surveyed and their claims as indices of fitness examined critically. It is found that most of them probably fail to quantify the property of fitness in any fundamental or universal sense. One measure, however, possesses logical properties which recommend it as possibly fundamental. It is expected time to extinction, sometimes also called “average extinction time” or “mean survival time”. The expected time to extinction of a population or genotype is intimately related to the probability of its occurrence in such a way as to ensure the validity of certain common inference patterns found in optimization, adaptationist, and similar types of evolutionary reasoning. Thus its application is more readily justified on logical grounds than would be the use of other more traditional measures of fitness. The traditional measures may, however, be sufficiently closely associated with expected time to extinction to justify their use under certain special conditions.