THE FITNESS OF FITNESS CONCEPTS AND THE DESCRIPTION OF NATURAL-SELECTION

''Fitness'' has been used to indicate a measure of general adaptedness , and to indicate a short-term measure of reproductive success. The fo rmer concept seems unproductive in evolutionary biology, but consensus on the exact form of the latter might be possible. Fitness as a short -term measure of reproductive success can be defined from the demograp hic recurrence equations for genotypic number; it refers to a genotype or to a genotypic combination, if genotypes interact. Fitness summari zes a model for genotypic demography for a given set of assumptions ab out the population and the genotypic and individual interactions withi n it. For a population growing at a constant rate, demographic genotyp ic fitness has the same shape as reproductive value at birth; but repr oductive value refers to a cohort of a genotype, while demographic gen otypic fitness refers to organisms of one genotype at one moment in ti me. This is a major conceptual difference, although the numerical iden tity between demographic genotypic fitness and reproductive value for a population growing at a constant rate explains why models of life hi story evolution based upon reproductive value are successful. The Seco ndary Theorem of Natural Selection (Robertson, 1968) predicts the sele ction response in mean trait value by the genetic covariance between t rait and fitness. Selection on a quantitative trait is often formulate d as involving the heritability and the phenotypic covariance between trait and fitness or the phenotypic selection gradient beta, the (part ial) regression of fitness on the trait. The change in the covariance between the genotypic and the phenotypic level introduces an assumptio n on the additivity of fitness. The selection gradient, as a regressio n, focuses on differences in fitness as derived from differences in th e trait. In the Secondary Theorem, trait and fitness play equivalent r oles. The Secondary Theorem implies a different understanding of the p rocess of selection from a phenotypic selection gradient and a heritab ility, on those two counts. Fitness might arise from the phenotype in interaction with the environment, but phenotype and fitness might both arise as consequences of development. The study of selection thus bec omes the study of the biological mechanisms underlying and generating the genetic covariance between phenotype and fitness.