The hypothesis that traits closely associated with fitness will generally possess lower heritabilities than traits more loosely connected with fitness is tested using 1120 narrow sense heritability estimates for wild, outbred animal populations, collected from the published record. Our results indicate that life history traits generally possess lower heritabilities than morphological traits, and that the means, medians, and cumulative frequency distributions of behavioural and physiological traits are intermediate between life history and morphological traits. These findings are consistent with popular interpretations of Fisher's (1930Fisher's ( , 1958 Fundamental Theorem of Natural Selection, and Falconer (1960, 1981), but also indicate that high heritabilities are maintained within natural populations even for traits believed to he under strong selection. It is also found that the heritability of morphological traits is significantly lower for ectotherms than it is for endotherms which may in part be a result of the strong correlation between life history and body size for many ectotherms.
INTRODUCTIONThe fundamental theorem of natural selection states: "The rate of increase in fitness of any organism at any time is equal to its genetic variance in fitness at that time" (Fisher, 1930). Fisher's theorem is axiomatic to much of current evolutionary research. It has been variously interpreted but is generally construed to imply that traits that have been closely and consistently associated with fitness will exhibit low additive genetic variances as a result of natural selection (e.g., Hegmann and Dingle, 1982; Lynch and Sulzbach, 1984; Riddel et cii., 1981). However, the validity of the fundamental theorem is dependent upon many assumptions that may not usually be met by natural populations (e.g., population equilibrium, weak selection, constancy of genotypic fitnesses over time, and independence of genotypic frequencies) (Charlesworth, 1987). The maintenance of low additive genetic variance has also been inferred to imply low heritability (in the narrow sense); "On the whole, characters with the lowest heritabilities are those most closely connected with fitness, while characters with the highest heritabilities are those that might be judged on biological grounds to be the least important as determinants of natural selection." (Falconer, 1960(Falconer, , 1981.) The most extensive compilation of data in support of this view is that presented in table 10.1 of Falconer (1981). However, these data are rather few and derived primarily from domestic animals, which may be partly inbred. Extension to wild, outbred organisms may be erroneous.It is possible that some significant amount of additive genetic variance can be maintained within natural populations, even for characters tightly connected to fitness; possible mechanisms include mutation (Lande, 1976; Turelli, 1984), heterozygote advantage (Falconer, 1981), frequency dependence (Bulmer, 1980), fluctuating environments (Ewing, 1979) and migration (Felsenstein, 197...
