The study by Mousseau and Roff examines the heritability of traits related to fitness in wild, outbred animal populations. Using 1120 narrow-sense heritability estimates, they find that life history traits generally have lower heritabilities than morphological traits, while behavioural and physiological traits have intermediate heritabilities. These findings support Fisher's and Falconer's interpretations of the Fundamental Theorem of Natural Selection, indicating that high heritabilities can persist for traits under strong selection. The heritability of morphological traits is lower in ectotherms than in endotherms, possibly due to the strong correlation between life history and body size in ectotherms. The study also explores how genetic variability differs across taxa. It finds that ectotherms have lower heritabilities for morphological traits than endotherms, but no significant differences in life history traits. The analysis of data from 75 species shows that heritability estimates are generally consistent across different methods, with no significant bias introduced by the method of estimation. The results suggest that traits closely associated with fitness tend to have lower heritabilities, consistent with classical genetic theory. However, the study also indicates that significant genetic variance can be maintained in natural populations for traits under strong selection, possibly due to mechanisms such as mutation, heterozygote advantage, frequency dependence, fluctuating environments, and migration. The findings support the idea that genetic variation is an important factor in evolutionary processes, and that the heritability of fitness components is generally lower than that of traits not closely related to fitness. The study concludes that much of what is generally accepted in evolutionary literature regarding genetic variance in natural populations is confirmed, but the mechanisms maintaining high heritabilities for fitness components remain unclear.The study by Mousseau and Roff examines the heritability of traits related to fitness in wild, outbred animal populations. Using 1120 narrow-sense heritability estimates, they find that life history traits generally have lower heritabilities than morphological traits, while behavioural and physiological traits have intermediate heritabilities. These findings support Fisher's and Falconer's interpretations of the Fundamental Theorem of Natural Selection, indicating that high heritabilities can persist for traits under strong selection. The heritability of morphological traits is lower in ectotherms than in endotherms, possibly due to the strong correlation between life history and body size in ectotherms. The study also explores how genetic variability differs across taxa. It finds that ectotherms have lower heritabilities for morphological traits than endotherms, but no significant differences in life history traits. The analysis of data from 75 species shows that heritability estimates are generally consistent across different methods, with no significant bias introduced by the method of estimation. The results suggest that traits closely associated with fitness tend to have lower heritabilities, consistent with classical genetic theory. However, the study also indicates that significant genetic variance can be maintained in natural populations for traits under strong selection, possibly due to mechanisms such as mutation, heterozygote advantage, frequency dependence, fluctuating environments, and migration. The findings support the idea that genetic variation is an important factor in evolutionary processes, and that the heritability of fitness components is generally lower than that of traits not closely related to fitness. The study concludes that much of what is generally accepted in evolutionary literature regarding genetic variance in natural populations is confirmed, but the mechanisms maintaining high heritabilities for fitness components remain unclear.