Mutation, selection and the heritability of complex traits.

It has been suggested that complex traits, like intelligence, have low heritabilities. This hypothesis stems from the idea that strong selection for higher intelligence has led to the fixation of genetic variation for this trait. The same hypothesis has been framed for complex sexual ornaments used in courtship display. These traits are also subject to directional selection, in this case caused by sexual selection. However, contrary to the hypothesis, comparative data shows that sexual ornaments have higher additive genetic variation than similar non-sexual traits. It appears that the number of variable genes and the effect per genetic locus have increased for sexual ornaments. Theory suggests this is due to selection for extreme phenotypes resulting in condition-dependent expression of sexual traits. Experimental work on the stalk-eyed fly, Cyrtodiopsis dalmanni, confirms that the male sexual trait (exaggerated eyespan) is more sensitive to environmental conditions than other non-sexual traits (wing dimensions and female eyespan). This environmental sensitivity has a genetic basis and environmental stress enhances genetic differences. It is likely that genetic variation in intelligence is maintained in a similar fashion.