Constraint-early/selection-late model of developmental divergence. Reduced divergence rates of embryonic genes relative to those of later stages are explained by purifying selection against the deleterious pleiotropic effects of mutation. Later stage genes are not simply 'less constrained', but experience unique selective pressures, such as sexual selection, driving accelerated divergence.
Artieri et al. BMC Biology 2009 7:42 doi:10.1186/1741-7007-7-42