The Evolution of Sexual Dichromatism in a Large Radiation of Landfowl: Re-examining Female-Biased Selection in Wallace's Model.

Sexual dichromatism is generally thought to arise from sexual selection favouring elaborately coloured males, as proposed in Darwin's model. Wallace offers an expanded perspective, emphasising that the evolution of cryptic and dim female colouration to evade nest predation significantly contributed to the origin of sexual dichromatism. However, previous studies on the evolutionary forces of sexual dichromatism examining Darwin's and Wallace's models have produced mixed results. Here, we re-examined Wallace's model of female-biased selection using the largest and most ecologically diverse family of landfowl (Phasianidae), known for its wide-ranging distribution and striking colour patterns. Our results demonstrate that the level of sexual dichromatism is negatively correlated with colour complexity in females but not males, and the evolutionary rates of sexual dichromatism are positively correlated with the evolutionary rates of colour complexity in both sexes. Furthermore, we show that female colour complexity is significantly associated with nest exposure, consistent with predation-related ecological constraints on female colour evolution. Together, these findings support Wallace's model by highlighting the importance of nest predation in shaping female colouration, while further suggesting that the evolution of sexual dichromatism reflects the dynamic interplay of natural and sexual selection.