Abstract: Rates of protein evolution (d_N/d_S) vary widely across the tree of life. In plants, both life-history traits and GC-biased gene conversion (gBGC) are thought to contribute to this variation, although disentangling their individual contributions remains a challenge. Using information on variation in life-history traits and molecular data in 148 species from Poaceae subfamilies Bambusoideae (mostly woody) and Pooideae (exclusively herbaceous), we investigated the relative importance of modes of reproduction and the non-selective forces of gBGC on protein evolutionary rates between the two subfamilies. Elevated rates of protein evolution associated with relaxed purifying selection were more evident in woody bamboos than in Pooideae and were better explained by reproductive modes than by traits that are likely proxies of effective population size. Although gBGC slightly reduced protein evolutionary rates in both subfamilies, its contribution had only a limited effect on molecular divergence between the groups. Forward simulations generally supported our empirical results on the influence of reproductive mode on selection and gBGC. Our findings from two sister lineages of the grass family provide evidence for association between protein evolution and life-history traits governing reproductive mode and enhance understanding of molecular evolution in plants with contrasting reproductive strategies.

Key words: life‐history traits, natural selection, Poaceae, protein evolutionary rate, reproductive mode