The Effects of Mutation and Selection on the Rate and Pattern of Molecular Evolution in Birds

By comparing sequence diversity and divergence on sex chromosomes one can study how the rate of evolution in affected by mutation and/or selection. The rate of mutation in male biased, meaning that relatively more mutations are created in the male germ line than in the female. Since the male mutation bias (αm) most likely is a consequence of the difference in cell divisions between male and female germ lines, life history characters that affect this difference should covary with αm. Indeed, we found a positive correlation between estimates of αm and increased generation times and increased intensity of sperm competition. We have also found that estimates of αm varied significantly between gametologous introns located on the sex chromosomes. This could be a consequence of the variation in substitution rates between loci. Population genetics theory predicts that both positive and negative selection reduce genetic diversity around a selected locus at a distance determined by the rate of recombination. Consequently, a non-recombining chromosome, like the female specific W chromosome in birds, selection is expected to have a large impact on sequence diversity. Indeed, in a large sequence screening we found only one segregating site among 7643 base pairs sequenced in 47 chicken females. Furthermore, we also found that deleterious substitutions are fixed in a higher rate for W- than Z-linked sequences, which is in agreement with the lack of recombination and strong genetic drift due to the low effective population size. Rarely non-synonymous mutations are beneficial for an individual, but when it happens, the mutation is positively selected and rapidly reaches fixation in a population. We have found that positive selection has been acting on the female reproductive protein, zona pellucida c in birds. This rapid evolution is likely a mechanism to prevent hybridisation