![]() Most studies have found that convergent phenotypes tend to evolve through changes within the same gene (Conte et al., 2012 Martin and Orgogozo, in press) or within a particular pathway (e.g., Chan et al., 2012 Tenaillon et al., 2012) in the absence of a reporting bias, these findings indicate that the number of permissible paths to a trait is often limited. In other cases, changes in distinct loci result in the evolution of the same trait (e.g., Chen et al., 1997 Hoekstra et al., 2006 Borowsky, 2008 Steiner et al., 2009). To date, cases of convergence have been found to occur through independent coding changes in the same gene (e.g., Mundy, 2005 Christin et al., 2008 Gross et al., 2009 Kingsley et al., 2009) through changes in distinct non-coding elements regulating the same gene (e.g., Miller et al., 2007) through distinct changes in the same regulatory element (e.g., Tishkoff et al., 2007) or even through changes in the same protein domains (e.g., Aminetzach et al., 2009 Feldman et al., 2012) or at the same amino acids (e.g., Li et al., 2010 Liu et al., 2010b Shen et al., 2012). Examples of convergent evolution, the independent acquisition of the same trait in different populations or species, may be highly informative with regard to questions of evolutionary predictability and constraint because these likely represent multiple solutions to similar selective pressures (see, e.g., Conway Morris, 2003 Christin et al., 2010 Losos, 2011). In natural populations, the number of possible genetic changes leading to a given trait is determined by the combination of two factors: the number of loci at which mutations influence the phenotype (the mutational target size) and the proportion of these mutations that do not have prohibitively deleterious consequences on other phenotypes (the extent of pleiotropy Stern and Orgogozo, 2008 Stern, 2011). Given the high similarity between the blue iris phenotypes in these species and that in humans, this finding implies that evolution has used different molecular paths to reach the same end. In the orthologous region, we found no variant that distinguishes the two lemur species or associates with quantitative phenotypic variation in Japanese macaques. Variation in an enhancer of OCA2 is primarily responsible for the phenotypic difference between humans with blue and brown irises. ![]() Yet whereas Japanese macaques and humans display continuous variation, the phenotypes of blue-eyed black lemurs and their sister species (whose irises are brown) occupy more clustered subspaces. Characterizing the phenotype across these species, we found that the variation within the blue-eyed subsets of each species occupies strongly overlapping regions of CIE L*a*b* color space. ![]() We investigated the convergent phenotype of blue iris pigmentation, which has arisen independently in four primate lineages: humans, blue-eyed black lemurs, Japanese macaques, and spider monkeys. How many distinct molecular paths lead to the same phenotype? One approach to this question has been to examine the genetic basis of convergent traits, which likely evolved repeatedly under a shared selective pressure.
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