Figure 4. Comparison of autosome and sex chromosome nucleotide diversity.

The ratio of sex chromosome to autosome diversity is plotted for the 29 species in which both estimates were available from the same population(s). Colors indicate the phylum to which the species belong. Within Chordata, open circles denote mammals and open triangles birds; within Arthropoda, open circles denote Drosophila species. The number of species in each group is given in parentheses in the legend. Within species, crosses represent the ratio estimated from different populations, with the median of the estimates shown as a triangle (birds) or circle (all other species). Solid horizontal lines indicate the median sex chromosome to autosome ratio for arthropods and chordates, colored as in the key. The black dashed line indicates where sex chromosome diversity equals three-fourths of autosomal diversity.