WHY ARE THE SEXES AS THEY ARE? MANY DATA, SOME PATTERNS, AND MANY UNSOLVED MYSTERIES 1

Recently, in all seriousness, I was asked if the sexes were different. I answered that I thought so. “But, on what evidence?” continued my inquisitor. I replied that my favorite came from the many studies showing that parents adaptively modify the sex ratios of their progeny, so even if you do not recognize sex differences, in a huge variety of species, parents respond—or their parents’ proxy through natural selection responds—to differences between the sexes. More evidence of sex differences is in this collection of papers,1 which explores more thoroughly than any previous book the distribution and adaptive significance of sexual size dimorphism (SSD), one of the most easily recognized forms of sex difference. SSD is about within-population differences in the sizes of females and males. The first part—the heart of the book—is about macroevolutionary patterns comprised of six taxonomic chapters that remind us who is larger; the second part contains microevolutionary studies, whose authors seek to understand the selective pressures favoring SSD, and the last is about mechanisms of sexual dimorphism. Many data inform the description of macroevolutionary patterns, though a few obvious groups need more work and the fish were notably absent. There is an obvious need for more microevolutionary studies of SSD like those in the second