The key to understanding diversity in nature is what happens in the embryo, says Carroll (Genetics/Univ. of Wisconsin-Madison), and he provides compelling proof.
One of the great revelations of comparative genome studies over the past 20 years has been the discovery that animals share certain sets of master genes and switches that determine the ultimate shape of the animal, from flies and centipedes to mice and men. The fruit fly, for example, has a set of “Hox” genes on a single chromosome ordered in such a way that when expressed, they shape the fly’s body from head to end. Mind-bogglingly, these same Hox genes, or multiples of them on different chromosomes, are found in vertebrates, mammals and humans—where they play the same roles. Such “tool-kit” genes, as Carroll calls them, and the all-important genetic switches that orchestrate where and when the tool-kit proteins are turned on, not only determine animal forms but more nuanced details. These discoveries, along with the realization that embryonic development builds on repeated modular forms (think of the multiple segments of the human spine) are also clues to complexity: Further tinkering in gene expression and timing can lead to new, specialized appendages like arms and legs or wings and webbed feet. Admittedly, taking in all the details of these discoveries in the early chapters can be heavy going, but if the reader persists, there are delights to come. In the latter half, Carroll neatly describes the development of eyespots on butterfly wings, stripes in zebras, circles on fruit flies and red hair on redheads. His final chapters tackle human evolution, providing an up-to-date reprise of current fossil finds and speculation on how unique human traits may have developed. All this is further fallout from the new field of “evo devo” (evolutionary developmental biology) and provides more fuel to fight the creationist/intelligent-design folks.
Deserves to find its way into schoolrooms across the nation.