Fig. 1. Chordate phylogeny, which illustrates evolutionary relationships and differing Hox cluster complements (from Carroll, 1988). Blue brackets indicate time spans within which particular duplication events are believed to have occurred. The times when the ray and lobe-finned fishes last shared a common ancestor, and when the ostariophysans and acanthopterygians last shared a common ancestor, are indicated in green (in millions of years ago; Mya).