Interesting exceptions to the general success of breeding programs come from Thoroughbred racehorses and greyhound dogs. Here, there appears to have been little improvement as judged by winning times over most of the past 50 years. For example, the record for the Kentucky Derby has stood since 1973, during which time milk yield in cattle has been increased nearly twofold. No hypotheses to explain this stasis seem fully convincing. One is simply that the base population was very small, limiting the amount of variation present; but there is evidence of substantial genetic variance for racing performance in horses, and mutations contribute to variation for other traits in other species. It is possible that mutations do occur for speed, but that almost all of them have deleterious pleiotropic effects, for example on leg strength or conformation. What Animal Breeding Has Taught Us about Evolution William G. Hill and Mark Kirkpatrick *********************************************** The BEE-L mailing list is powered by L-Soft's renowned LISTSERV(R) list management software. For more information, go to: http://www.lsoft.com/LISTSERV-powered.html Guidelines for posting to BEE-L can be found at: http://honeybeeworld.com/bee-l/guidelines.htm