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

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