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amounts of genetic separation. Most of these breed-specifi c dif erences have
nothing to do with the artifi cial selection that generated the breeds. They
are simply genetic accidents, the inevitable result of dividing a gene pool up
into small subpools. But scientists are beginning to track down some of the
more meaningful genetic dif erences among breeds that dog breeders have
produced through their deliberate artifi cial selection.
For example, it made sense for the fi rst agriculturalists to select for small
dogs. Small dogs can sound the alarm against intruders and catch rats as
ef ectively as large ones, and they don't eat as much. A genetic consequence
of that selection has now been found by Nathan Sutter of the National Insti-
unique allelic form of an insulin-dependent growth factor gene. The “small
dog” allele has been subject to stronger recent selection than the other alleles
at this genetic locus that are carried by large dogs. Over the past few thou-
sand years the small dog allele has increased in numbers in dog populations,
driven by artifi cial selection.
Alleles for smallness must have arisen many times in wolf populations
by mutation, but they were soon lost because small wolves would have been
at a disadvantage. This seems to have remained true even in dog populations
up to the time of the agricultural revolution. Before that time, dogs were as
large as wolves. Small dogs fi rst appear in the fossil record in the Middle East
and Europe about 10,000 to 12,000 years ago, at the same time as the spread
of agriculture.
Around that time dog breeders selected for small dogs, which pulled the
small dog allele up in frequency. As the allele became commoner it dragged
along other genetic dif erences that lie close to it on the same chromosome.
It was this cluster of accompanying genetic dif erences that provided Sutter
with evidence that the small dog allele has increased in dog populations as a
result of recent artifi cial selection.
Enormous behavioral dif erences among dog breeds have also been pro-
duced by millennia of deliberate selection. These dif erences provide us with
great opportunities for understanding the genetics of behavior. Herding,
pointing, and retrieving behaviors can now be subject to genetic analysis.
