The red fox (Vulpes fulva) exists in the wild in three coat colour phases; red, cross, and silver or black. These three phases result from the action of one pair of alleles, the homozygotes being silver and red and the heterozygote being cross. At least two different mutations have occurred giving rise to the Canadian gene in eastern Canada and the Alaskan gene in western Canada. The mixing of these two mutant genes complicates the gene frequency analysis.The proportions of the three colour phases are shown to vary with (1) locality, (2) state of population cycle, (3) population trend, (4) migration pressure. Of these factors the variations with locality and population trend are fairly satisfactorily explained by selection but the fluctuation of colour phase proportions with the population cycle is not. On the other hand all the facts can be explained by a migration theory, with or without selection. By migration, a mixture of native and migrant populations with different gene frequencies is obtained. Such migrations tend to be rhythmic since they are connected with the population cycle. In the year that migration took place the pelt returns reveal aberrant gene frequencies or colour phase ratios. In the following years the gene frequencies quickly approach equilibrium that may be at the premigration level or at a new one depending upon the success of the migrants in establishing themselves in the breeding population.The marked diminution in the percentage silver and cross is due to the rapid population increase in an area of low frequency of the silver producing gene and the migration of this type into areas of higher frequency.
The Effects of selection and human preference on coat colour gene frequencies in urban cats
