The phenotypic markers, coat color, pattern and hair length, of natural domestic cat populations observed in four cities (Barcelona, Catalonia; Palma Majorca, Balearic Islands; Rimini, Italy and Buenos Aires, Argentina) were studied at a microgeographical level. Various population genetics techniques revealed that the degree of genetic differentiation between populations of Felis catus within these cities is relatively low, when compared with that found between populations of other mammals. Two different levels of sampling were used. One was that of "natural" colonies of cat families living together in specific points within the cities, and the other referred to "artificial" subpopulations, or groups of colonies, inhabiting the same district within a city. For the two sampling levels, some of the results were identical: 1) little genic heterogeneity, 2) existence of panmixia, 3) similar levels of expected heterozygosity in all populations analyzed, 4) no spatial autocorrelation, with certain differentiation in the Buenos Aires population compared to the others, and 5) very high correlations between colonies and subpopulations with the first factors from a Q factor analysis. Nevertheless, other population genetic statistics were greatly affected by the differential choice of sampling level. This was the case for: 1) the amount of heterogeneity of the FST and GST statistics between the cities, which was greater at the subpopulation level than at colony level, 2) the existence of correlations between genic differentiation statistics and size variables at subpopulation level, but not at the colony level, and 3) the relationships between the genetic variables and the principal factors of the R factorial analysis. This suggests that care should be taken in the choice of the sampling unit, for inferences on population genetics to be valid at the microgeographical level.
