Fig. 5

Observed multilocus genotypic distributions in some populations of mixed composition (white, front row), compared with expectations in local panmictic equilibrium populations (black, middle row) and in non-interbreeding mixtures of two taxa (shaded, back row). The picture shows the distribution of the observed hybrid index defined as the total count of M. trossulus-specific alleles (T, see Table 2) at the four strongly differentiated allozyme loci Est, Gpi, Odh and Pgm. The expected distributions are calculated in a hypothetical mixture of the parental taxa, defined by the 4-locus allele frequencies (bi-allelic pooling) and the estimated mixing index X as in Fig. 4, and represent the extreme unimodal and bimodal contact zone structures, respectively. Frequencies of each distribution (Y-axis) sum to 1. X, population-level mixing index. R′/R′_max, average two-locus genotypic disequilibrium as a proportion of that in a non-interbreeding mixture (from Fig. 4, cf. Nikula et al. 2008), N, sample size