Gene trees and species trees. (a) Gene tree based on mitochondrial COI and 16S sequences from representative individuals of 5 Physa populations, with Aplexa as outgroup. Jackknife values are given at the nodes. For sequence codes and genbank numbers see Wethington and Lydeard  and Wethington et al. . (b) Species tree modelling hybrid viability. The solid line is a hypothetical locus "J," a match at which allows unimpeded hybridization, and the dashed line a second locus "K," a match at which allows partial hybridization. (c) Species tree modelling the evolution of F1 fertility as a single locus. (d) Species tree modelling mating compatibility. Two loci are hypothesized, a solid "J" and a dashed "K," a match at only one of which is sufficient for unrestricted mating.
Dillon et al. BMC Evolutionary Biology 2011 11:144 doi:10.1186/1471-2148-11-144