MOLECULAR PHYLOGEOGRAPHIC EVIDENCE FOR MULTIPLE SHIFTS IN HABITAT PREFERENCE IN THE DIVERSIFICATION OF AN AMPHIPOD SPECIES

An earlier study of ours demonstrated polygenic control of habitat pre ference between sympatric populations of an estuarine amphipod (Stanho pe et al. 1992). Knowledge of the ecological history in this estuary s uggested that two new and distinct habitat types (wood debris and Fucu s) had been formed immediately adjacent to each other, in an area whic h was previously the habitat type common to the rest of the estuary (b ank). This suggested the possibility that the ancestral population had been split into two resource specialists (disruptive selection on hab itat preference). The genetic relatedness of these three populations ( the proposed ancestor and the two proposed descendants) and six others occupying the same three habitat types were investigated on a regiona l geographic scale, using an extensive set of genomic DNA RFLPs. These data were combined with measures of habitat preference (including gen etic tests) in the additional populations. A very strongly supported p hylogeographic tree, unequivocally supported a shift in habitat prefer ence in the wood-debris population of this estuary. The data did not, however, support the hypothesis of disruptive selection on the ancesto r (bank) with a consequent split into two resource specialists. Instea d, it clearly indicated that the occupants of the Fucus habitat type w ere members of a habitat specific race, and thus their presence in thi s estuary reflected the expansion of an old resource base. Furthermore , the combined RFLP and habitat preference data, for all nine populati ons, indicated that the same polygenically based shift in habitat pref erence that had occurred in the original estuary had occurred independ ently, in another estuary, 700 km removed, that had similar environmen tal circumstances.