SEPARATING POPULATION-STRUCTURE FROM POPULATION HISTORY - A CLADISTIC-ANALYSIS OF THE GEOGRAPHICAL-DISTRIBUTION OF MITOCHONDRIAL-DNA HAPLOTYPES IN THE TIGER SALAMANDER, AMBYSTOMA-TIGRINUM

Nonrandom associations of alleles or haplotypes with geographical loca tion can arise from restricted gene flow historical events (fragmentat ion, range expansion, colonization), or any mixture of these factors. In this paper, we show how a nested cladistic analysis of geographical distances can be used to test the null hypothesis of no geographical association of haplotypes, test the hypothesis that significant associ ations are due to restricted gene flow, and identify patterns of signi ficant association that are due to historical events. In this last cas e, criteria are given to discriminate among contiguous range expansion , long-distance colonization, and population fragmentation. The abilit y to make these discriminations depends critically upon an adequate ge ographical sampling design. These points are illustrated with a worked example: mitochondrial DNA haplotypes in the salamander Ambystoma tig rinum. For this example, prior information exists about restricted gen e flow and likely historical events, and the nested cladistic analyses were completely concordant with this prior information. This concorda nce establishes the plausibility of this nested cladistic approach, bu t much future work will be necessary to demonstrate robustness and to explore the power and accuracy of this procedure.