Genetic Distances and Ordination: The Land Snail Helix aspersa in North Africa as a Test Case

We examined the efficiencies of ordination methods in the treatment of gene frequency data at intraspecific level, using metric and nonmetric distance measures Neis and Rogers genetic distances, χ² distance. We assessed initial processes responsible for the geographical distribution of the Mediterranean land snail Helix aspersa. Seventeen enzyme loci from 30 North African snail populations were considered in the present analysis. Five combinations of distance/multivariate analysis were compared: correspondence analysis CA, nonmetric multidimensional scaling NMDS on Neis, Rogers, and χ² distances, and principal coordinates analysis on Rogers distances. Configuration of the objects resulting from ordination was projected onto three dimensional graphics with the minimum spanning tree or the relative neighborhood graph superimposed. Pre and postordination or clustering distance matrices were compared by means of correlation methods. As expected, all combinations led to a clear west versus east pattern of variation. However, the intraregional relationships and degree of connectivity between pairs of operational taxonomic units were not necessarily constant from one method to another. Ordination methods when applied with Neis and Rogers distances provided the best fit, with original distances r = 0.98 compared with UPGMA clustering r 0.75. The Nei/NMDS combination seems to be a good compromise distortion index d_t = 10 between Rogers/NMDS, which produces a more confusing pattern of different tiation d_t = 24, and χ²/CA, which tends to distort large distances d_t = 31. NMDS obviously provides a powerful method to summarize relationships between populations, when neither hierarchical structure nor phylogenetic inference are required. These findings led the discussion on the good performance of NMDS, the appropriate distances to be used, and the potential application of this method to other types of allelic data such as microsatellite loci or data on nucleotide sequences of genes.