. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 394 Bulletin Museum of Comparative Zoology, Vol. 148, No. 8 Table 7. Matrix of Mahalanobis D" distances for samples of the hybrid zone at rocky point. 305 306 307 308 260 305 306 307 308 260 bridization confirm the localization of in- teraction. Sample 310, collected 400 m from the coast between sample 305 (the pure ribby beginning of the hybrid zone) and sample 306 lies among mottled samples, but near the pe
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 394 Bulletin Museum of Comparative Zoology, Vol. 148, No. 8 Table 7. Matrix of Mahalanobis D" distances for samples of the hybrid zone at rocky point. 305 306 307 308 260 305 306 307 308 260 bridization confirm the localization of in- teraction. Sample 310, collected 400 m from the coast between sample 305 (the pure ribby beginning of the hybrid zone) and sample 306 lies among mottled samples, but near the peripheiy of mottled and inter- mediate forms. Sample 309, about 600 m inland from 310, is well within the C. ben- dalli cluster and shows no signs of inter- mediacy. We then performed a discriminant anal- ysis on samples of the hybrid zone, using D/DA, a program written by John Rhoads, Dept. of Anthropology, Yale University (see Gould et al., 1974 for more details). In the field, we had concluded that the morpho- logical effects of hybridization are confined to a small, coastal area at Rocky Point (Fig. 5). We therefore performed our analysis on the five samples collected along this mile of coast (from south to north, 305, 306, 307, 308, and 260). The table of univariate ANOVA's (Table 6) shows that the best discriminators are measures of ribbing and shell size—scarcely surprising .since shells of C. hendalli are characteristically smaller and more copi- ously (though more weakly) ribbed than those of C. ahacoense. As a first indication of evenly clinal pat- terns, the matrix of Mahalanobis D- dis- tances (an overall measiue of similarity based on all characters with variance and covariance adjustments) exhibits a smootli morphological transition along the geo- graphic axis of collections (Table 7). Fig- ure 10 represents a plot of all samples against the first two discriminant axes. The first axis, which encompasses fully % ( per cent) of all information, arrays the sam-.
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