QUANTIFYING GEOGRAPHIC VARIABILITY OF A LINEAGE: THE GEOMETRIC MORPHOSPACE OF MULINIA

Mulinia has been an ubiquitous mactrid bivalve within the Atlantic Coastal Plain since the Pliocene. Qualitative observations suggest there is geographic morphological variability in both past and present populations. However there are no cited attempts to quantify the degree of geographic variability and determine if the magnitude remains constant within a lineage. This study aims to quantify the variability seen within populations of Mulinia congesta and its likely descendant Mulinia lateralis. Specimens were used from bulk research and museum collections from Virginia to Florida. M. congesta specimens were sampled from Pliocene formations co-eval temporally to the Yorktown Formation. M. lateralis specimens were sampled from Pleistocene formations co-eval temporally to the James City Formation and Flanner Beach Formation.

Each valve was digitally imaged and x-y coordinates for fifteen landmarks and pseudolandmarks were acquired. Procrustes method was used to simultaneously fit the landmark points and derive shape coordinates that are invariant in respect to variations in size, rotation, and position of specimens. Tangent coordinates were used to derive principle component ordination to compare the overall shape differences between shells from different regions, and size was estimated using shell length and centroid size.

Preliminary results based on 789 specimens of M. congesta from 16 localities suggest that geographic morphospace varies significantly and there is a latitudinal trend based on the amount of specimens misclassified during a jackknife crossvalidation. For specimens originally from Virginia, 9% of them were misplaced into North Carolina, while none were misclassified with specimens from Florida. For Florida specimens, 8.33% were misclassified as Georgia, while 2% were misclassified as Virginia specimens. The amount of variation between geographic regions is comparable to the amount interspecific variation between the two species, when 68 M. laterialis specimens were analyzed. The geographic variability in morphology of Mulinia exceeds notably environmental variability and may be comparable in level to interspecific variability within the genus. Geographic gradients may thus exert influence on morphology that is comparable to morphological effects of speciation.