Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Geographic variation was studied in fox squirrels, Sciurus niger, and gray squirrels, S. carolinensis, by comparing patterns of differentiation within and between these two sympatric species. Patterns of variation were examined in light of the lower Mississippi River as a potential barrier to dispersal and gene flow in these squirrels. Differences within and between species were analyzed using morphologic (15 cranial and mandibular measurements) and allozymic (35 presumptive gene loci) characters. Geographic patterns of variation among populations were apparent in the morphology and allozymes of both species; patterns of differentiation in morphologic and allozymic characters are similar between species; however, morphologic variation is not congruent with allozymic variation within either species. Fox squirrels and gray squirrels each vary morphologically so that, within each species, individuals that inhabit the Mississippi River floodplain and delta region are smaller than animals from adjacent regions. This size variation may be a nongenetic response to environmental factors, or it may reflect regional differences in selective regimes, and thus may represent genetic variation among populations. Available data are insufficient to distinguish between these two causal mechanisms. Allozymically, fox squirrels and gray squirrels exhibit similar patterns of differentiation; within each species, there are differences among eastern and western populations, as defined by their geographic location relative to the present Mississippi River channel. Thus, the Mississippi River and associated habitats may have been (and may still be) a barrier to gene flow in these species. This study provides considerable evidence that the lower Mississippi River has influenced morphologic differentiation in fox and gray squirrels and that the river has impeded (and may still impede) gene flow in these species. The role of the river as a barrier to dispersal and gene flow may have resulted from direct effects; the Mississippi River may be a substantial physical barrier to tree squirrels. It is also highly likely that the river has affected dispersal and gene flow in tree squirrels indirectly due to environmental and vegetational shifts that occurred in the alluvial valley of the Mississippi River during the late Quaternary.