CRANIAL DIFFERENTIATION AND EVOLUTION IN THRICHOMYS APEREOIDES (RODENTIA, ECHIMYIDAE)

Thrichomys apereoides is an echimyid rodent which ranges in distributi on from north-eastern and central Brazil into Paraguay, and currently five subspecies are recognized. Recent morphometric analyses of popula tion samples formally assignable to T. a. laurentius and T. a. inermis , which occur in north-eastern Brazil, have shown that a major group o f populations including both subspecies differ in cranial shape from a single population allocated to T. a. laurentius. In this study we emp loyed mathematical models of evolutionary quantitative genetics to ass ess the role that random drift and selection may have played in the ev olution of cranial shape differences in T. apereoides. The hypothesis of evolution due to drift was rejected and the selective forces necess ary to account for shape differences were estimated. Minimum selective mortalities of the order of 10(-3) per generation were sufficient to explain the observed morphologic differentiation.