CONTRASTING MODES OF EVOLUTION IN TWO GROUPS OF PEREZIA(MUTISIEAE; COMPOSITAE) OF SOUTHERN SOUTH AMERICA

Summary Two species groups of the closely knit South American section of the genus Perezia occur in the Andes of southern America but differ conspicuously in the number of species they contain, and the habitat and amount of morphological specialization of their constituent species. These differences reflect the contrasting evolutionary histories of the two species groups. The evolutionary mode most significant in forming each group has been, in turn, determined by the age, geographical extent and vicissitudes of the habitat in which each group occurs. In one case, the habitat is old, covers a large continuous geographical area and has remained relatively stable since the arrival of the ancestral stock well before the Pleistocene. In the second case, the habitat is young, of discontinuous and often limited extent and has undergone numerous rapid alterations since its inception at the end of the Pliocene. In the first case the Pereziae have followed a course of phyletic evolution; in the second, a mode of true speciation.