Abstract
between the. need for fitness, securing immediate survival, and for flexibility, as the basis of evolutionary change. The degree of harmony achieved between these alternatives affects to a large extent not only the lifespan of any evolutionary line, but also the amount of diversification and evolutionary progress of which it is capable. The considerable variability in genetic systems of both plants and animals (e.g., see Stebbins 1950; Mayr 1963; White 1973) is evidence for the fact that no one level of compromise is equally advantageous for all organisms. According to a widely appreciated idea, every or group of has established a particular genetic system, effecting a certain level of compromise, as a result of its genetic potentialities and of the action of natural selection. Here I consider facts and express opinions on the genetic system and its evolution in the free-living, ciliated marine protozoa, Euplotes mninuta and Euplotes crassus. These represent two cosmopolitan morphospecies (Tuffrau 1960; Borror 1962; Curds 1975) whose separateness has recently been challenged. On the basis of multivariate analyses of selected cortical structures, Gates (1978) argued that populations of the aforementioned (along with those of the other nominal marine species, Euplotes vannus and Euplotes mutabilis) belong to a single evolutionary grouping of morphologically inseparable populations. Such a viewpoint aligns with that of Genermont (1976), Genermont et al. (1976), and Machelon (1978), at least as far as the vannus-crassus-mutabilis group is concerned. They investigated the relationship between morphologic differentiation and the degree of genetic isolation between clusters of populations within this group. The French authors concluded that (1) mutabilis is not a valid species, but a cortical variant appearing among clones of several genetically isolated groups of populations; (2) the populations in the vannus-crassus group can be grouped into at least three biological species not separable on the basis of the classic morphological criteria; that is, there exists a complex of E. vannus-like ciliates apparently not separable morphologically, but which exhibit various de
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