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THE GENUS: A MACROEVOLUTIONARY PROBLEM

Although much of the recent work on macroevolution has centered on the fossil record (Gould, 1980), a few studies have addressed this concept using the morphology of living species as a data base (Ricklefs, 1980; Lemen and Freeman, 1981). The neontological record does lack the element of time that allows a researcher to follow change through time, but this deficiency is counterbalanced by the fact that living species represent one perfect slice of time. Admittedly systematists have not been able to collect and classify all extant species, but tests using the neontological record in the better known groups such as birds and mammals will not suffer the problems of incomplete data sets or uncertain chronologies as badly as is probable with the paleontological record. One way to use neontological data is to study how species are arranged in morphological space. The evolutionary diversification of a clade can be seen as a tree that spreads with morphological change and branches with cladogenesis. In such a vision living species are one cross-sectional slice of the tree. The problem is to find what this cross section reveals about the structure of the tree. Perhaps the best way to start visualizing this approach is by considering that all clades ultimately trace their ancestry to a single species. Therefore, one can imagine a clade evolving from one species to many and filling morphological space through time in a certain way that is based on the number of species in the clade and the individual morphologies of the species. The specific morphologies of these species are not predictable because they are the products of the unique environment, history and stochastic processes that species face. However, if all clades share certain fundamental evolutionary processes, then some repeated patterns that depend on these shared processes might be expected in all clades. This paper centers on the macroevolutionary problem surrounding the concept of the vertebrate genus. We have been fascinated by the apparent tendency of members of a genus to have the same shape in contrast to the great differences in shape among genera at the family level. If this is true, genera would be considered shape conservative groups. Our initial view of this contrast in shape variation within and among genera leads us to question whether the same evolutionary processes that produce genera can simply be extended to produce families. To approach this question two things need to be done. First, the morphology of genera must be quantified to yield a more exact idea of how the morphological variation of a family is partitioned into genera. And second, evolutionary models need to be built that make different assumptions about how evolution proceeds. Our quantification of the morphology of genera will involve looking at size and shape variation at the familial and generic levels with multivariate methods. The evolutionary models will be used to make predictions of size and shape variation in families and genera under different evolutionary assumptions. As will be presented below, our models make clearly different predictions of how species will