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199O.-Evolution is the unifying concept of biology. The study of evolution can be approached from a within-lineage (microevolution) or among-lineage (macroevolution) perspective. Phylogenetic systematics (cladistics) is the appropriate basis for all among-liieage studies in comparative biology.

Phylogenetic systematics enhances studies in comparative biology in many ways. In the study of developmental constraints, the use of such phylogenies allows for the investigation of the possibility that ontogenetic changes (heterochrony) alone may be sufficient to explain the perceived magnitude of phenotypic change. Speciation via hybridization can be suggested, based on the character patterns of phylogenies. Phylogenetic systematics allows one to examine the potential of historical explanations for biogeographic patterns as well as modes of speciation. The historical components of coevolution, along with ecological and behavioral diversification, can be compared to the explanations of adaptation and natural selection. Because of the explanatory capabilities of phylogenetic systematics, studies in comparative biology that are not based on such phylogenies fail to reach their potential.