Orthogenesis

Abstract: Orthogenetic evolutionary patterns may appear within taxa as a result of developmental constraints on the expression of genetic variation. Metachromism is a theory of variation in mam? malian coat color that predicts an orthogenetic evolutionary pattern. This theory was based in part on studies of interspecific and geographic variation in the coat colors of New World monkeys. We tested the theory of metachromism in tamarins (Saguinus) using phylogenetic information derived from mitochondrial DNA sequences. Parsimony analysis of mitochondrial DNA sequences from the D-loop and cytochrome b genes supports recognition of Saguinus as a monophyletic genus composed of two major clades. Coat colors for 16 chromogenetic regions of the tamarin coat were examined using the molecular phylogeny to test predictions of the metachromatic the? ory. Our results support the hypothesis of an orthogenetic evolutionary pattern for coat color in tamarins. (Orthogenesis; phylogeny; coat colors; Saguinus', tamarins, mitochondrial DNA.) Orthogenesis denotes a linear pattern of evolution. Historically, orthogenesis was conceived as an internal momentum of variation that propelled evolution along a particular morphological pathway without regard to natural selection (Bowler, 1983). Orthogenesis was invoked as the driving force behind the evolution of seemingly nonadaptive characters and as an expla? nation for the appearance of structures considered useless in their incipient stages (Jepsen, 1949). Although early theories of orthogenesis were discredited during the evolutionary synthesis (Bowler, 1983), de? velopmental constraints on the phenotypic expression of genetic variation may lead to orthogenetic evolutionary patterns (Riedl,

Abstract: -In the panbiogeographic evolutionary synthesis, orthogenesis is referred to as a process which involves an inherent tendency to vary in certain directions, as ancestors vicariate or break apart into descendants. Orthogenesis has origins in the studies of form, but is given a biogeographic perspective in panbiogeography. The panbiogeographic use of orthogenesis is compatible with the origin of the term as presented by Haacke and Eimer. Misinterpretations of orthogenesis describing it as mystical, teleological and linear are invalid. The orthogenetic aspect of evolution was recognized by Darwin as "laws of growth," but was neglected in favor of natural selection. Although an internal component to evolution is recognized by contemporary biologists, it is often considered to be secondary to natural selection. Where recognition is given to an internal tendency for evolution to proceed without requiring the action of natural selection, terms such as "constraints," "bias," and "potential" may imply orthogenesis. [Orthogenesis; panbiogeography; biogeography; vicariism; evolution; dispersal.] The orthogenetic idea that evolution has its own inexorable internal momentum forcing lineages beyond the limits of natural selection is obviously absurd [Dawkins and Krebs, 1979:507]. In due time all theories defending orthogenesis were refuted [Mayr, 1982a:530]. I can accordingly understand that some naturalists may wax "religious," and speak of orthogenesis in mystical terms; others indulge in philosophical abstractions in regard also of the same orthogenesisions in regard also of the same orthogenesis etc. Naturally, I do not see the need for my part to follow in their company, but I do believe that, whatever we intend and believe, we had all better be clear about the substances here in play [Croizat,