/awr'theuh jen"euh sis/, n.1. Biol.a. Also called orthoselection. evolution of a species proceeding by continuous structural changes in a single lineage without presenting a branching pattern of descent.b. a theory that the evolution of a species in a continuous, nonbranching manner is due to a predetermined series of alterations intrinsic to the species and not subject to natural selection.2. Sociol. a hypothetical parallelism between the stages through which every culture necessarily passes, in spite of secondary conditioning factors.[1890-95; < NL; see ORTHO-, GENESIS]
* * *▪ biologyalso called straight-line evolutiontheory that successive members of an evolutionary series become increasingly modified in a single undeviating direction. That evolution frequently proceeds in orthogenetic fashion is undeniable, though many striking features developed in an orthogenetic group appear to have little if any adaptive value and may even be markedly disadvantageous. A variety of theories have attempted to explain orthogenesis, but with only partial success. One underlying view in orthogenesis, as espoused by some paleontologists, is that variation in nature is directed toward fixed goals and that species evolve in a predetermined direction irrespective of selection.
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