Orthogenesis

Orthogenesis is the hypothesis that evolution proceeds in a predetermined direction — that life is pulled toward particular outcomes by forces independent of natural selection. The term encompasses theories from Lamarck's innate drive toward complexity to paleontological proposals of evolutionary trends too consistent to be random.

Orthogenesis was rejected by the Modern Synthesis because it violated a core commitment: that variation is undirected with respect to fitness. But some paleontological data — trends toward increasing body size, repeated evolution of similar forms — suggest that developmental constraints and environmental constancies can produce directional patterns. The challenge is distinguishing true directionality from the appearance of directionality produced by biased variation.

Orthogenesis was dismissed as teleology, but the phenomena that motivated it — directional trends, evolutionary channels — have returned as evolutionary constraints and developmental bias. The name was discredited; the problem never went away.