Orthogenesis

Orthogenesis is the discredited theory that evolution follows a predetermined direction because of an inherent tendency, independently of natural selection.

Theodor Eimer: A German zoologist who argued that evolution followed inherent, often linear directions. His writings helped establish orthogenesis as an explicit evolutionary theory.
Natural selection: Differential survival and reproduction cause heritable traits to become more or less common in populations. Orthogenesis proposed direction without the differential reproduction central to this mechanism.
Irish elk: An extinct giant deer known for its exceptionally large antlers, which reached about 3.5 meters across. Its antler enlargement was once cited as a lineage-wide trend toward an extreme.
Orthoselection: A proposed sustained directional pattern of selection within an evolving lineage. The term separates a directional outcome of selection from orthogenesis's proposed internal drive.
Edward Drinker Cope: An American paleontologist who proposed evolutionary laws including an inherent tendency toward increased complexity. His idea of the law of acceleration became one version of orthogenetic thinking.
Mutationism: The view that mutations, rather than natural selection, play the leading role in evolutionary change. It offered a rival explanation for change that did not require a predetermined evolutionary trajectory.
Titanothere: An extinct group of large herbivorous mammals, some with prominent paired horns on their snouts. Successive fossil forms with larger horns became a classic example of apparent directional evolution.
Evolutionary progress: The idea that evolution shows an overall increase in complexity, adaptation, or some other favored measure. Orthogenesis supplied a mechanism for progress that did not depend on natural selection.
Alpheus Hyatt: An American paleontologist who described evolutionary sequences as progressive, but also subject to decline. His recapitulation theory treated lineages as having internally directed developmental trajectories.
Genetic drift: Random changes in allele frequencies caused by chance sampling across generations. It demonstrates that evolutionary change can lack any inherent directional tendency.

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