Orthogenesis is a historical concept in evolutionary biology that proposed a directed form of evolution. This idea suggested that organisms evolve in a specific, predetermined direction.
Defining Orthogenesis
Orthogenesis, derived from Greek words meaning “straight origin,” posited that evolution follows a predetermined path. This theory suggested an intrinsic driving force within organisms, guiding their development in a specific, non-random direction, independent of environmental pressures or natural selection. Proponents believed that once a species began evolving in a particular direction, it would continue along that trajectory due to this inherent momentum.
This concept implied a “goal-oriented” evolution, where species progressed toward fixed endpoints or increasing complexity. Unlike Darwinian natural selection, which emphasizes random variations and survival based on environmental fitness, orthogenesis focused on internal forces driving evolutionary change.
The Rise and Appeal of Orthogenesis
Orthogenesis gained popularity among some scientists and thinkers in the late 19th and early 20th centuries. This period predated a full understanding of genetics and the mechanisms of inheritance that would later form the modern synthesis of evolution. The intellectual climate allowed for alternative explanations to Darwin’s theory of natural selection, which some found too random or lacking purpose.
Many found orthogenesis appealing because it offered a more orderly and purposeful view of evolutionary change. It seemed to align with philosophical or religious ideas of progress, suggesting that life was inherently striving towards higher, more complex forms or a specific goal. Paleontologists, in particular, found the idea compelling as they observed what appeared to be directional changes in fossil records, especially in invertebrate lineages, which seemed to show gradual and constant progression. Influential figures such as Theodor Eimer and Edward Drinker Cope championed these ideas, arguing for inherent trends in evolution.
Why Orthogenesis Was Rejected by Science
Orthogenesis was rejected by the scientific community because it lacked empirical support and contradicted mounting evidence from genetics and observed evolutionary patterns. The theory proposed internal, predetermined forces guiding evolution, but no verifiable mechanism for such forces was ever identified. American paleontologist George Gaylord Simpson, for example, criticized orthogenesis by describing its proposed mechanism as a “mysterious inner force.”
The development of genetics in the early 20th century, culminating in the modern evolutionary synthesis of the 1930s and 1940s, provided a robust framework that contradicted orthogenetic ideas. This synthesis demonstrated that evolution is primarily driven by natural selection acting on random genetic mutations, gene flow, and genetic drift. These processes result in a branching, non-linear pattern of descent, rather than the straight-line progression proposed by orthogenesis.
Observed evolutionary changes in the fossil record were found to be richly branching and complex, not unilinear as orthogenesis suggested. The idea that organisms would continue along a set path, even if it led to their extinction, was difficult to reconcile with the adaptive nature of evolution driven by natural selection. Evolutionary biologist Ernst Mayr effectively discouraged the use of the term “orthogenesis” in scientific discourse by 1948, noting its implication of “some supernatural force.”