---
title: "History of evolutionary thought"
chunk: 10/14
source: "https://en.wikipedia.org/wiki/History_of_evolutionary_thought"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:59:38.552937+00:00"
instance: "kb-cron"
---

The concept of evolution was widely accepted in scientific circles within a few years of the publication of Origin, but the acceptance of natural selection as its driving mechanism was much less widespread. The four major alternatives to natural selection in the late 19th century were theistic evolution, neo-Lamarckism, orthogenesis, and saltationism. Alternatives supported by biologists at other times included structuralism, Georges Cuvier's teleological but non-evolutionary functionalism, and vitalism.
Theistic evolution was the idea that God intervened in the process of evolution, to guide it in such a way that the living world could still be considered to be designed. The term was promoted by Charles Darwin's greatest American advocate Asa Gray. However, this idea gradually fell out of favor among scientists, as they became more and more committed to the idea of methodological naturalism and came to believe that direct appeals to supernatural involvement were scientifically unproductive. By 1900, theistic evolution had largely disappeared from professional scientific discussions, although it retained a strong popular following.
In the late 19th century, the term neo-Lamarckism came to be associated with the position of naturalists who viewed the inheritance of acquired characteristics as the most important evolutionary mechanism. Advocates of this position included the British writer and Darwin critic Samuel Butler, the German biologist Ernst Haeckel, and the American paleontologist Edward Drinker Cope. They considered Lamarckism to be philosophically superior to Darwin's idea of selection acting on random variation. Cope looked for, and thought he found, patterns of linear progression in the fossil record. Inheritance of acquired characteristics was part of Haeckel's recapitulation theory of evolution, which held that the embryological development of an organism repeats its evolutionary history. Critics of neo-Lamarckism, such as the German biologist August Weismann and Alfred Russel Wallace, pointed out that no one had ever produced solid evidence for the inheritance of acquired characteristics. Despite these criticisms, neo-Lamarckism remained the most popular alternative to natural selection at the end of the 19th century, and would remain the position of some naturalists well into the 20th century.
Orthogenesis was the hypothesis that life has an innate tendency to change, in a unilinear fashion, towards ever-greater perfection. It had a significant following in the 19th century, and its proponents included the Russian biologist Leo S. Berg and the American paleontologist Henry Fairfield Osborn. Orthogenesis was popular among some paleontologists, who believed that the fossil record showed a gradual and constant unidirectional change.
Saltationism was the idea that new species arise as a result of large mutations. It was seen as a much faster alternative to the Darwinian concept of a gradual process of small random variations being acted on by natural selection, and was popular with early geneticists such as Hugo de Vries, William Bateson, and early in his career, Thomas Hunt Morgan. It became the basis of the mutation theory of evolution.

=== Mendelian genetics, biometrics, and mutation ===

The rediscovery of Gregor Mendel's laws of inheritance in 1900 ignited a fierce debate between two camps of biologists. In one camp were the Mendelians, who were focused on discrete variations and the laws of inheritance. They were led by William Bateson (who coined the word genetics) and Hugo de Vries (who coined the word mutation). Their opponents were the biometricians, who were interested in the continuous variation of characteristics within populations. Their leaders, Karl Pearson and Walter Frank Raphael Weldon, followed in the tradition of Francis Galton, who had focused on measurement and statistical analysis of variation within a population. The biometricians rejected Mendelian genetics on the basis that discrete units of heredity, such as genes, could not explain the continuous range of variation seen in real populations. Weldon's work with crabs and snails provided evidence that selection pressure from the environment could shift the range of variation in wild populations, but the Mendelians maintained that the variations measured by biometricians were too insignificant to account for the evolution of new species.
When Thomas Hunt Morgan began experimenting with breeding the fruit fly Drosophila melanogaster, he was a saltationist who hoped to demonstrate that a new species could be created in the lab by mutation alone. Instead, the work at his lab between 1910 and 1915 reconfirmed Mendelian genetics and provided solid experimental evidence linking it to chromosomal inheritance. His work also demonstrated that most mutations had relatively small effects, such as a change in eye color, and that rather than creating a new species in a single step, mutations served to increase variation within the existing population.

== 1920s–1940s ==

=== Population genetics ===

The Mendelian and biometrician models were eventually reconciled with the development of population genetics. A key step was the work of the British biologist and statistician Ronald Fisher. In a series of papers starting in 1918 and culminating in his 1930 book The Genetical Theory of Natural Selection, Fisher showed that the continuous variation measured by the biometricians could be produced by the combined action of many discrete genes, and that natural selection could change gene frequencies in a population, resulting in evolution. In a series of papers beginning in 1924, another British geneticist, J. B. S. Haldane, applied statistical analysis to real-world examples of natural selection, such as the evolution of industrial melanism in peppered moths, and showed that natural selection worked at an even faster rate than Fisher assumed.
The American biologist Sewall Wright, who had a background in animal breeding experiments, focused on combinations of interacting genes, and the effects of inbreeding on small, relatively isolated populations that exhibited genetic drift. In 1932, Wright introduced the concept of an adaptive landscape and argued that genetic drift and inbreeding could drive a small, isolated sub-population away from an adaptive peak, allowing natural selection to drive it towards different adaptive peaks. The work of Fisher, Haldane and Wright founded the discipline of population genetics. This integrated natural selection with Mendelian genetics, which was the critical first step in developing a unified theory of how evolution worked.

=== Modern synthesis ===