The Discovery of Genetics Challenges Darwin's Theory
As the 20th century dawned, geneticists learned that discontinuous variations could arise by mutation and be transmitted to offspring via stable non-mixing factors: the rules of transmission of these factors constitute Mendel’s laws. A more revolutionary discovery, from the perspective of evolutionary theory, was that slight variations in quantitative traits that emerge reliably every generation— like the "fluctuations" on which Charles Darwin built his theory— were not heritable. This result was shown in a series of breeding experiments carried out by the Danish biologist Wilhelm Johannsen. From mixtures of different true-breeding varieties of beans of different sizes, selection on a breeding population could be used to sort out the large from the small varieties, but would not change their heights, even though fluctuations in size continued to appear each generation, following the familiar normal distribution.
This result was understood widely as a direct threat to the "Natural Selection" theory of Darwin, who argued in The Origin of Species that
"Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being; and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection, if it be a true principle, banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure." (Ch. 4, OOS)
Darwin knew that discontinuous variations or "sports" occurred, and that their effects were heritable, but he argued that such changes would not be important in evolution, which must occur gradually according to the doctrine of natura non facit salta (see gradualism). In Darwin's theory, infinitesimal hereditary variation arises automatically in response to the effect of "altered conditions of life" on "the sexual organs"; whenever conditions change, adaptation happens automatically (and by infinitesimal increments) as selection preserves fluctuations that fit the new conditions. That is, Darwin proposed a mechanism of automatic evolution, based on automatic variation that would always be present when needed.
However, genetics showed that the kind of variation that arises automatically in response to altered conditions is not genetic variation, but non-heritable environmental variation. Heritable variation, by contrast, arises spontaneously by events of "mutation". This is how the discovery of genetics forced a re-appraisal of the mechanism of evolution — a re-appraisal that led to the rise of "mutationism".
Read more about this topic: Mutationism
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