Demise of Mutationism and Rise of The Modern Synthesis
While the mutationist view was very popular in the first 3 decades of the 20th century, it was replaced eventually by the Darwinian view expressed in the Modern Synthesis. In 1902 G. Udny Yule argued that a trait reflecting effects of multiple Mendelian characters could show a normal distribution. Thus, even though variations that arise in response to altered conditions are environmental and non-heritable (contrary to Darwin's assumptions), some of the continuous variation in natural species could have a genetic basis, and could serve as the Mendelian basis for Darwinian gradualism. Nevertheless, the synthesis of Mendelian genetics and Darwinism later put forth by R. A. Fisher and others did not develop immediately, for various reasons: it could be doubted that natural selection was sufficiently powerful to act on infinitesimal differences; it could be doubted that natural populations had enough heritable variation to support a Darwinian view; a common (erroneous) belief at the time (following Francis Galton's notion of regression to the mean) held that even heritable fluctuations could not lead to large or qualitative changes; and some advocates of Darwinism, such as Karl Pearson, refused to accept Mendelian genetics. A key conceptual innovation of the Modern Synthesis, crucial for its acceptance, was the "gene pool" concept, which argued that natural populations "maintain" abundant heritable variation through a combination of recombination, mixis, recessivity, heterosis and balancing selection.
At the time of the Darwin centennial in Cambridge in 1909, Mutationism and Lamarckism were contrasted with Darwin's “Natural Selection” as competing ideas; 50 years later, at the University of Chicago centennial of the publication of The Origin of Species, mutationism (like Lamarckism) was no longer seriously considered.
Nevertheless, after another 50 years, evolutionary biologists are re-considering the mutationist view.
Read more about this topic: Mutationism
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