July 22, 1910 (a Friday)

On this date, the American embryologist and geneticist T(homas) H(unt) Morgan reported in the journal Science:

In a pedigree culture of Drosophila which had been running for nearly a year through a considerable number of generations, a male appeared with white eyes.

White-eyed and wild type Drosophila.

For the science of genetics, the portent of the white mutation was enormous. Quickly, additional sex-linked mutants were discovered by Morgan and his students. By 1913 Sturtevant, with unsurpassed intuition, constructed the first linear genetic map of the X chromosome, followed by Bridges’ cytogenetic proof in 1916 of the chromosomal theory of inheritance. In 1915, Morgan, Sturtevant, Calvin Bridges and H. J. Muller wrote the seminal book The Mechanism of Mendelian Heredity. By 1925 the vast amount of information accumulated by Morgan and his students in less than 15 years was summarized in the monograph The Genetics of Drosophila. Documented therein are those fundamental principles of genetics derived from the study of Drosophila, principles that have withstood the test of time and that are included in all contemporary textbooks of genetics.

Interestingly, the period from approximately 1875 to 1925 has been called “the eclipse of Darwinism.” The phrase refers to the circumstances prior to the modern evolutionary synthesis when evolution was widely accepted in scientific circles but relatively few biologists thought that natural selection was its primary mechanism. In fact, many biologists considered natural selection to have been a wrong guess on Darwin’s part, and during his early career, Morgan had been one of them. In Evolution and Adaptation (1903), he had argued the anti-Darwinist position that selection never could produce wholly new species by acting on slight individual differences.

However, after discovering many small stable heritable mutations in Drosophila, Morgan had gradually changed his mind.  Since Morgan (1915) had ‘solved the problem of heredity’, he was in a unique position to examine critically Darwin’s theory of natural selection.  On February 24, 1916, Morgan began a series of lectures that would later be the basis of a book he published entitled A Critique of the Theory of Evolution (1916). The subsequent lectures occurred on March 1, 8, and 15.  He discussed questions such as:

  • Does selection play any role in evolution?
  • How can selection produce anything new?
  • Is selection no more than the elimination of the unfit?
  • Is selection a creative force?

After eliminating some misunderstandings and explaining in detail the new science of Mendelian heredity and its chromosomal basis, Morgan concluded that “the evidence shows clearly that the characters of wild animals and plants, as well as those of domesticated races, are inherited both in the wild and in domesticated forms according to the Mendel’s Law.”  “Evolution has taken place by the incorporation into the race of those mutations that are beneficial to the life and reproduction of the organism.”  “Injurious mutations have practically no chance of becoming established.”  Far from rejecting evolution as the title of his 1916 book may suggest, Morgan not only laid the foundation of the science of genetics, but by doing so, he also laid the theoretical foundation for the mechanism of evolution: natural selection.  Heredity was an essential  requirement of Darwin’s theory of natural selection, but Darwin had a wrong theory of heredity. Therefore, Darwinism could not progress without a correct theory of genetics. Morgan furnished that foundation, which is why his  work was so important for the neo-Darwinian synthesis, despite his criticism at the beginning of his career.

References:

  • Bridges, C . B., “Nondisjunction as proof of the chromosome
    theory of heredity” Genetics 1: 1-52, 107-163 (1916).
  • Green, M.M., “The ‘Genesis of the white-eyed mutant’ in Drosophila melanogaster: A reappraisalGenetics 142: 329-331 (Feb. 1996).
  • Morgan, T.H., “Sex-limited inheritance in Drosophila” Science 32: 120-122 (1910).
  • Morgan, T.H., A Critique of the Theory of Evolution (Princeton, NJ: Princeton University Press, 1916).
  • Morgan, T.H., Sturtevant, A.H., Muller, H.J. and C. B. Bridges, The Mechanism of Mendelian Heredity (New York, NY: Henry Holt, 1915).
  • Morgan, T. H., Bridges, C.B. and A.H. Sturtevant, “The
    genetics of Drosophila” Bibliogr. Genet. 2: 1-262 (1925).
  • Sturtevant., H., “The linear arrangement of six sex-linked
    factors in Drosophila as shown by their mode of association” J.
    Exp. Zool.
    14: 43-59 (1913).
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