It now became possible to understand the hereditary variation that is found throughout the biological world and that forms the basis of the theory of evolution. Genes are normally copied exactly during chromosome duplication. Rarely, however, changes (mutations) occur in genes to give rise to altered forms, most—but not nU—of which function less well than the wild-type alleles. This process is necessarily rare; otherwise, many genes would be changed during every cell cycle, and offspring would not ordinarily resemble their parents. There is, instead, a strong advantage in there being a small but finite mutation rate; it provides a constant source of new variebility, necessary to allow plants and animals to adapt to a constantly changing physical and biological environment.
Surprisingly, however, the results of the Mcndelian geneticists were not avidly seized upon by the classical biologists, then the authorities on the evolutionary relations between the various forms of life. Doubts were raised about whether genetic changes of the type studied by Morgan and his students were sufficient to permit the evolution of radically new structures, like wings or eyes. Instead, these biologists believed that there must also occur more powerful "rnacromutations," and that it was these events that allowed great evolutionary advances.
Gradually, however, doubts vanished, largely as a result of the efforts of the mathematical geneticists Sewfill Wright, Ronald A. Fisher, and John Burden Sanderson Haldane, They showed that, considering the great age of Earth, the relatively low mutation rates found for DrosophUn genes, together with only mild selective advantages, would be sufficient to allowr the gradual accumulation of newr favorable attributes. By the t030s, biolngists began to reevaluate their knowledge on the origin of species and to understand the work of the mathematical geneticists. Among these new Darwinians were biologist Julian Huxley
(a grandson of Darwin's original publicist, Thomas Huxley), geneticist Theodosius Dobzhansky, paleontologist George Gaylord Simpson, Emd ornithologist Ernst Mayr. In the 1940s all four wrote major works, each showing from his special viewpoint how Mendelianism and Darwinism were indeed compatible.
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