Evolutionary Notes: A species produced in an instant
Thursday 15 April 1999
Population geneticists, being Darwinians, rejected Eldredge and Gould's "model of punctuated equilibria". For them, experiments had long demonstrated that mutation constantly fuels the variation upon which natural selection acts. The process of species change never stops. Many paleontologists also claimed that, if fossils are collected properly, one sees a picture of gradual transformation.
How came one view of evolution to dominate the field? When Darwin published On the Origin of Species in 1859, Thomas Henry Huxley admonished him for embracing gradualism. Like other comparative anatomists, Huxley was a saltationist, for whom the emergence of species was a profound event. Darwin incorporated Jean Lamarck's ideas on use and disuse into his theory of blending inheritance, pangenesis. But the geneticists of the late 1800s and early 1900s - such as William Bateson, Hugo de Vries, and the young Thomas Hunt Morgan - rejected these conceits, as well as natural selection.
For them, variation is not continuous, species arise rapidly via large- scale organismal reorganisation, and, as for selection, if a new feature doesn't kill you, you have it. Ironically, Morgan later revived Darwinism by claiming that he had demonstrated gradual change experimentally: if, in the laboratory, you could change the length of fruit-fly wings over many generations, imagine what natural selection in the wild could do over millions of years?
Via mathematical manipulation, Ronald Fisher melded Morgan's idea with an expanded version of Darwinian natural selection, with organisms' features having purpose and selection opting for the most advantageous. This Darwinism became the bailiwick of the founders of the Evolutionary Synthesis - among them Theodosius Dobzhansky, Ernst Mayr, and George Gaylord Simpson - who proposed that large-scale systemic mutations quickly create new species.
But the language of evolution that emerged from the Synthesis had lost the fundamentals of heredity on which the early geneticists had based their theories. Non-lethal mutations arise in the recessive state and are thus inaccessible to selection. Only after many generations of the mutant recessive being passed on will there be a sufficient number of heterozygotes with a dominant wild and the new allele to produce homozygotes with two recessive alleles, in whom the effects of the mutation can then be expressed. At some point, a recessive may be converted to the dominant state.
The result of this process is that a new feature will appear suddenly, without a trail of transitional forms and in a number of homozygotes, who, along with heterozygotes, will continue to produce individuals with the new feature. But, depending on the genes behind the features, the impact of mutation and inheritance will be worlds apart. In terms of, say, structural genes for eye colour or height, the result is individual variation. But in terms of regulatory genes involved in the development of, for instance, fins versus limbs with feet, or animals that are radially or bilaterally symmetrical, the effect will be profound.
This is the stuff of evolution, which produces new species, without geographic isolation, in an instant.
Dr Jeffrey Schwartz is the author of `Sudden Origins: fossils, genes and the emergence of species' (published in May, John Wiley & Sons, pounds 19.99)
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