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Offline Buckaroo Banzai

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Mutacionismo, eclipse do Darwinismo, a nova síntese, e o futuro
« Online: 17 de Dezembro de 2015, 22:09:53 »
Série interessante de posts sobre a história da teoria evolutiva e algumas distorções nela.

O último post tem a série de links para os posts precedentes:

The Mutationism Myth is a story told in the literature of neo-Darwinism, regarding the impact of the (re)discovery of Mendelian genetics a century ago. In this story, the discoverers of genetics (characterized as laboratory-bound geeks) misinterpret their discovery, thinking it incompatible with natural selection; the false gospel of these "mutationists" brings on a dark period that lasts until the 1930s, when theoretical population geneticists prove that genetics is the missing key to Darwinism; Darwinism is restored, and there is peace and unity in the land.

[...] An entirely different, but similarly prescient, model is found in T.H. Morgan's 1916 book:

    "If through a mutation a character appears that is neither advantageous nor disadvantageous, but indifferent, the chance that it may become established in the race is extremely small, although by good luck such a thing may occur rarely. It makes no difference whether the character in question is a dominant or a recessive one, the chance of its becoming established is exactly the same. If through a mutation a character appears that has an injurious effect, however slight this may be, it has practically no chance of becoming established.
    If through a mutation a character appears that has a beneficial influence on the individual, the chance that the individual will survive is increased, not only for itself, but for all of its descendants that come to inherit this character. It is this increase in the number of individuals possessing a particular character, that might have an influence on the course of evolution." (187-189)

This is an abbreviated framework for understanding evolution under the "new mutations" or "mutation-limited" view that is now commonplace in molecular evolution. A new mutation arises and may "become established- we would say "become fixed" or "reach fixation" in population-genetics jargon- with a probability (not a certainty) that depends on its effects. If its effects are injurious, is has practically no chance of being established, and so on.

Morgan's verbal description is remarkably accurate. Later, in the 1920s, Haldane, Wright, and Fisher began to work out some approximations for the probability of fixation of a new mutant allele. For newly introduced neutral alleles,  (substitute 2N for diploids), where N is the population size, and this value is not affected by recessivity or dominance, just as Morgan says; for a newly introduced beneficial allele, , where s is the selective advantage; for a significantly deleterious allele, the probability of fixation is vanishingly small.


To the extent that there was a distinctive "mutationist" perspective on evolutionary genetics that was rejected for its non-Darwinian implications, this was it. While Haldane, Fisher and Wright worked out the theory2 for the probability of fixation of a new mutation, they didn't use this knowledge for anything important, because evolution by new mutations was not part of their theory1 of evolution. Instead, Morgan's view of evolution as a series of mutation-fixation events was rejected by the Modern Synthesis as the "lucky mutant" view, and was ignored for nearly 50 years; Kimura popularized a neutral version of this view, which remained associated with neutral evolution for another 30 years; and in the past 10 years, Morgan's perspective is emerging as a more general view that may serve as the basis for models of adaptation (e.g., Orr, 2002).


Thus, Bateson interpreted quantitative characters precisely as we do today, as the result of overlaying environmental fluctuation on a discrete distribution of genetic types. This interpretation is not due to little Ronny Fisher, the 12-year-old boy who would grow up to be a founder of mathematical population genetics and would declare that genetics was the key to Darwin's theory7, but to Bateson and other geneticists, including Danish botanist Wilhelm Johannsen and the Swedish geneticist Herman Nilsson-Ehle. [....]


Darwin's 20th-century followers responded to the Mendelian threat— which (at least partially) called for a stochastic, non-infinitesimal, mutation-driven view of evolution and adaptation— by developing the Modern Synthesis (aka "modern neo-Darwinism" or the "New Synthesis"), a new theory that purported to be consistent both with genetics and with Darwin's 19th-century view of evolution as a process of infinitesimal change controlled, initiated and directed by selection.

The development of this theory, which went on to dominate the 20th century, was based on 3 innovations. The first innovation was to redefine Darwinism. The version of "Darwinism" that the MS restored was not the one that the Mendelians rejected. Instead, Darwinism 2.0 was "Darwinian" in emphasizing the pre-eminence of selection, leaving out the Darwin's non-Mendelian laws of heredity. The second innovation was the notional "gene pool", a populational buffer that insulates ÇevolutionÈ from effects of mutation by churning and mixing and "maintaining" abundant variation. The "gene pool" concept provided a foundation to reject the "lucky mutant" view and argue against Mendelian heterodoxies, e.g., given that evolution begins with the "gene pool", selection (not mutation) initiates evolution, and chooses its direction from the abundance of possibilities.

Finally, the MS included an integrated view of causation in which continuous shifts in allele frequencies are seen as the common currency of causal effects. That is, a factor is identifiable as an evolutionary "force" to the extent that it is capable of causing mass-action shifts in allele frequencies. This view appeared to justify the claim that selection is the driving force in evolution, and that mutation is not a potent force, but merely serves to supply "raw materials" to the "gene pool".

Thus, while the Mutationism Myth wrongly suggests that the MS reconciled genetics and selection (instead, the Mendelians accomplished this), it correctly suggests that the MS restored a "Darwinian" view, and that arguments from population genetics were the key to this restoration, though (as we'll find out later) the crucial arguments from population genetics were based less on mathematics than on metaphors and metaphysics.

Offline Jack Carver

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Re:Mutacionismo, eclipse do Darwinismo, a nova síntese, e o futuro
« Resposta #1 Online: 19 de Dezembro de 2015, 16:52:52 »
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