Tuesday, February 28, 2006
Common objections to 'Internal Evolutionary Mechanisms' (1)
In their paper "The Spandrels of San Marco and the Panglossian Paradigm" Gould and Lewontin briefly described the European concept of Bauplan ('bodyplan') which, in its 'strong' form, speculates:
"But the important steps of evolution, the construction of the Bauplan itself and the transition between Bauplane, must involve some other unknown, and perhaps 'internal,' mechanism."
An internal mechanism cannot be 'mystical' because if one exists then it would be testable. This suggests the concept ought to evoke no greater uncertainty than that which would be appropriate to the words of Einstein: "If we knew what we were doing, we wouldn't call it research, would we?".
[The following also appears as "An error in associating Lamarck with 'Adaptive Mutations?"]
In 1640 Galileo Galilei wrote a letter to Fortunio Liceti in which he said:
"If Aristotle were to see the new discoveries recently [made] in the heavens, whose immobility he had asserted, because no alteration had previously been seen in them, he would now without doubt state the contrary." ['Galileo Galilei - Towards a Resolution of 350 Years of Debate', Paul Cardinal Poupard].
The above statement highlights the danger of placing dependence on words frozen in time without taking into account how different those words might be if their author had had access to the discoveries that have since been made.
Lamarck, for example, published his "Zoological Philosophy" in 1809 and is today popularly associated with "the inheritance of acquired characteristics" whereby organisms somehow direct their own evolution. On the basis of Galileo's words, however, it could be argued that had Lamarck been alive in the 1890s, over thirty years after publication of Darwin's "On the Origin of Species", his views would have progressed from the moment in time in which they had been caught.
With access to the discoveries and discussions that occured throughout the 19th Century it is conceivable that Lamarck might even have reached broad agreement with J. Mark Baldwin over the latter's proposal of an indirect factor in evolution, known today as the "Baldwin Effect", and described in the 1896 paper "A New Factor in Evolution" [American Naturalist].
Pure speculation ,of course, but if sufficient to illustrate a general principle (that "words frozen in time should be differentiated from those carved in stone") then the inappropriateness of interpreting new discoveries or proposals in 'Lamarckian terms' is readily apparent.
"Evolution is a process that results in heritable changes in a population spread over many generations."
This is a good working scientific definition of evolution; one that can be used to distinguish between evolution and similar changes that are not evolution. Another common short definition of evolution can be found in many textbooks:
"In fact, evolution can be precisely defined as any change in the frequency of alleles within a gene pool from one generation to the next."
- Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974
One can quibble about the accuracy of such a definition (and we have often quibbled on these newsgroups) but it also conveys the essence of what evolution really is. When biologists say that they have observed evolution, they mean that they have detected a change in the frequency of genes in a population.
Individuals do not evolve, but if shared circumstances 'triggered' individual internal evolutionary mechanisms in a subset of a population then this could cause similar genetic changes to appear in their progeny. Thereby causing a "change in the frequency of alleles within a gene pool from one generation to the next."
Extract from the transcript (click here) of a radio program in which Robyn Williams (ABC: Australian Broadcasting Corporation) talked to Ted Steele (author of "Lamarck's Signature"):
Steele: ...First, in 1885, three years after Darwin's death, a German biologist, August Weismann, responding to the challenge of Darwin's Theory of Pangenesis, erected his now famous 'barrier' between the somatic cells and germ cells. 'Weismann's Barrier' was assumed to protect the germ cells from any type of genetic change within the body.
The bulk of Weismann's experimental refutation focused on testing whether acquired parental mutilations could be inherited.
He is most famous for his work on chopping off the tails of rats shortly after birth. He then showed in breeding experiments extending over many generations, that such tail chopping at birth never produced a tailless offspring.
Critics of this experiment have pointed out that such experiments did not test Lamarck's idea. A short tail caused by chopping is a modification that was not produced by the rat. In contrast, Lamarck believed that only modifications produced by a response of the rat to the environment would be inherited.
Although the criticism is valid the most important point is that Weismann wasn't testing for a specific internal evolutionary mechanism whereby it had been proposed that 'acquired characteristics' could be inherited.