Lamarckism
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[edit] Definition
Lamarckism was an early attempt to explain evolution. The essential idea was that qualities acquired during the course of a lifetime, such as the development of a muscle as a result of exercising it, were heritable, and that this would explain the adaptation of organisms to their environment.
[edit] Lamarck
His ideas about evolution were published in 1809 in his Zoological Philosophy.
[edit] Lamarck in his own words
Everything which nature has made individuals acquire or lose through the influence of conditions to which their race has been exposed for a long time and, consequently, through the influence of the predominant use of some organ or by the influence of the constant disuse of this organ, nature preserves by reproduction in the new individuals arising from them, provided that the acquired changes are common to the two sexes or to those who have produced these new individuals. [...]
In this matter of habits, it is remarkable to observe the result in the peculiar form and height of the giraffe (camelo-pardalis). We know that this animal, the largest of the mammals, lives in the interior of Africa and dwells in those places where the earth, almost always arid and without grass, requires the animal to browse on the foliage of trees and constantly to try hard to reach that foliage. As a result of this habit, maintained for a long time in all the individuals of its race, the animal's front limbs have become longer than those at the back, and its neck has grown longer to such an extent that the giraffe, without rearing up on its hind legs, lifts its head and reaches up to six metres in height (close to twenty feet). (Jean-Baptiste Lamarck, Zoological Philosophy [1])
[edit] Discussion
A number of objections to Lamarckism might be raised.
One quibble put forward at the time is that circumcision has not reduced the natural size of Jewish foreskins. However, circumcision is a modification to the body, not an adaptation of the body to conditions, so this objection is not actually a counterexample to Lamarckism. In the same way, the artificial modification of experimental animals (cutting off the tails of successive generations of mice, for example, an experiment actually performed by August Weismann) proved nothing against Lamarckism.
Another objection might be to point out that certain acquired characteristics are most definitely not inherited, such as immunity from measles. However, Lamarck might have replied that his hypothesis did not require that all acquired traits should be heritable, but only those traits he was trying to explain.
We might complain that we see no evidence that (for example) stretching one's neck causes a permanent extension of its length, to which Lamarck might have replied that the effect he's talking about is small but cumulative.
Or we might argue from incredulity that such modifications could, at best, produce simple anatomical changes to existing structures, but could never produce new structures; in which case we would look silly, because successive small modfications acted on by natural selection do in fact have this effect.
No, the real argument against Lamarck is that genes don't work in the way that his theory requires, as we now know. The genotype determines the phenotype, including its ability to adapt over the course of life. Such modifications do not affect the underlying genetic material.
Lamarck has the misfortune to be remembered chiefly for a mistake. In fairness to him, it was not a silly mistake. Evolution does, after all, occur, and Lamarck's idea that it might work by the gradual cumulative modification of a lineage was in fact correct. Moreover, Lamarckian evolution would look, superficially, like evolution by natural selection; for many purposes the two models are broadly equivalent in their predictions.
Until scientists knew how genetics worked, there was a fair possibility that a Lamarckian mechanism might exist, and just conceivable that this would be sufficient to explain evolution. The kind of knowledge needed to prove Lamarck right or wrong was simply not available to scientists of his day.
