Getting “Lamarckian” right
Lamarck’s theory of evolutionary adaptation invokes the inheritance of adaptive responses that emerge by effort. Over a century ago, this mechanism was rejected by geneticists familiar with results on mutation and inheritance, e.g., Cuenot (1909) rejects Lamarckian modification on the grounds that hereditary variants emerge suddenly, rather than being brought on gradually by use and disuse.
This topic would deserve no further comment, except that the false idea of a rebirth of a Lamarckian mode of evolutionary change has been stirred up repeatedly by contemporary scientists guided by the false dichotomy of Lamarck vs. Darwin.
Lamarck’s theory of adaptation is based explicitly on two laws, literally the Première Loi and the Deuxième Loi (see the wikipedia article on Lamarck’s view). The first law invokes the beneficial effects of use and disuse on the strengthening or weakening of organs. The second law asserts that the responses brought on by use and disuse are preserved by reproduction. The two laws together provide an automatic, generalized mechanism of cumulative adaptation. In Lamarck’s thinking, this mode of adaptive modification explains the fit of organisms to their circumstances, but adaptation itself is of secondary evolutionary importance relative to the tendency for organisms to increase their complexity, climbing the scala naturae.
With regard to his theory of adaptive modification, Lamarck was particularly interested in the law that he lists first. To account for it mechanistically, he proposes that the sentiment intèrieur of organisms causes them to respond to adaptive needs (besoins), and this causes “vital fluid” to be directed differentially to the relevant parts of the body, causing them to strengthen or expand. For instance, by striving to swim, turtles, otters and frogs increase the area of their paddle-like appendages; by striving to reach higher leaves, giraffes increase the length of their necks.
The second law, the inheritance of acquired characters or traits (IAC or IAT), was conventional wisdom in Lamarck’s time, as pointed out by historians such as Richard Burkhardt, or Michael Ghiselin in The Imaginary Lamarck: A Look at Bogus “History”. Heredity, development, and growth were understood as manifestations of the same underlying process mediated by growth-directing substances circulating in the body. Under this view, it is natural to assume that any modifications that take place organically (i.e., bodily responses rather than damage) must be heritable. Darwin’s views were based on this way of thinking. In the Origin of Species, Lamarckian modification is the second major means of evolutionary modification, less important than “natural selection”, but more important than direct modification by the environment (to check this in a searchable online copy, find where Darwin invokes “disuse”, “use”, or “habit”, noting that the latter two yield mostly false positives).
In standard histories, we learn that support for Lamarckism faded as Mendelian genetics became accepted as the basis of heredity. This is certainly true, but a second shift in thinking is also highly relevant. Everyone who works with animals knows that adaptive responsiveness is pervasive. Organisms adapt to conditions within their lifetimes via bodily changes: the blacksmith’s arms grow stronger with use, the fox grows thicker fur as winter comes on, the bacterium turns on expression of a catabolic pathway when the substrate becomes available, the plant grows toward the light and so captures even more of it. Thousands of examples could be given. Textbooks sometimes call this “physiological adaptation”, as distinct from evolutionary adaptation.
Adaptive responsiveness (the capacity for physiological adaptation) is clearly a special property of living things. We do not see it in rocks or pieces of furniture. Before the 20th century, it seems to have been accepted as a mysterious but clearly factual property, innate to living matter. Then scientists began to re-interpret it as a heterogeneous collection of topical adaptations, each with a separate historical explanation, e.g., the capacity to form callouses in response to abrasion was re-interpreted as a specially evolved capacity, and not as some kind of inherent adaptive property of integuments. I don’t know exactly when this change happened historically, but it is clearly underway in Morgan (1903).
When adaptive responsiveness was reinterpreted as a heterogenous collection of topical adaptations, this permanently ruined the prospects for a meaningful Lamarckian explanation of adaptation. If the potential for a specific adaptive response (e.g., callous formation) evolves first, and then drives further evolution via the combination of this specific response with IAC (e.g., further evolution of callouses but not anything else), this is a different and rather unsatisfying theory.
That is, Lamarck’s theory is a general theory of adaptation when IAC is pervasive and adaptive responsiveness (AR) is innate. But if AR reflects a large collection of adaptions that arose by some second mechanism of adaptation such as Darwin’s “natural selection”, then this is quite a different theory, and it is only a general theory of adaptation to the extent that the second mechanism is general.
Why is Lamarck’s name invoked by contemporary scientists wishing to draw attention to some heterodoxy? In some cases, Lamarck’s name is associated with the unconventional belief that mutation mechanisms have become adapted so as to improve the chances for further adaptation. In other cases, his name is invoked by those who attribute an evolutionary importance to non-Mendelian means by which environmentally-induced effects such as methylation states are inherited.
From the above discussion, calling these things “Lamarckian” is a mistake. If epigenetic inheritance provided for the blacksmith’s stronger arms to be passed on to his children, allowed the necks of giraffes to grow longer with effort, and explained other classic examples of adaptive responsiveness, this would be a vindication of a means of modification proposed by Lamarck and shared by Darwin. More generally, if contemporary mechanisms for environmental or behavioral modification of inheritance were linked pervasively with adaptive responsiveness, this would be a contemporary re-birth of Lamarckism— but no one is actually arguing that. No “Lamarckian” today is actually a Lamarckian.
In making this point, I am not original. Multiple philosophers who have explored the “random mutation” issue— Sarkar (1991, pp. 239 to 240), Millstein (1997, p. 134), and Merlin (2010, p. 11)— all make the same point, though inexplicably, most of them continue to make the same false contrast of “Darwinian” and “Lamarckian” that Ghiselin (1994) shreds to pieces. A fine example of this false contrast in contemporary literature would be when Yosef, et al (2016) suggest that
Over 150 years ago, Darwin claimed that selection pressure does not induce changes in organisms, but only acts as a filter that allows the organisms acquiring these changes to propagate more efficiently (Darwin 1859). This perspective was in contrast to Lamarck’s theory, claiming that the selection pressure actually shapes organisms and allows them to propagate more efficiently (Lamarck 1809).
This utterly distorts the views of both Darwin and Lamarck. It is precisely Darwin’s view to propose that “altered conditions of life” induce “indefinite variability”, and to reject the idea that the struggle for life is merely a filter.
Equating Lamarckism with IAC is a mistake because IAC was not Lamarck’s idea, and because Lamarck’s mechanism of adaptation requires both of his laws, i.e., the combination of IAC and AR.
Equating Lamarckism with a contemporary rejection of the random-mutation doctrine, e.g., on the basis of CRISPR-Cas, is also a mistake. This mistake requires more explanation. Over a century ago, Darwin’s most ardent followers, the neo-Darwinians, rejected Lamarckian modification, focusing exclusively on what Darwin called “natural selection”. Though Darwin’s minor modes of modification invoked non-random variation, his original conception of “natural selection” did not, but relied on abundant slight variations in every trait, which blend together in a hereditary stream shaped by the struggle for life. Darwin was very deliberate in insisting that “chance” variations do not play a role in shaping the outcome of evolution, but only supply material, and this became a doctrine of neo-Darwinism maintained when it was fused with Mendelism in the Modern Synthesis.
CRISPR-Cas is clearly a case of instructive mutation, a direct contradiction of the random mutation doctrine of modern neo-Darwinism. However, we don’t call something “Lamarckian” just because it is not neo-Darwinian. Lamarck’s theory is a specific theory, and the mechanistic details of CRISPR-Cas activity are so distant from it, that equating them is silly.
IMHO, invoking “Lamarckian” mechanisms is not just a matter of getting history wrong. The fake conflict between Darwin and Lamarck distorts contemporary issues and steals credit from contemporary scientists. For instance, there is a contemporary debate about the “mutation is random” doctrine largely because, in 1988, Cairns et al mounted a challenge to it, with ongoing reverberations, including intellectual heirs in a “directed mutations” research community that continue to hold non-mainstream views on the nature and role of mutation in evolution. If we were to bring those ancient authorities back from the dead to have a discussion about the legitimacy of “random mutation”, the implications of epigenetic inheritance, and so on, they would have nothing useful to say, and would have to be sent to university for years just to be able to understand the issues.
The challenge of understanding mechanisms of variation, and their role in evolution, is emphatically not an ancient controversy between Lamarck and Darwin, nor IMHO is there much to be gained from studying the profoundly confusing statements in historical works from most of the 20th century.
For those readers with a burning desire to introduce Lamarck’s name into contemporary discourse, a fun and defensible way to do so is to offer a correction whenever someone uses “Darwinism” or “Darwinian” to refer to belief in evolution generally (You mean, “Lamarckism”? You mean, is he still with the “Lamarckian Sciences Cluster” at U. Chicago?), or someone attributes to Darwin the idea of a unified branching genealogy of life (You mean “Lamarck’s tree of life“?), as Lamarck has priority in both cases.
Millstein RL. 1997. The Chances of Evolution: An Analysis of the Roles of Chance in Microevolution and Macroevolution. University of Minnesota.
Sarkar S. 1991. Lamarck Contre Darwin, Reduction Versus Statistics: Conceptual Issues in the Controversy Over Directed Mutagenesis in Bacteria. In: Tauber AI, editor. Organism and the Origins of Self. Netherlands: Kluwer Academic Publishers. p. 235-271.