Darwin’s Lamarckism vindicated?

Darwin rejected his own theory in favor of Lamarckian evolution. Epigenetics now suggests he was partly right.

by Dr Robert W. Carter

Published: 1 March 2011 (GMT+10)

It generally surprises people when they learn that Darwin disavowed his own theory of evolution in favor of a Lamarckian view of inheritance (a belief that acquired, rather than genetic, characteristics, can be passed on). Today, Lamarckian inheritance is making a comeback in the field of epigenetics.

With few exceptions,1 Darwin pulled no punches in his Origin of Species (published 1859), claiming that all current species descend from common ancestors and that natural selection acting on morphological diversity is the driving agent of change. This is what most people think of when they think about Darwin’s ideas of evolution.

Nearly a decade after Origin was published, Darwin produced Variations of Plants and Animals Under Domestication (1868).2 In this book, he first suggests a model of inheritance he called “pangenesis”. While trying to explain the mode of inheritance and the source of variation, Darwin imagined “corpuscles” being produced by the various body parts in response to environmental stresses. These corpuscles would travel to the gonads to be passed to the next generation. This idea of pangenesis is Lamarckian to the core. In The Descent of Man (1871),3 he attempts to lay out his reasoning and evidences (with more of a focus on the former) for his idea that man and apes have a common ancestor. But in so doing, he also answers several objections people had to his thesis in Origin. While addressing the critiques of his contemporaries, some of whom claimed that his ideas in Origin were overly dogmatic, he backed off the idea that natural selection was the only engine of evolution. In its place, he says that a combination of natural selection, operating on variation caused by the hypothetical and Lamarckian process of pangenesis, and sexual selection are the driving forces. He also alludes to group selection and kin selection (two “modern” theories that have been debated for decades). Sexual selection theory has been controversial since its inception and has taken huge setbacks recently,4 and his pangenesis idea is demonstrably false. One is left wondering what, exactly, people mean when they say “Darwinian evolution”!

This is an example of the way Darwin worked. He was a theorist, not an experimentalist. In fact, there is no record of him ever doing a controlled experiment in the Baconian tradition..

In Descent, he used sailors and watchmakers as examples of pangenesis, claiming sailors tended to become farsighted and watchmakers nearsighted due to their respective occupations, and that these acquired traits are inheritable. Without further analysis, he claimed this as evidence for his idea. This is an example of the way Darwin worked. He was a theorist, not an experimentalist.5 In fact, there is no record of him ever doing a controlled experiment in the Baconian6 tradition. Rather, he says his methodology was to come up with an idea and then amass evidence in its favor.7 He tried valiantly in Descent to build up a case for the evolution of man, but never really proved anything. Worse, his vacillation on the mechanism of evolution weakened his arguments significantly and exposed them for the weak hypotheses that they were.

Pangenesis is essentially modified Lamarckism, where environment influences the individual and its offspring. For decades, geneticists have resisted any notion that the environment of the parents could affect the phenotype of the progeny. It simply wasn’t part of the basic formula of the neo-darwinian synthesis (genes + mutation = variation). In recent years, however, enough evidence has emerged to launch an entirely new field called “epigenetics”. This new field has brought evolutionary theory full circle.

If the environment can affect phenotype in an inheritable manner, individuals will be falsely targeted. Then, when the epigenetic modification is reset, natural selection is sent back to square one and the ‘less-fit’ individuals were selected against in vain.

Epigenetics deals with non-permanent modification of the genome. Like turning on and off a light, genes can be turned on and off as well. The main engine for this is called DNA methylation, where a methyl group (–CH 3 ) is attached to specific cytosine residues. The bulky methyl group attached to the DNA blocks the transcription machinery, so a methylated gene is effectively silenced. Methylation is strongly associated with the environment. Thus, the environment can affect the way an organism behaves. More importantly, evidence is slowly accumulating that methylated genes can be transmitted from one generation to the next.8 Even more importantly, methylation can be reversed. This field has given us terms like “epialleles” (“epi” means “upon”) and “paramutation” (“para” means “alongside”) to describe non-permanent variation and non-permanent changes, respectively, that occur in the genomes of various organisms.9 Examples of epigenetic inheritance range from petal number in plants to coat color in mice. In the mouse example, in certain instances the coat color of young mice is affected by the diet of the mother. The coat color is also able to pass to the grand-mice, but the effect wears off over the generations if the diet is changed. The removal of the environmental trigger allows the methylation patterns to change back to the original state.10

As epigenetics is becoming established as a force to be reckoned with, it is simultaneously challenging strict, orthodox Darwinism. Why? Because non-permanent changes act like a moving target for natural selection. By definition, the ability of natural selection to influence the distribution of genes in the next generation depends on phenotype (the physical characteristics of an organism). If the environment can affect phenotype in an inheritable manner, individuals will be falsely targeted. Then, when the epigenetic modification is reset, natural selection is sent back to square one and the ‘less-fit’ individuals were selected against in vain. Darwinian evolution cannot handle this. For example, say that the diet of mice generates epigenetic changes in the offspring that result in a coat color that gives good camouflage. Mice without that coat color could be at a disadvantage. However, there is no difference in the underlying genes that determine coat color. The mice have different colors because of something they ate. Some individuals would be eliminated that had no genetic disability and, when the effect wears off, all that selection would have been wasted.

In fact, considering that epigenetic changes seem to be designed to have a significant effect, they seem to be more powerful than most mutations, which generally have little to no effect on the individual.11 ‘Natural selection’ needs to be able to ‘see’ mutations, but most mutations are so weak as to be invisible. Epigenetics adds a lot of noise, making it even harder for natural selection to operate. How can evolution proceed if natural selection cannot remove mutations? That is one puzzle that epigenetics creates.

Just like Darwin’s claims that a mixture of forces (natural selection, sexual selection, and Lamarckism) causes evolution to proceed, the modern evolutionist believes in a mixture of forces (natural selection, neutral drift,12 and, now, epigenetic modification). Thus, we have come full circle and evolutionary theory is as unresolved as ever. Epigenetics does not explain all variation, of course, but the epigenetic landscape is just starting to open up before us. The potential for reversible environmental influences on genetic expression is huge. Darwin has been partly vindicated for his rejection of strict ‘Darwinian’ evolution, but will the rest of his idea hold up? Will natural selection be able to see the genetic forest through the epigenetic trees?