Epigenetics and Lamarckian Evolution
Has Transgenerational Epigenetics Vindicated Jean-Bapstsiste Lamarck?
When people hear the term “evolution” all they think about is Charles Darwin, the famed English naturalist who, in 1859, gave us the most notable mechanism of the evolutionary process. However, fifty years prior to Darwin’s publication of On The Origin of Species, Jean-Baptiste Lamarck had formulated his own theory on the mechanism of evolution. Lamarck’s theory has been overshadowed by Darwin’s, and a majority of biologists — until recently — have ridiculed it.
Lamarck proposed the inheritance of acquired characteristics. This being the most prominent postulation of his theory, it argues that traits acquired by organisms in their lifetime can be passed on to the next generation. This mechanism is illustrated in his hypothetical story of how giraffes got their long necks.
Anyone with some basic understanding of Mendelian genetics would easily see something wrong with this mechanism of inheritance. We have long understood that the units of heredity — the genes — are what give rise to traits of an organism. Viewed under genetics, the only way the giraffes’ offspring could possibly inherit the long necks is if, somehow, the stretching of the parent giraffes’ necks led to changes in their DNA sequence. Thus, the dominance of Darwinian evolution in tandem with Mendelian genetics in twentieth century biological thought effectively put Lamarckism to rest.
The emergence of epigenetics, however, is showing us that there is more to heredity than the simplistic ‘genes equals to phenotype’ model. Evidence for this has been accumulated in studying identical twins — they result from the same fertilized egg that, shortly after, splits into two. In essence identical twins share the same genetic information and are expected to be phenotypically the same. It would therefore be quite strange if they were to be born with different traits. Such discordance has been documented in the X-linked genetic disorder, Duchenne muscular dystrophy, where one of the twins was born with this severe muscle-weakening condition while the other was very healthy. Other studies have also pointed out discordance of orofacial clefts in identical twins.
Extensively, Prader-Willi syndrome and Angelman syndrome are two vastly different syndromes with vastly different symptoms, but both are a result of a deletion from the same chromosomal region of chromosome 15. The only difference is that the former manifests when offspring inherit this chromosomal deletion from the father while the latter manifests when offspring inherit the same deletion from the mother. Clearly there is a lot more happening at the molecular level than previously understood.
In its simplest form, epigenetics means on top of the gene. It is now widely understood that chemical substances can tag along a DNA molecule. This tagging along does not change the underlying DNA sequence, but rather alters the activity and/or expression of the particular gene. One of the most common epigenetic mechanisms is DNA methylation, a process where methyl groups are added to the DNA molecule. In her book, The Epigenetics Revolution, professor Nessa Carey has extensively shown the role of DNA methylation in gene expression.
The raging debate among biologists on the revival of Lamarckism has been based on whether these acquired changes in gene expression can be passed on to subsequent generations — this is known as trangenerational epigenetics. One author remarked that nothing in biology is more provocative than suggesting that Jean-Baptiste Lamarck may have been right.
The refutation of transgenerational epigenetic inheritance has been based on the understanding that epigenetic marks accumulated from the previous generation are erased off the genome just after fertilization, as well as during development of primordial germ cells.
Extensive experimental studies, however, have been dedicated to showing the retention of some epigenetic marks. Professor Trygve O. Tollefsbol rightly points out that if epigenetic marks were completely erased with each new generation, as was originally thought by some, Lamarckian epigenetic inheritance would not be feasible. Professor Fatimah Jackson further goes on to say that environmentally induced epigenetic changes are known to persist, in various mammalian models, at least 3 to 4 generations beyond their initial onset.
The most cited example of transgenerational inheritance is the Dutch famine that occurred during the winter of 1944–45. There exists insurmountable evidence that the nutrition of pregnant women actively influences the development of the fetus. It has been documented that the effects of the Dutch famine — not discussed here — extended to the grandchildren of the women who experienced the famine. Professor Jackson, who has also highlighted on comparative studies from The Gambia, argues that data gathered from the Dutch famine show that mothers who were subjected to famine had children with epigenetic changes in genes involved in the pathogenesis of obesity and diabetes.
This is not to say the responsibility solely lies on the mother. Överkalix studies have shown some sex-specific effects. In these studies, the paternal grandfathers’ diet is linked to the cardiovascular mortality risk rate of their grandsons. Perhaps the saying ‘we are what we eat’ should consider that our parents’ diets may also have contributed to what we are.
The effect of vinclozolin on mice up to the fourth generation is yet another example of transgenerational inheritance. When vinclozolin — a fungicide — gets into mice it prevents testosterone from transmitting its usual signals to the cell, blocking normal effects of the hormone. Exposure of pregnant rats to this chemical substance during the development of the embryo leads to male offspring being born with testicular defects that are evidenced to persist across generations. These are but some of the many examples that are given to validate transgenerational epigenetics.
The novelty of epigenetics means that there is a lot of aspects that are not yet well understood. There is still a lot more evidence being collected. Regardless, the take away is that there is more to heredity than was earlier understood. As it turns out, the environmental conditions in which our parents and/or grandparents survived in might have had profound effects on the phenotypes that we presently exhibit. Likewise, whatever we do now may potentially have an effect on subsequent generations.
While the debate on the validity of Lamarckism still rages, there is now good evidence that some genes — imprinted genes — do escape epigenetic reprogramming and thus maintain epigenetic states established in the gametes of the parents.
Perhaps Lamarck had the right idea in mind but merely lacked the correct explanation. All in all, professor Carey puts it best when she says it would be a mistake to look back on Lamarck and only mock.
