If we wanted to summarise a century and a half of evolutionary theory in one paragraph we might say:
Random variation in genes creates phenotypic variation in individuals. Some individuals will survive better than others in a particular environment, and these individuals are likely to have more offspring. These offspring may inherit the same advantageous genetic variation as their parent, so they too will have increased breeding success. Eventually, over a huge number of generations, separate species will evolve.
The raw material for random variation is mutation of the DNA sequence of the individual; his or her genome. Mutation rates are generally very low, and so it takes a long time for advantageous mutations to develop and to spread through a population. This is especially the case if each mutation only gives an individual a slight advantage over its competitors in a particular environment.
This is where the Lamarckian model of acquired characteristics really falls over, relative to Darwinian models. An acquired change in phenotype would somehow have to ‘feed-back’ onto the DNA script and change it really dramatically, so that the acquired characteristic could be transmitted in the space of just one generation, from parent to child. But there’s very little evidence that this happens, except occasionally in response to chemicals or irradiation which damage DNA (mutagens), causing a change in the actual base-pair sequence. Even these mutagens only affect the genome at a relatively small percentage of base-pairs and in a random pattern, so these still can’t drive inheritance of acquired characteristics in any meaningful way.
The overwhelming body of data argues against Lamarckian inheritance, so there’s very little reason for individual scientists to work on this experimentally. This isn’t surprising. After all, if you are a scientist interested in the Solar System, you could choose to investigate the hypothesis that at least some parts of the Moon are made of cheese. But to do so would mean that you wilfully ignored the large body of evidence already present against this – hardly a rational approach.
There’s also possibly a cultural reason that scientists have shied away from experimental investigations of the inheritance of acquired characteristics. One of the most notorious cases of scientific fraud is that of Paul Kammerer, who worked in Austria in the first half of the 20th century. He claimed that he had demonstrated the inheritance of acquired characteristics in a species called the midwife toad.
Kammerer reported that when he changed the conditions in which the toads bred, they developed ‘useful’ adaptations. These adaptations were structures on their forelimbs called nuptial pads, which were black in colour. Unfortunately, very few of the specimens were retained or stored well, and when a rival scientist examined a specimen he found that India ink had been injected into the pad. Kammerer denied all knowledge of the contamination and killed himself shortly afterwards. This scandal tainted an already controversial field[39].
One of the statements in our potted history of evolutionary theory was the following, ‘An acquired change in phenotype would somehow have to ‘feed-back’ onto the DNA script and change it really dramatically so that the acquired characteristic could be transmitted in the space of just one generation, from parent to child.’
It’s certainly hard to imagine how an environmental influence on the cells of an individual could act at a specific gene to change the base-pair sequence. But it’s all too obvious that epigenetic modifications – be these DNA methylation or alterations to the histone proteins – do indeed occur at specific genes in response to the environmental influences on a cell. The response to hormonal signalling that was mentioned in an earlier chapter was an example of this. Typically, a hormone like oestrogen will bind to a receptor on a cell from, for example, the breast. The oestrogen and the receptor stay together and move into the nucleus of the cell. They bind to specific motifs in DNA – A, C, G and T bases in a particular sequence – which are found at the promoters of certain genes. This helps to switch on the genes. When it binds to these motifs, the oestrogen receptor also attracts various epigenetic enzymes. These alter the histone modifications, removing marks that repress gene expression and putting on marks that tend to switch genes on. In this way, the environment, acting via hormones, can change the epigenetic pattern at specific genes.
These epigenetic modifications don’t change the sequence of a gene, but they do alter how the gene is expressed. This is, after all, the whole basis of developmental programming for later disease. We know that epigenetic modifications can be transmitted from a parent cell to a daughter cell, as this is why there are no teeth in your eyeballs. If a similar mechanism transmitted an environmentally-induced epigenetic modification from an individual to their offspring, we would have a mechanism for a sort of Lamarckian inheritance. An epigenetic (as opposed to genetic) change would be passed down from parent to child.
It’s all very well to think about how this could happen, but really we need to know if acquired characteristics can actually be inherited in this way. Not how does it happen, but the more basic question of does it happen? Remarkably, there appear to be some specific situations where this is indeed taking place. This doesn’t mean that Darwinian/Mendelian models are wrong, it just means that, as always, the world of biology is more complicated than we imagined.
The scientific literature on this contains some confusing terminology. Some early papers refer to epigenetic transmission of an acquired trait but don’t seem to have any evidence of DNA methylation changes, or histone alterations. This isn’t sloppiness on the part of the authors. It’s because of the different ways in which the word epigenetics has been used. In the early papers the phrase ‘epigenetic transmission’ refers to inheritance that cannot be explained by genetics. In these cases, the word epigenetic is being used to describe the phenomenon, not the molecular mechanism. To try to keep everything a little clearer, we’ll use the phrase ‘transgenerational inheritance’ to describe the phenomenon of transmission of an acquired characteristic and only use ‘epigenetics’ to describe molecular events.
Some of the strongest evidence for transgenerational inheritance in humans comes from the survivors of the Dutch Hunger Winter. Because the Netherlands has such excellent medical infrastructure, and high standards of patient data collection and retention, it has been possible for epidemiologists to follow the survivors of the period of famine for many years. Significantly, they were able to monitor not just the people who had been alive in the Dutch Hunger Winter, but also their children and their grandchildren.
This monitoring identified an extraordinary effect. As we have already seen, when pregnant women suffered malnutrition during the first three months of the pregnancy, their babies were born with normal weight, but in adulthood were at higher risk of obesity and other disorders. Bizarrely, when women from this set of babies became mothers themselves, their first born child tended to be heavier than in control groups[40][41]. This is shown in Figure 6.1, where the relative sizes of the babies have been exaggerated for clarity, and where we’ve given the women arbitrary Dutch names.
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If you want to know more, try Arthur Koestler’s highly readable though exceptionally partisan book,