Thursday, April 19, 2018

The end of the selfish gene

The German biologist August Weismann (1834-1914) was one of the most influential biologists of the late nineteenth century. His most important contribution was the theory of germinal plasma, also called in his honor Weismannism. According to this theory, there are two classes of cells in all multi-cellular living organisms (see Figure 1):
Figure 1
  • Somatic cells, represented in the figure by an S, that make up most of the body and do not play any role in inheritance.
  • Germ cells, represented in the figure by a G: the gametes, ovules and sperm, which pass the genetic information to the next generation.

In the figure, the horizontal arrows represent the transmission of the germ between different generations, while the leaning arrows represent the building of the soma (the body of the living beings) by the germ. It was assumed that genetic information cannot pass from the soma to the germ, which eliminated the possibility of the inheritance of acquired characters: the inheritance in the style of Lamarck, who proposed a theory of biological evolution before Darwin. Biologists called this impossibility Weismann's barrier.
In 1958 Francis Crick, one of the discoverers of the double helix structure of DNA, published a famous article (On Protein Synthesis) where he enunciated what has come to be called the Central Dogma of Molecular Biology, which can be summarized thus:
All the information is in the genes. This information can be transferred from nucleic acid to nucleic acid, or from nucleic acid to protein, but never from protein to protein, or from protein to nucleic acid.
Figure 2 represents the Central Dogma in a schematic way. In this case, G represents the genes, which are transmitted between different generations of living beings (represented by the horizontal arrows). P, on the other hand, represents proteins. It will be noted that both figures are identical. In reality, Crick’s Central Dogma is nothing but Weismannism adapted to modern terminology.
Figure 2
The theory of the Selfish Gene, proposed by Richard Dawkins in 1976, is just another way of expressing Weismannism. Indeed, if the genes (or the germ, as Weismann called it) is the only thing that is transmitted from generation to generation, while natural selection acts on the soma (for it is individuals who are subject to the dangers of the environment), it seems logical to conclude that the subjects of evolution are genes, while somas (adult individuals) would only be the means by which genes get to reproduce. Their role would be, therefore, secondary.
Stephen Jay Gould
Another important biologist took issue against the theory of the Selfish Gene: Stephen Jay Gould, who in 1972 proposed the theory of punctuated equilibrium in evolution, which holds that long periods of evolutionary stagnation are followed by short periods of rapid changes. After almost two decades of debate between Gould and Dawkins, in the early 1990s Dawkins seemed to have won the game.
As we saw in the previous post, the situation took a complete turn in the following years. Epigenetics, plus the advances that have taken place in the study of embryonic development, have led to the conclusion that the Central Dogma of Molecular Biology is false. There are many ways in which the extra-chromosomal environment can influence the development of living beings and the final phenotype of adults. Both possible cases can happen:
  • Two different genotypes can give rise to the same phenotype.
  • The same genotype can give rise to two different phenotypes. Two spectacular examples are the worm and the butterfly, or the different cell lines of the same adult individual (such as neurons and epithelial cells), each of which is capable of reproducing itself by mitosis and remaining identical to itself, although different strains share exactly the same genome.
On the other hand, proteins regulate gene expression, so there is feedback from proteins to nucleic acids. Consequently, the Weismann barrier does not exist and the relationship between germ and soma is much more complex than Weismann and Crick had thought, following Figure 3.
Figure 3
It is evident, by the simple observation of this figure, that the theory of the selfish gene can no longer be defended. Adult individuals do not play a secondary role in evolution, but a major role, as important (or more) as genes. In the end it will turn out that it was Gould, rather than Dawkins, who was right. Even Lamarck could emerge from the ostracism where Darwin and Weismann relegated him. But this is how science progresses: pointing out the errors of previous theories and fighting against vested interests, which are often very powerful.

The same post in Spanish
Manuel Alfonseca
Third post in a series of four

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