Why organic development is not all in the genes

 

I RECENTLY spoke in a debate on genetic engineering at TCD. In preparing my presentation I was led to ponder again on the overall significance of genetics. Genetic manipulation is a very topical area and people have high expectations that great benefits are just around the corner for example in the genetic therapy of disease.

Certainly nobody questions the fundamental significance of the genes. And rightly so. But there is a danger that this constant focus on the power of genetics will foster the distorted view that we are totally determined by our genes. Our genetic blueprints are very determining, but they are not the whole story. The environment is also determining. This is clearly argued in a beautifully written essay by R.C. Lewontin in a recently published book, Hidden His to flees of Science (Ed. R.B. Silver - Grants Publications, 1997).

R.C. Lewontin is a geneticist and Professor of Biology at Harvard University. He has written extensively on population genetics and human diversity and he is a trenchant opponent of the proposition that heredity explains measured differences in IQ between races. Such differences are attributed to environmental and other factors. Ideologically Lewontin is left wing, and left wing philosophy greatly favours the idea of human malleability under environmental influences. He may unconsciously tend fob overemphasise the environment and under emphasise the gene. However, I find the arguments that he presents in the essay reasonable and impressive.

In the world of classical physics, it was envisaged that everything could be precisely determined given a knowledge of sufficient data on the then known bodies and particles. The French mathematician Pierre Simon Laplace (1749-1827) declared that if he knew the precise momentum and position of every particle in the universe he could accurately predict the entire future of the universe. We know from 20th century developments in quantum physics and chaos theory that Laplace's forecast is wrong. In a biological re echoing of Laplace, James Watson, the co discoverer of the structure of DNA, recently stated that if he had the complete DNA sequence of an organism and a large enough computer, he could compute the organism. James Watson's prediction is just as over optimistic as that of Laplace.

At they beginning of the 19th century there was a debate between preformationism and epigenesis, two competing ideas about what determines the development of an embryo. The preformationists believed that the adult was already present in minuscule (the homunculus) within the fertilised egg. Indeed many believed the homunculus was present in the sperm and entered the egg on fertilisation. Investigators examined sperm under the microscope and "saw" the homunculus crouching inside the sperm. They published detailed drawings. (My students enjoy this story, but I don't think they really believe me.) The epigeneticists claimed only that an ideal plan of the adult was present in the egg, and that this blueprint dictated the formation of the adult as development progressed.

The views of the epigeneticists prevailed. We now identify the blueprint of the epigeneticists with the genes made of DNA. However, as Lewontin points out, this is a simplistic picture. Development is not simply an unfolding of an internal plan, the environment outside also plays an important part.

The development of most organisms is a consequence of the interaction between the inside of the organism and the outside environment. What eventually emerges as a mature organism is the result of the unique interaction between the organism's genes and the temporal sequence of environmental signals through which it has passed. Except in general terms, one cannot predict in advance from the DNA sequence alone what the organism will look like.

This is illustrated by a classical experiment in plant genetics. Seven individuals of the plant achillea were each cut into three pieces. One piece from each plant was planted at low elevation, one at intermediate elevation, and one at high elevation, and the pieces regrew into new plants. The result is shown in the illustration. In each vertical column the plants grew from three pieces of the same original plant and therefore they are each genetically identical.

It is obvious that the relative growth of the different plants cannot be predicted when the environment is changed. Overall there is no predictability from one environment to the next. There is no "largest" genetic type. The results described for achillea have been duplicated with every organism where it is possible to duplicate the genetic constitution and to test the identical individuals in different environments.

Another source of variation in development is "developmental noise". Most biological organisms, including ourselves, show bilateral symmetry of form. The symmetry is never perfect. You can easily check this out by standing naked in front of a mirror and making careful comparisons - if your spouse finds you doing this, please feel free to blame me. Even a tiny fly will have a different number of sensory bristles on its left and right sides. Both sides have the same genes and the difference between sides is a result of random events (noise) in the timing of division and movement of individual cells.

This noise is inevitably present during the development of the brain and the associated cognitive functions. According to Lewontin it is not possible to say how much of the difference in cognitive function between individuals is caused by genetic differences, how much by different life experiences, and how much is attributable to random developmental noise. In other words, although I have absolutely no talent as a sculptor or painter, that would not necessarily preclude me from having an identical genetic makeup to Michelangelo - I exaggerate to make the point.

Lewontin criticises the conceptual framework of Darwinian evolution as being simplistic in an important aspect, i.e. the absolute segregation of the internal environment of an organism from thee external environment of the world.

In the Darwinian scheme the eternal world sets problems for organisms which must solve them relying on random internal mutations. The outside and inside spheres are not seen as interpenetrating each other. But Lewontin points out that the environment as a concept is meaningful only in the context of life species and that the environment for one species, e.g. an earthworm, is a completely different thing to the environment for another species, e.g. a bird.

In other words, environment consequences for an organism are a consequence of its genes. This complements the already described situation where the information needed to specify an organism is contained not only in its genes, but in the environment.

Organisms are also constantly changing their environment. This has the most radical effect over time. Billions of years ago, our atmosphere contained no oxygen. It now contains 18 per cent oxygen. This transformation was effected by biological life on earth. Mankind is not in any way unique in changing the environment.

The inside of biological organisms and the outside environment are not two sealed compartments that bounce off each other. They each exert more interpenetrating effects on each other. Lewontin would like this to be taken into account in formulating a more complete account of biological evolution.