Thursday, November 10, 2005

The road from Darwin's theory of natural selection to the Francis-Crick DNA structure was a long one indeed...one that took more than a 100 years to traverse. And although most scientists, including Darwin himself, had a hunch that there was a hereditary substance that passed on characters through generations, it was only in the 1950s that this was narrowed down to the deoxyribonucleic acid (DNA) of the chromosome. However, once the mystery of the DNA, its structure and its role in passing on information started to get unraveled, biological thought became extremely gene-centered. That adaptation could occur only through natural selection of chance genetic variations became the biologist's mantra. However recent advances in molecular biology are causing a gradual change in our concepts of heredity, genes and evolution.

For the last two decades Eva Jablonka and Marion Lamb have been trying to understand the manner in which biological systems operate and transmit information through generations. In Evolution in Four Dimensions: genetic, epigenetic, behavioral and symbolic (MIT press) they lay out some startling and conceptual theories that are changing the way in which we view heredity and evolution. What Jablonka and Lamb posit is that in addition to genetic systems in evolution (i.e. information passed through the DNA) other non-DNA inheritance modalities such as those based on epigenetic, behavioral and symbolic (language and culture based) forms also exist. Together these four provide all the variations within which natural selection acts and evolution proceeds. In other words these pathways intersect and form a tangled web through which the transmittal of biological information occurs.

Examples of the epigenetic systems are specialized cells, such as cells from the liver, heart, kidney etc. of an individual that despite possessing the same genetic information are completely different. This is possible because of the "information" introduced at a developmental phase (usually embryogenesis or the period of embroyonic development) that tells them which genes need to be turned off or on. Interestingly these cells can transmit this information of their specialization to their daughter cells.

A highly recommended read, this book is a must for lovers of genetics and evolutionary biology. Simple to read and well illustrated, the authors use examples from day to day life to prove their point. They are, however, on slippery ground with the last two hereditary systems, namely behavioral and symbolic, atleast at far as biologists are concerned. Jablonska and Lamb argue that if two sets of populations have different lifestyles then it is possible that over thousands of years they would evolve differently; that is to say that their behavioral patterns may be significantly diverse. Could a new invention and its social diseemination possess the potential to alter an organism's long term behavior en masse? Examples in the animal world are very few and far between. Again the role that symbolic inheritance, the fourth hereditary system, plays in evolution of a species is somewhat controversial. Symbolic implies the transmittal of information by acquiring and organizing knowledge and by thinking and communication through symbols such as language. The bank of acquired information ensures that every successive human generation does not have to reinvent the wheel. But the million dollar question to ask would be whether or not symbolic transmittal can add a developmental dimension to a species: a dimension that functions irrespective of the environment?

Diammetrically opposite to this cocktail of hereditary systems is the gene centric thinking. The other day there was a report on BBC on an article that appeared in New Scientist. In a study done in the UK, the appearance of 59 women was monitored daily over a period of 6 weeks and linked to their estrogen levels. Two composite pictures were created: one was an amalgamation of pictures of 10 women with the highest estrogen levels and the other was 10 pictures of women with the lowest levels. And lo and behold....higher estrogen translated into greater attractiveness. And we had a battery of experts linking all this to evolution and so on and so forth. As expected, there were queries at press conferences on whether or not estrogen injections would affect attractiveness. More importantly the study also showed that make up could indeed "make up" what you had lost in terms of the estrogen lag from your foremother.

Now these sort of sweeping statements linking one gene or protein product with a trait may sound very mind blowing but in reality these are nothing but broad statements painted with a broader brush. What geneticists actually study is the correlation between a gene and the possibility that it might affect a particular index related to a trait....whether it acts alone or in combination with other genes and products is unknown.

The human biological system is not a binary code operating on 2 genes. For a long time now we have known the relationship between DNA and genes and that between genes and proteins. We also know that the DNA or the linear sequence of units where each position in the sequence can only be occupied by any one of a set of four nucletides determines the protein that will be made. And that one wrong nucleotide can produce a defective or dysfunctional protein that may sometimes cause a disease or a set of diseases. However this does not mean that a single gene always codes for a product and changes in that gene and hence that product will cause changes in phenotype. And what it certainly does not mean is that a computer print out of one's genes (with a light brush stroke of the environment) would determine characters or physical traits in a person. These traits such as appearance etc. are the sum or combination of various genes producing one or more proteins under different conditions (environment), all of which might interact in various ways.

Coming back to the BBC report, let me also clarify (lest I sound overtly critical) that estrogen does play a very important role in terms of regulating the expression of a variety of genes involved in development, metabolism, and reproduction. In addition, estrogen-estrogen receptor complex affect (but do not exclusively control) cardiovascular protection, bone preservation, neuroprotection, and proliferation of various cell types. So clearly estrogen is important in certain indices that can be measured by some biophysical parameters but to link it to a concept such as attractiveness is a bit far fetched. For starters, attractiveness cannot be measured in the way a biophysical index can be....although it is still very difficult to link genes to these indices. If you dissect attractiveness...it is a function of a number of genes and their products which probably act through multiple interactions and which have certainly been switched on or off much before puberty....so there are eyes, cheekbones, hair, eyebrows, skin tone, muscle and many more. Cellular and developmental networks are too complicated to predict what effect high estrogen may have on all of these.

Perhaps the time has come for all of us to realize that the interpretation of biological information involving genes and their protein products is complex and depends upon countless factors. Add to that an environment that we do not even know how to define. It is only when we start to understand the networks and dynamics of how multiple genes, their products and the environment interact that we may be able to get a fuzzy glimpse of the whole puzzle.

5 Comments:

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