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Resonance

April 2004
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Reflections

In this issue, honouring the great Indian palaeobotanist Birbal Sahni, we have chosen to print an abridged and slightly edited version of the Presidential Address delivered by him at the Annual Meeting of the National Academy of Sciences, India, on 5th March 1938, an address reproduced in full in the journal Current Science, Vol 61, 1991, pp.594-600. It is an interesting address to read with the benefit of a further 65 years of evolutionary research, for the topic Birbal Sahni chose to discuss then is still a live issue in evolutionary biology today. Basically, the issue is that of the repeated pattern of the sudden (in geological terms) appearance of new species in the fossil record, after a long period of stasis, without evidence of any intermediate forms that would be expected under the notion of evolution as a gradualistic process of transformation of one species into another. This issue came into prominence once again in 1972 when Niles Eldredge and Stephen Jay Gould put forward the theory of punctuated equilibrium to explain this pattern in the fossil record (see Resonance 2002 Vol 7, No.11, pp.2-5).

It is worthwhile, when reading this piece, to remind oneself of the year – 1938. The Neo-Darwinian synthesis was far from complete, population genetics theory was yet to be appreciated by the majority of evolutionists, and the clear distinction made today between mechanisms and rates of micro-evolution (adaptive evolutionary change within populations of a species) and macro-evolution (origin of new species, genera etc.) was far fuzzier. These were also the days when Darwinism was equated with gradualism, the notion that new species arose only by the slow, incremental transformation of pre-existing species. In fact, gradualism is not central to a Darwinian view of adaptive evolution as occurring largely through natural selection, and is no longer considered an important part of Darwinism today. Even in Darwin’s time, his friend T H Huxley had tried to convince him to drop his emphasis on gradualism, calling it a ‘mill-stone’.

What I found most interesting about this address by Birbal Sahni is the intimate look it gives us at the workings of a first-rate scientific mind. Faced with the empirical evidence of sudden appearance of new flora in fossil strata, Sahni discusses with clarity, and rejects, the possible explanation that the data could be explained as mere imperfections in the fossil record. He then goes on to critically discuss various speculative hypotheses that had been proposed to explain the observed patterns in the fossil strata. Finally, he goes beyond his own field of specialization to the relatively new field of experimental cytogenetics, and marshals evidence supporting his hypothesis that adverse climatic conditions may, in fact, lead to major changes in chromosome number, thus facilitating the rapid evolution of new species. This is a great example of a scientific mind at work: examining data critically, proposing hypotheses, and marshalling evidence from disparate fields of research in order to put the hypotheses to empirical test. The notion of environmental change somehow facilitating rapid evolution also resonates today in light of recent findings on the unmasking of genetic variation in stressful conditions, and the conversion of non-additive to additive genetic variance during population bottlenecks or environmental change. Stasis and rapid evolutionary change, as well as evolvability, and its evolution under different environmental scenarios, are today topics receiving much attention from evolutionary geneticists, and Sahni’s early speculations about the role of cytogenetic changes in periods of rapid evolutionary change are a sobering reminder that we often keep re-discovering, under ever newer names, the same basic ideas and insights that have periodically occurred to our many predecessors in the quest for understanding that is science.

Amitabh Joshi

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Revolutions in the Plant World

Birbal Sahni

Presidential address delivered at the annual meeting of the
National Academy of Sciences, 5 March 1938.

As the subject of my address today I have chosen a small theme embraced by that ocean of ideas that we call evolution. The particular aspect that I propose to deal with is revolutions in the plant world.

As in the history of nations, so also in that of plants and animals, we find that after a period of gradual change, which we generally call evolution, there comes inevitably a revolution – a period of rapid transition, when things begin to move faster and on a different plane. These revolutions in the organic world are the landmarks of geological history. Each of them marks a large-scale extinction of plant and animal life as well as a more or less sudden appearance of forms of life previously unknown. So striking is this fact of the sudden appearance of new species, genera and families that it is in sharp conflict with the Darwinian doctrine of natural selection as the only or even the chief explanation for the origin of new forms of life. Evolution in the sense of a gradual, orderly process of change is an undisputed fact. But evolution in this gradual sense is not the whole of organic evolution as revealed by the geological record. Periodic revolutions are an integral and essential part of evolution, and it may well be that they form the more important part, so far as the creation of new forms is concerned. At all events the orthodox idea of natural selection through the gradual accumulation of continuous variations utterly fails to explain some of the glaring facts of palaeontology.

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