Evolution.

Society for Experimental Biology

Date:: 1953

Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Evolution. Source: Wellcome Collection.

31/484 (page 7)

THE ORIGIN OF LIFE 7 (1) The appearance of droplets of water on charged ions in a Wilson cloud chamber. The event here may be a single radioactive disintegration, but because the increment of condensation on to the drop is (within limits) an increasing function of the size of the drop, the event becomes magnified many million times. (2) The formation of crystals of solid in a liquid, where the increment of crystallization is an increasing function of the size of the crystal nucleus. (3) The circulation of vapour in an unstable atmosphere, where the increment of volume to the ascending air mass is an increasing function of the velocity of ascent, which, in tum, depends on the volume of the rising air. (4) Truly autocatalytic chemical reactions. (5) The initial increase in quantity of active intermediate chemical com¬ pounds in a branching chain reaction (Semenov, 1935; Laidler, 1950). (6) Linked autocatalysis in living systems. (7) The spread of a favourable mutation through a population. (8) The development of ideas in the human brain (Langmuir, 1943 ; this analogy has been elaborated by Pringle, 1951). Example (7) serves to emphasize that the occurrence of divergent phenomena in evolution is not the whole story. The 'steady-state' equi¬ librium condition in which organisms and populations exist involves a balance between divergent (birth) and convergent (death) processes, which occur at equal rates when the state is truly steady; natural selection (differential death rates) is as important as the increase of a mutation as a factor in evolution (compare Herbert Spencer's 'simple', 'compound' evolution). But the existence of divergent processes nevertheless dis¬ tinguishes this type of steady-state equilibrium from that occurring in such systems as, for example, a liquid-containing tank with equal inflow and outflow, in which no divergent phenomenon is present and in which small- scale fluctuations produce no more effect on the macroscopic behaviour of the system than they do in a true static thermodynamic equilibrium. A balance between birth and death processes occurs in all the examples given above ; for instance, the growth of water droplets in a cloud chamber is ultimately balanced by evaporation, and the circulation in the lower atmosphere is limited by the existence of a stable layer higher up in which the ascending air columns disperse. In all these balanced systems con¬ taining a divergent element the prerequisites for an evolutionary process are present, though a progressive change may not occur unless there are alternative stable states in which the system can exist. An ' evolution ' does, however, take place in the atmosphere during the course of each day, leading to a definite sequence of structural changes in the pattern of