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.

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xiv FOREWORD islands have characteristic distributions. The authors ascribe these dif¬ ferences to selection rather than drift. The Irish male pattern dis¬ tribution differs from the English one. This is an extensive analysis of a problem complementary to the intensive analysis of a single Panaxia population. Heuts's paper on the blind white cave fish Caecobarbus eertsii of the Belgian Congo probably contained more surprises than any other read at the symposium. These fish have a very low basal metabolism, and an extremely low growth rate. The growth rates, both of the whole body and of different organs, differ widely between the populations of different caves. Heuts thinks that the slow growth is a consequence of food shortage, and that 'all the variations considered are due to variations in the body growth rate'; the eyes, being positively allometric organs, are highly inhibited, while tactile organs of an archaic type, which are negatively allometric, do not regress as in normal fish but persist into adult life. He also thinks that cave animals, because they live in a very constant external environment, have largely lost the capacity for regulating their internal environments. Some of Heuts's conclusions will not be accepted readily, but they may be a major contribution to evolution theory. Two papers dealt with tropical botany. I. Mantón and her colleagues have made chromosome counts on most of the fern species of Ceylon. Polyploidy and hybridization turn out to be commoner than in western Europe. This disproves the theory that polyploidy is an adaptation to cold. Ferns seem to be evolving more rapidly in the tropics than in temperate climates. Mantón thinks that this is a general rule for all living organisms. Holttum described the physiology of plants adapted to a very uniform climate. Some species grow and flower continuously. Others respond to stimuli of irregular occurrence, such as sudden storms on hot days. Some deciduous trees have periods of a year, some of more or less than a year. These facts suggest a programme of future research on Turessen's lines to determine how different ecotypes differ with regard to such responses. Thoday defined the fitness of a unit of evolution (which may be anything from a group of potentially hybridizing species to an apogamous individual) as the probability that it will leave descendants after a hundred million years. So we cannot be dogmatic about the fitness of any organism living since the lower Cretaceous ! And the advent of man has drastically altered the criteria of fitness. The components of fitness in this very broad sense include genetic stability, which means that highly adapted organisms will generally produce progeny like themselves, genetic flexibility, which means capacity to change the genetic constitution, and phenotypic flexibility, that