An Introduction to the Invertebrates, Second Edition - tiera.ru

2 THE PROCESS OF EVOLUTION: NATURAL SELECTIONcompetition for priority is an encouraging example of decencytranscending competition.Darwin’s argument was as follows:1. Living things tend to multiply. There are more offspring thanparents and, if unchecked, their numbers would increase ingeometrical ratio.2. The progeny cannot all survive, because resources (food, space,etc.) are insufficient.Therefore there will be competition for survival, a ‘struggle forexistence’ between individuals of the same species.3. Living things vary; the progeny are not all identical and somewill be better equipped for survival than others.Therefore ‘favourable variations would tend to be preserved, andunfavourable ones to be destroyed. The result of this would be theformation of new species’ (The Autobiography of Charles Darwin, ed.Norah Barlow, Collins 1958, p. 120).To describe this process of natural selection Herbert Spencer usedthe phrase ‘survival of the fittest’. The phrase needs to be qualifiedif misunderstanding is to be avoided: firstly, it is not mere survivalbut differential reproduction that is required and, secondly, ‘fittest’does not refer to general health and strength but to some preciseadvantage in particular circumstances in a particular environment.Adaptation consists in the perpetuation of such an advantage downthe generations.Here at once was Darwin’s greatest difficulty. For natural selectionto work, advantageous changes had to be inherited. In Darwin’s timeheredity was assumed to involve the blending of the features of thetwo parents, and Darwin was much worried by the criticism (from anengineer, Fleeming Jenkin) that any system of blending inheritancewould remove the advantage in a few generations. The solution wasat hand, but never known to Darwin. Gregor Mendel had alreadyshown that heredity was particulate, but his work was not publiciseduntil 1900.1.2 What was Mendel’s theory of heredity?Mendel’s ‘atomic theory’ of heredity was based on his experiments oncrossbreeding garden peas. He deduced that hereditary factors areconstant units, handed down unchanged from parent to offspring,and that these units occur as ‘allelomorphic pairs’, the two membersof each pair representing two contrasting characters. At sexualreproduction when gametes (spermatozoa and ova) are formed, onlyone factor of each pair can enter a single gamete. When gametes fuseto form a ‘zygote’ the factors, one from each parent, are combined.One factor in a pair may be ‘dominant’ over the other, which is called