8 EVOLUTION OF LIFE 1.6- 



6. What is it that heredity transmit? 



What is passed on is a coded pattern or plan controlling the 

 organization of the life processes of the next generation. The plan 

 takes the physical form of a series of molecules of deoxyribose 

 nucleotides (DNA) in the chromosomes. These by the specific 

 arrangements of the four types of base that they contain somehow 

 organize the proper linear sequences of the twenty or so amino-acids 

 that make up the proteins of each species. By the emergence at the 

 proper time during development of the appropriate proteins, enzyme 

 systems are produced that ensure the development and functioning of 

 the embryo and later the adult. We cannot fully understand how all 

 these processes are regulated but we see in outline how it all follows 

 if the DNA molecules provide a code from which natural selection 

 has chosen in the past those items that are suitable to provide viable 

 organisms for a particular environment. 



The organization of life is very rarely identical in any two indi- 

 viduals; there is, therefore, a considerable range of potential patterns 

 resident in all those animals of a population that are capable of mating 

 together. The sum of those variants of the hereditary materials con- 

 stitutes the pattern or mould, as it were, of the life of the whole species. 

 Evolution consists in a change in this hereditary genotype, producing, 

 of course, a new set of adults. The genotype probably rarely stays for 

 long quite the same. Even in species that do not seem to be changing 

 rapidly there are continual adjustments, for example in the power to 

 produce antibodies or to manufacture enzymes. Evolution, proceeding 

 by mutation, recombination, and selection, is not some remote or 

 rare thing occurring only sporadically. It is a 'physiological' process as 

 much as is a change in respiration rate or in number of red cells, but 

 it has a longer time course than these. Evolution is the process by 

 which the whole population adjusts its control system to meet chang- 

 ing needs. Over long periods of years these adjustments produce the 

 new forms of life that appear as, say, the first fishes, or land animals 

 or mammals. Our aim is to try to discover the conditions under 

 which each new main group of vertebrates arose and so to understand 

 the processes that have been at work, modifying the basic organization. 



We must therefore direct our studies continually to populations, 

 rather than to single individuals, thinking of all the creatures of a 

 kind, spread out wherever a suitable habitat for them occurs. They 

 will not all be alike genetically, and the circumstances of the lives of 

 some members of the group may become sufficiently dissimilar to 



