Repetitive and non-repetitive DNA sequences and a speculation on the origins of evolutionary novelty.

Recent experimental information on DNA sequence repetition is reviewed, and the significance of both repetitive and non-repetitive sequence considered. Included are a summary of data on the distribu tion of genome sizes in animals, new experiments on interspecific DNA homology, the distribution of sequence frequencies, and the interspersion of repetitive sequences within the genome. Aspects of the process of evolution are considered at the level of change in the DNA. The process by which novel structure and function could have arisen during evolution are considered speculatively in terms of the authors' gene regulation theory (Britten and Davidson, 1969). INTRODUCTION I N RECENT years experiments have · shown that the genome of higher organisms contains a large amount of order which is manifested as repeated nucleotide sequences in the DNA (Britten and Kohne, 1968 a, b). Recently we have considered the possible implications of this order for the mechanism of control of genetic activity and have constructed a model for gene regulation (Britten and Davidson, 1969; also see below). H ere we consider some of the implications for the processes of evolution and examine in more detail some of the experimental evidence which has led to the development of the regulation model. In the first four sections we take up nonrepetitive DNA and the term "complexity"; the growth of the genome during evolution; the origin and divergence of repeated sequences; and their arrangement in the genome. In the final sections we shall make a proposal for the origin of evolutionary novelty and discuss the evolution of regulatory processes. The purpose, as in our previous papers (Britten and Davidson, 1969; Davidson and Britten, 1971) is to construct a conceptual scheme which can be tested. vVe hope, naturally, that an