Comparative studies of DNA sequences provide opportunities for testing the neutral and the selection theories of molecular evolution. In particular, the separate estimation of the numbers of synonymous and nonsynonymous substitutions is a powerful tool for detecting selection of the latter. The difference in the patterns of these two types of substitutions of mammalian genes turned out to be in accord with the slightly deleterious or nearly neutral mutation theory for nonsynonymous changes. Interaction systems at the amino acid level were (...) suggested to be responsible for such nearly neutral, or very weak, selection. Synonymous substitutions are not strictly neutral, but because of their minute effect, random drift predominates such that the rate of substitution is only slightly less than the completely neutral prediction. It was concluded that the strictly neutral theory has not held up as well as the nearly neutral theory, yet remains invaluable as a null hypothesis for detecting selection. On the other hand, the main difference between the nearly neutral and the traditional selection theories is that the former predicts rapid evolution in small populations, whereas the latter predicts rapid evolution in large populations. (shrink)

Recent analysis of the contribution of replication slippage to genome evolution shows that it has played a significant role in all species from eubacteria to humans. The overall level of repetition in genomes is related to genome size and to the degree of repetition that can be measured within individual ribosomai RNA genes, suggesting that the entire genome accepts simple sequences in a concerted manner when its size increases. Although coding sequences accept simple sequences much less readily than non‐coding sequences, (...) they accept some repeats, particularly (CAG)n, preferentially. This may have consequences for the evolution of the genes involved in trinucleotide expansion diseases and the transcriptional networks of which they may form a part. (shrink)