MOLECULAR EVOLUTION AND THE PHENOTYPE

Abstract

In terms of the acquisition of important and relevant data, molecular evolution is proceeding more rapidly than any other area of evolutionary biology. The reason is simple. Most of the data is produced by workers whose interest is in isolating and sequencing genes from their favorite organisms, without any particular interest in their evolution. These data are being greatly augmented by those from genomic sequencing projects. Stored in sequence databases, the resource then awaits the attention of molecular evolutionists. From comparisons between species of known phylogeny, the rates and processes of molecular change can be established, and, through an understanding of these processes, the use of molecular characters as a guide to phylogeny can be optimized. Indeed, just as molecular data are a tool in systematics, so evolutionary comparisons are demonstrating their worth in investigating the functions of DNAs and amino acid sequences. In general, DNA sequences that are of functional importance evolve slowly relative to non-functional sequences. This result, arising from the lack of selective constraint on the latter, allows the conservation of sequence to be a guide to functionality. Thus, for example, at the level of DNA, sequences important in the control of gene expression can potentially be discerned from other intergenic sequences by virtue of their low evolutionary rates. More distant phylogenetic comparisons allow the identification, among amino acid sequences, of those residues involved in functions shared between the distantly related groups of organisms being compared. Molecular biology, like all of genetics, has learned the advantages of model systems, experimentally tractable organisms that are studied, and from which inferences are drawn of high generality. The basis of such inference is homology, the sharing of characteristics over what are sometimes wide phylogenetic distances as a result of the preservation of features from a common ancestor. Thus, when genes are now isolated and sequenced, the discovery of homologous genes in other groups strengthens the justification of the model organism approach, and also the choice of the particular species being investigated (and thereby may greatly assist in helping the investigators find funding). Indeed, it seems likely, for example, that a large proportion of genes in humans will turn out to have identifiable homologues in insects. The sequence conservation which keeps homologues recognizable over such evolutionary times implies that, among animals, a large proportion of the network of genetic interactions, which ultimately results in the phenotype, has been conserved. One important question thereby raised is the extent