ABSTRACT
The classical genetic approach to the analysis of evolution was based on a mathematics that reduced the entire process to the study of changes in gene frequencies within populations. In effect, the developmental interactions responsible for producing evolutionary changes were ignored. In Neo-Darwinian theory, selection, coupled sometimes with neutral drift, was regarded as the only mechanism responsible for the spread and establishment of biological novelty. In treating the genome simply as a collection of individual genes, however, this theory fails to come to terms with the implications of the turnover of multigene families implicit in the concept of molecular drive. It is simply not adequate to graft such recent molecular findings onto conventional evolutionary theory. Thus two decades of analysing the polymorphisms that are evident in single genes has given no insight into the origin of morphological change. The signal failure of this approach has, in turn, led to recourse to ‘explanations’ in terms of ‘regulatory’ genes and their rôle in generating evolutionary change.
Molecular analyses of genome structure, by contrast, have highlighted the rapid flux that characterises the genomes of eukaryotes. In addition it has drawn attention to the fact that most of these genomes are choked with DNA sequences that represent nothing more than evolutionary debris. These sequences are the natural outcome of processes involving the replication, recombination, amplification, insertion, excision and conversion of the DNA molecules within a genome. Many of these sequences are mobile, and most of them make no significant contribution to developmental programmes and so do not impinge on phenotype.
Molecular analyses of the variation in multigene and repetitive DNA families, both within and between species, as well as the experimental addition of specific cloned DNA sequences to a genome, emphasise that many DNA sequences are naturally apostolic and missionary oriented since they continuously convert each other. These findings have provided the basis for the theory of molecular drive which has added a new dimension to the 264spread of variation through a population and has directed attention to the need to examine the developmental rôle of multigene families.
Molecular analyses are also now focusing attention on executive genes, as opposed to these which service and maintain universal metabolic back-up systems. One class of these executive genes, typified by the Bithorax complex of Drosophila, has been shown to have homologies in all major classes of vertebrates. The structure and mode of expression of such executive genes draws attention to the concept of developmental circuits and the ways in which alterations in circuitry lead to phenotypic changes of evolutionary significance. Many genes affecting early embryogenesis in Drosophila have also now been cloned, and the localisation of their transcripts and protein products has revealed unexpected distributions.
Finally, the genetic analyses of neural cell lineages in the roundworm Caenorhabditis elegans, the monoclonal antibody studies on neuronal pathfinding in the grasshopper, and the recombinant DNA analyses of RNA populations in the vertebrate brain, have revealed both unexpected simplicities and complexities of neuronal developmental lineages.
These developmental insights, stemming from reductionist molecular approaches to the genome, offer for the first time to provide an objective basis for defining and analysing the modes of morphological change that play a major rôle in evolution.
