Topological and Quantitative Relationships in Evolving Genomes

Abstract

The network properties of the circuitry (topology) of gene interactions are to be distinguished from the quantitative aspects of these interactions. The difference between homologous and analogous morphological structures is best founded on this distinction. It is proposed that gene interaction topologies change very slowly during evolution, while quantitative aspects of gene action and interaction change faster. “Islands” of extremely old sectors of gene interaction topology may have been preserved. Cases of apparent convergent evolution of morphological features thus may contain a component of parallel evolution, which renders the independent appearance of such similar structures more plausible. Two types of gene interaction topology, a trans-spatiotemporal interaction topology and a stage-and-tissue-specific interaction topology need to be distinguished. Pathways for changes in gene interaction topology include gene duplication, heterochrony (the switch of the activity of a gene from one developmental time to another) and hetero histosis (the switch of the activity of a gene from one tissue to another). It is surmised that even higher organisms are built on the basis of a surprisingly small number of genes controlling polypeptides with significantly different functions, though in the same context the importance of the presence of a multiplicity of only slightly diverged gene duplicates is recognized. Even quantitative aspects of gene action and interaction are often conserved over rather long evolutionary periods. When quantitative relationships between gene activities change during evolution, they mostly are expected not to affect the network properties of the gene interaction system.