Evolutionary Change in Genomic Regulatory Organization: Speculations on the Origins of Novel Biological Structure

Abstract

In considering the mechanisms which could have led to the amazing variety of biological structures in the animal world, it has seemed reasonable to focus on the evolution of genomic regulatory pathways ((50), and Britten, this volume). Biological structure is the immediate result of developmental processes in which sets of diverse structural genes are expressed as ontogenically functional units. Such units must include genes that control the cell divisions in given tissue anlage or cell lineages, the nature of the products formed in the cells, their interactions with adjacent cells, and so forth. Relatives of most known structural genes seem to occur in a very wide phylogenetic range of creatures, while the specific patterns of development in which they are utilized are characteristic of each taxonomic group of organisms. If we understood the genomic organization underlying these specific ontogenic regulatory patterns, we might be in an excellent position to construct a useful theory of evolutionary invention at the DNA level. Unfortunately, such understanding still lies beyond current knowledge, though perhaps not very far beyond. In this essay I have chosen somewhat arbitrarily to consider two forms of hypothetical genomic regulatory organization. For neither of these is there yet any very strongly convincing evidence, particularly in regard to ontogenic regulatory coordination. Nonetheless, it is interesting to examine the implications for both kinds of proposed regulatory organization of various kinds of genomic alteration known to occur in evolution at a relatively high rate. This heuristic exercise shows that it is not difficult to imagine evolutionary mechanisms that could have had large scale functional effects on ontogenic patterns, and thus have led to the appearance of novel biological structures.

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