Observing the very similar chemical constituents of diverse life forms, biologists long ago realized that life (on Earth, at least) is a single subject. This edifying realization did not require knowledge of anything about genes and was basically understood even by Aristotle and others in ancient times. However, we now have genetic knowledge, and it confirms the unity of life in exquisite detail and adds whole new sets of phylogenetic relationships that go beyond and in some ways are independent of those of species.
Genes play a variety of roles. The differentiation of complex organisms into a variety of tissues depends on the fact that cells typically only express a fraction of their genes in any given context. This requires mechanisms to regulate context-specific gene expression, which as we have seen largely resides in the genome. Despite the diversity of genes and mechanisms, general statements can be made that apply broadly, from single-celled organisms to plants and animals. Chapter 4 described generalities concerning the ways DNA carries various kinds of "information," Chapter 5 described ways in which the modular nature of the genome is used to produce complex function, and Chapter 6 described many of the diverse characteristics of cells and their behavior. Table 7-1 provides a few reminders, summary pointers, and general principles. In this chapter, we will discuss some of the types of genes that have arisen in terms of mechanisms of gene action.
Our objective is to identify different kinds of gene action and to describe a few salient features that can help identify generalizations about phenogenetic relationships. In each case, there is a diversity of genes, always involving at least one gene family (usually several). It would not be possible (for us) to enumerate all of these, but there would also be no point to that. They share the general characteristics of gene families: divergent sequence, divergent but related function, and so forth. Online references are available and easily located by keyword searching; they include sequences, functional and evolutionary aspects, protein structure and
Genetics and the Logic of Evolution, by Kenneth M. Weiss and Anne V. Buchanan. ISBN 0-471-23805-8 Copyright © 2004 John Wiley & Sons, Inc.
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