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For the first part of the 20th century, the study of genetics was considered by many biologists to be separate from, or even irrelevant to, the processes by which darwinian evolution occurred. It did not seem possible that natural selection—if it was the mechanism of speciation—could work gradually, as Darwin had suggested, through the discrete particles of mendelian inheritance, whose known changes caused discretely different, or worse, grossly disruptive changes to the organism. However, as genetic understanding grew, it became possible to see how a unified theory of biology might work. By the 1930s, a group of leading biologists proposed what they referred to as the modern, or evolutionary, synthesis (Mayr 1982) that united the study of taxonomic relationships among species, the fossil record, and the theory of genetic inheritance into a single formal theory of evolutionary biology.

Before this time, it was difficult to have a rigorous, quantitative theory about the pace or nature (sometimes called the "tempo and mode") of evolution, and the theory was largely conceptual. But an assumption made by the modern synthesis, with widespread implications, was that genes are the fundamental elements of life— much as atoms are the units of chemistry and physics—and that evolution is to be explained in principle in terms of the processes of genetic change. The subsequent discovery of the nature and inheritance of DNA and its function as a protein and regulatory coding system greatly strengthened the gene-based view of life and provided a general research approach that predominates biology today.

Whatever the inherited material, if it is variable and particulate, so that each variant can be identified and not blend quantitatively with other variants, then the behavior of such variation over time and place can be quantified. If genes are the root units of biological causation, then the behavior of genetic variation over time will illustrate—and will be—evolution. The formal mathematical theory of evolu-

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.

tion which describes this is called population genetics. Of course, organisms are more than genes, but to the extent that phenotypes (traits) can be ascribable to specific genotypes, it should be possible to subsume phenotypic evolution under the same theory. Here, we will present some of the basic principles of population genetics (thorough treatments are given by Gillespie 1998; Hartl and Clark 1997; Hedrick 2000; Lynch and Walsh 1998) and then discuss some of the subtleties that arise when considering that evolution works through phenotypes rather than directly through genotypes. We will refer to aspects of the nature of genes that will be explained specifically in Chapter 4 for readers not familiar with them.

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