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This is a survey of current knowledge and practise emphasising the possibilities of isozymes being marker genes in solving a variety of problems important to population geneticists, evolutionists, systematists and plant breeders. Identity values of species pairs are correlated with the geologically determined recency of the island on which they occur. This result suggests the possibility that, in terms of the geological time scale, speciation is always a rapid process, but that after the species have become distinct, they may continue to diverge more with respect to isozymes than in morphology. Several comparisons of morphological and isozyme patterns of variability in both plants and animals have shown that they are often poorly correlated with each other. The use of isozymes as markers has been a new approach of considerable value to those plant breeders who have employed it. In this volume, Doebley has shown that it can aid greatly our understanding of crop plant evolution. The careful research of Stuber has used the method for attacking one of the most important and difficult problems in plant breeding - the location, number and nature of the genes that contribute to patterns of quantitative inheritance. These results should improve greatly efforts to increase grain yield. Dealing with an entirely different type of economic plant, fruit trees, in which the problems of improvement by breeding are intensified by the great length of their generations, Torres has shown that using marker isozymes provides valuable short cuts.
