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nization of European tree species from refugia subsequent to Pleistocene glaciation, and such studies have been instructive in understanding the origin and domestication of the crop cassava. Currently, several technical limitations hinder the widespread application of a genealogical approach to plant evolutionary studies. However, as these technical issues are solved, a genealogical approach holds great promise for understanding these previously elusive processes in plant evolution.

In the following succinct statements, G. L. Stebbins presents what would become the framework for the study of plant evolutionary mechanisms for the next 50 years (Stebbins, 1950).

Individual variation, in the form of mutation and gene recombination, exists in all populations; . . . the molding of this raw material . . . into variation on the level of populations by means of natural selection, fluctuation in population size, random fixation and isolation is sufficient to account for all of the differences, both adaptive and non-adaptive, which exist between related races and species . . .

The problem of the evolutionist is . . . evaluating on the basis of all available evidence the role which each of these known forces has played in any particular evolutionary line . . .

A central thesis of Stebbins' seminal book, Variation and Evolution in Plants (Stebbins, 1950), is the notion that to understand evolution we must examine its action at the level of populations within species. This reasoning may seem obvious to contemporary readers, but at the time of Stebbins' writing, the importance of population-level processes for evolution was far from apparent. Stebbins' elucidation of this connection is one of his most enduring contributions to plant evolutionary biology.

Fifty years ago, the study of plant evolution was necessarily concerned with the phenotype, much of which is subject to selection. Morphology, karyotypes, and fitness components are central traits for understanding evolution and adaptation, but they limit which evolutionary processes can be studied. In his book, Stebbins discusses such events as fluctuations in population size, random fixation (genetic drift), and isolation as all affecting the process of evolution (see passage quoted above; Stebbins, 1950). However, the study of these mechanisms requires markers that are not under selection.

In the years after the publication of Stebbins' book, among the first major technical advances in evolutionary biology were the development of protein electrophoresis and the identification of allozyme variation in natural populations. Many allozymes are selectively neutral, and thus, for

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