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Rare plants have long fascinated evolutionary biologists, who developed a wealth of theories concerning the causes and consequences of rarity (Darwin 1859; Fernald 1926; Stebbins 1942, 1980; Cain 1944; Drury 1980). Only recently, however, have the genetics and population biology of these taxa been documented in detail. Often the most informațive studies involve comparisons between closely related rare and com-mon congeners (Mehrhoff 1983; Kruckeberg and Rabinowitz 1985; Karron 1989). Such species pairs are likely to have similar ecological features and phylogenetic histories. Thus they may approximate “natural history experiments,” with the common species serving as a control.