to have played a role in contributing to the differentiation of some taxa and to have sharpened boundaries in the Sierra Nevada (17). Instead, in California there has been a history of extensive geomorphological evolution coinciding with the history of the Ensatina complex. The time and space dimensions of the diversification are interconnected. The history of this complex has probably featured substantial isolation, differen

FIG. 7. Historical biogeographic interpretation for the Ensatina complex. Five zones of secondary interaction are shown. 1, Interaction of klauberi and eschscholtzii. 2, Complex interaction between northern and southern platensis and of these interactors with xanthoptica in the central Sierra Nevada. 3, Interaction of oregonensis and northern platensis in the Lassen Peak area. 4, North Bay interaction of oregonensis and xanthoptica. 5, South Bay interaction of oregonensis and xanthoptica and of xanthoptica and eschscholtzii.

tiation, and multiple recent acts (Fig. 7). In effect, there are rings within rings in this complex, resulting from many levels of history being manifest in a single complicated pattern of variation, expressed somewhat differently at the three levels investigated to date—DNA sequences, allozymes, and color pattern. While the complex appears to be in a state of incipient species formation, which makes taxonomy problematic, it provides an instructive evolutionary example.

I thank M.Frelow, T.Jackman, D.Nguyen, C.Schneider, and K.P. Yanev for their extensive laboratory assistance, and C. Brown and many other individuals who have helped me in field work. Illustrations are by K.Klitz. I have benefited from discussions, comments on the manuscript, or both, with R.Bello, C.Brown, M.Garcia Paris, C. Haddad, R.Highton, T.Jackman, S.Kuchta, M.Mahoney, D.Parks, C.Schneider, R.Stebbins, M.Wake, K.Yanev, and K.Zamudio. The manuscript was improved by comments from two anonymous reviewers. This work was supported by grants from the National Science Foundation and the Gompertz Professorship.

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