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426 THE WILSON BULLETIN l Vol. 97, No. 4, December 1985 contra Felsenstein 1984) for describing phylogenies, we find that they reveal structure in the genetic data and are helpful in “visualizing” genetic distances. Also, because of the differing assumptions of each technique (e.g., relative homogeneity of rates of allozymic change), different kinds of information are revealed. In the present study, great congruence of pattern was achieved. The UPGMA and WPGMA clustering procedures (Sneath and Sokal 1973) for example, yielded virtually identical results; therefore, only the former is illustrated (Fig. 1A). All diagrams clearly show the close alliance of V. olivaceus and V. “chivi,” the more distant attachment of V. Jlavoviridis as a sister group to the first pair of forms, and the great separation of C. gujanensis from the clade comprised of the three forms of Mreo. We feel confident, therefore, that the consistent pattern of branching achieved in these analyses was not a simple result of the methodology employed (Presch 1979). That is, the branching pat- tern was independent of assumptions regarding the nature of the data. DISCUSSION AND CONCLUSIONS Age of the Taxa Several researchers (e.g., Nei 1975, Sarich 1977, Yang and Patton 198 1, Gutierrez et al. 1983) have attempted to determine the absolute timing of cladogenetic events by various calibrations applied to Nei’s D values among existing species. Gutierrez et al. (1983) for example, used the mid- Miocene date of a fossil quail, Cyrtonyx cooki (Wetmore 1934), in con- junction with average genetic distances between modern Cyrtonyx mon- tezumae and its Odontophorine sister taxa, to derive a calibration value of t = 26.3 x 106D, where t is the time since divergence and D is Nei’s (1978) genetic distance. Such calibrations are dependent on the existence of a molecular clock (Wilson et al. 1977, Thorpe 1982), that is, a process by which nucleotide substitutions and, therefore, allelic differences among populations and species accumulate stochastically through mutation and drift in a uniform, time-dependent fashion. The recent finding of Bar- rowclough et al. (1985) that patterns of genetic divergence in diverse taxa of birds agree with the predictions of a neutral, mutation-drift process (Kimura 1979, 1982) lends credence both to the existence of a clock and to these dating attempts. Nonetheless, several problems attend the use of such calibrations: (1) the relative imprecision of the dating of most fossils, including that of the quail mentioned above; (2) the fact that large standard errors are usually associated with estimates of genetic distance; and (3) the typical lack of independent means, beyond general stratigraphic date or degree of morphologic change, by which to judge the validity of divergence times

Johnson and Zink l GENETIC RELATIONSHIPS AMONG VIREOS 427 A. UPGMA rcc = 997 V 0 OlWaCeus(Red-eyed Vlreo) B. 1 1 042 036 030 0 24 0 18 0 12 006 0 Rogers’ D 150 46 d37 b WAGNER TREE % S.D. = 20.5 C 0 315 Million years before present f i 082 076 Vo d/versus (“Chivl” Vlreo) V flovovirid~s (Yellow-green V~reo) C q~,a”ens,s (Rufous-browed 1 V Pepper-Shrikel 030 V 0 olivaceus 023 V o dtversus flu YO vk/d;s * C gqanensa 1 I I , I I I I 1 1 I 0 002 004 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 C. F-M TREE % S D.= 187 Distance from root FIG. 1. (A) Phenogram based on Rogers’ D-values (Table 3) and derived by the UPGMA method. The high cophenetic correlation coefficient (rcc = 0.977) indicates very close agree- ment between the distances shown in the phenogram and the original distance matrix. See text for the derivation and interpretation of the time scale. (B) Distance Wagner Tree rooted at the outgroup, Cyclarhis gujanensis. The F-value (Farris 1972) is 0.002 and the cophenetic correlation is 1.00. This analysis produced no negative branches. (C) Branching diagram derived by the method of Fitch and Margoliash (1967). Branch lengths are in units of Rogers’ D (x 100). The tree is “rooted” (see Farris 1972) at Cyclurhis gujunensis. Of 6 F-M Trees examined, the one illustrated best summarized the original matrix based on the lack of negative branches and lowest percentage standard deviation.

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