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R.M. Zink and J. K1icka
mum likelihood tree, both with and without a molecular clock assumption, for the
21 species that are represented by ca. 1000 bp of cytochrome b. For the analysis in
which a molecular clock was enforced, we rooted the tree at the midpoint; rooting
on an ingroup taxon results in a spurious result (Klicka and Zink 1998). To assess
whether the sequences are evolving in a clock-like fashion, a log-likelihood ratio
test (LRT) can be performed by comparing twice the difference between the loglikelihoods for each tree to a chi-squared distribution with n-2 df, where n is the
number of taxa (Huelsenbeck and Rannala 1997). A nonsignificant chi-squared
value indicates no significant departure from a clock-like pattern of sequence evolution. We recognize that this test has been questioned (Goldman 1993), and we
also question the computation of the df; however, it is the most commonly used test
available.
Table 1. Gamma-corrected Kimura two-parameter mtDNA
distances between 21 pairs of avian sister species. These values are not corrected for within species variation
Piranga olivacea-llldoviciana
Passerina cyanea-amoena
P. cyanea-versicolor
Sialia mexicana-sialis
Cardinalis cardinalis-sinuatus
Calcarius lapponicus-mccownii
C. lapponicus-ornatlls
Oporonis philadelphia-tolmiei
o. philadelphia-agilis
Spizella breweri-pallida
Pheucticus melanocephalus-ilidovicialllls
Cyanocitta cristata-stelleri
Polioptila melanura-nigriceps
P. melanllra-californica
Pipilo aberti-crissalis
P. aberti-fuscus
Toxostoma rllfum-longirostre
T. lecontei-redivivum
T. lecontei-crissalis
T. redivivllm-crissalis
T. bendirei-cinereum
0.07
0.08
0.08
0.06
0.11
0.11
0.12
0.02
0.09
0.07
0.05
0.17
0.06
0.05
0.03
0.03
0.08
0.07
0.07
0.06
0.02