Ke. Omland, EXAMINING 2 STANDARD ASSUMPTIONS OF ANCESTRAL RECONSTRUCTIONS - REPEATED LOSS OF DICHROMATISM IN DABBLING DUCKS (ANATINI), Evolution, 51(5), 1997, pp. 1636-1646
Although phylogenetic reconstruction of ancestral character states is
becoming an increasingly common technique for studying evolution, few
researchers have assessed the reliability of these reconstructions. He
re I test for congruence between a phylogenetic reconstruction and a w
idely accepted scenario based on independent lines of evidence. I used
Livezey's (1991) phylogeny to reconstruct ancestral states of plumage
dichromatism in dabbling ducks (Anatini). Character state mapping rec
onstructs monochromatic ancestors for the genus A,rns as well as most
of its main clades. This reconstruction differs strongly from the wide
ly accepted scenario of speciation and plumage evolution in the group
(e.g., Delacour and Mayr 1945; Sibley 1957). This incongruence may occ
ur because two standard assumptions of character state reconstruction
are probably not met in this case. Violating either of these two assum
ptions would be a source of error sufficient to create misleading reco
nstructions. The first assumption that probably does not apply to duck
s is that terminal taxa, in this case species, are monophyletic, Many
of the widespread dichromatic species of ducks may be paraphyletic and
ancestral to isolated monochromatic species. Three lines of evidence
support this scenario: population-level phylogenies, biogeography, and
vestigial plumage patterns. The second assumption that probably does
not apply to duck plumage color is that gains and losses of character
states are equally likely. Four lines of evidence suggest that dichrom
atic plumage might be lost more easily than gained: weak female prefer
ences for bright male plumage, biases toward the loss of sexually dich
romatic characters, biases toward the loss of complex characters, and
repeated loss of dichromatism in other groups of birds. These seven li
nes of evidence support the accepted scenario that widespread dichroma
tic species repeatedly budded off isolated monochromatic species. Drif
t and genetic biases probably caused the easy loss of dichromatism in
ducks and other birds during peripatric speciation. In order to recove
r the accepted scenario using Livezey's tree, losses of dichromatism m
ust be five times more likely than gains. The results of this study ca
ution against the uncritical use of unordered parsimony as the sole cr
iterion for inferring ancestral states, Detailed population-level samp
ling is needed and altered transformation weighting may be warranted i
n ducks and in many other groups and character types with similar attr
ibutes.