Sexual signals are often diagnostic of closely related species, and their taxonomic value at higher
phylogenetic levels is often discussed. Complex signals such as birdsong are potentially rich in information
regarding evolutionary history. However, natural or sexual selection, if pervasive enough, might
distort that information. This has seldom been evaluated. We tested how ecological effects impact on the
phylogenetic information of birdsong using a real case of flawed taxonomy: the reciprocal polyphyly of
the genera Serinus and Carduelis. Major axes (principal components) of between-species song variation
were not related to body size or habitat type. The main axis of song variation, reflecting syllable
complexity and song length, was related to ecological indexes of sexual selection (latitude and ecological
breadth), resulting in a positive latitudinal gradient in song elaboration. Song significantly supported
monophyly over polyphyly for these genera, despite some song data having high phylogenetic signal
(relative to the correct, polyphyletic phylogeny). Thus, the presence of high phylogenetic signal did not
guarantee reliable taxonomic information at the genus level. The misleading phylogenetic information
appears related to the latitudinal gradient in song elaboration plus the traditional Serinus and Carduelis
spp. differing in latitude. We conclude that selection, in this case sexual selection, can pervasively distort
the phylogenetic information even of complex and potentially information-rich signals.