Various types of sexual signals, from cuticular hydrocarbons of
arthropods to the plumage and songs of birds, tend to evolve fast
and are often diagnostic of closely related or cryptic species (Mullen
et al. 2007; Price 2007). Since sexual signals and ornaments can
mediate reproductive isolation, these traits are biologically meaningful
delimiters at this low taxonomic level. But their taxonomic
value is uncertain at higher phylogenetic levels. Fast evolution may
increase the chances of homoplasy (similarity caused by convergent
evolution rather than common ancestry), especially for sexual
signals that can be lost and regained evolutionarily (Wiens 2001).
Furthermore, sexual signals often have behavioural components
(e.g. songs, displays). It has frequently been argued that behaviour
tends not to be taxonomically useful owing to greater evolutionary
plasticity, but there are many cases of phylogenetically informative
behaviour