been avoided. But formal biological names and classifications have their own historical legacies that cannot be ignored, and taxonomic stability also is highly important in systematics. One way to circumvent name changes and yet still implement the temporal-banding philosophy would be to attach a time clip (Fig. 15.2) to each extant taxon for which a reliable date of origin has been established from molecular-genetic or other evidence (Avise and Mitchell, 2007). For example, the familiar generic names Drosophila and Pan could be retained and merely time-clipped (with F:eo and C:pc) to signify their highly different evolutionary ages. Extant taxa for which origination dates remain unknown would lack time clips, but this too would convey important information by notifying the reader that a taxon’s evolutionary age might be a worthy topic for additional investigation. After time clips become available for many organismal groups, it would be a simple matter for anyone to identify, sort, and compare even disparate kinds of taxa according to their approximate dates of evolutionary origin.
A temporal-banding scheme (especially as implemented in the time-clip format) could offer systematics and the biodiversity sciences several substantial benefits [elaborated in Avise and Johns (1999) and Avise and Mitchell (2007)]. It would standardize biological classifications and thereby dramatically increase their comparative information content. It would both foster and facilitate comparisons of evolutionary rates in numerous genetic and phenotypic attributes (because absolute time is the denominator in any rate equation, and the time-clipped taxon names would specify approximate dates of clade origin). It would retain the well-established Linnaean ranking system, including familiar taxonomic names, yet simultaneously enable systematists to incorporate substantive new phylogenetic knowledge, as it becomes available, into a biological classification. It would promote the often neglected notion that every phylogenetic tree has a temporal as well as a cladistic dimension and that both are important subjects for investigation. It should engage and foster collaborations among many of the biodiversity sciences in a community-wide phylogenetic mission to chart and interpret the temporal as well as cladogenetic dimensions of the planet’s evolutionary heritage.
If the network model (e.g., Fig. 15.1b) proves to be more nearly correct for many taxonomic groups, then the challenges for systematics and evolutionary biology will be entirely different (McCarthy, 2008). First, phylogeneticists would have to admit that their dream of reconstructing a branched tree of life had been merely a pipedream, and they would have