Assignment Given the Tree Model Develop a Standardized Classification Scheme

The two basic functions of biological taxonomy are to (i) provide a universal system for information storage and retrieval, and (ii) encapsulate an evolutionary interpretation of biological diversity (Mayr, 1982). Unfortunately, current biological classifications are grossly nonstandard-ized because: (i) the species in named taxa are typically united by some unspecified mix of similarity by resemblance and similarity by descent, and (ii) even when the nested taxonomic ranks in a Linnaean hierarchy do register bona-fide nested clades the rankings remain noncomparable across different kinds of organisms (because no serious attempt has ever been made to normalize assayed characters, equilibrate taxonomic assignments, or even adopt any standardized criteria for taxonomic ranking). For example, some taxonomic genera such as Drosophila are an order of magnitude older than others such as Gorilla or Pan, and, because of an apples-versus-oranges problem, a taxonomic rank (such as a genus) shared by fruit flies and primates implies nothing about whether such taxa are similar with respect to genetic, phenotypic, or any other aspect of evolutionary diversity. As noted by de Queiroz and Gauthier (1992), ''No scientific enterprise, least of all one that considers the promotion of nomenclatural universality as one of its primary objectives, can accept the inconsistencies and ambiguities current in biological taxonomy.'' Or as phrased by Hennig (1966), ''If systematics is to be a science it must bow to the self-evident requirement that objects to which the same label is given must be comparable in some way.''

This state of affairs could, in principle, be rectified if systematists were to adopt absolute geological time as the universal evolutionary yardstick against which to standardize taxonomic assignments for extant clades of known age. The basic idea, proposed by Hennig (1966) and elaborated by Avise and Johns (1999), is that extant species that separated from a common ancestor in a specified window of evolutionary time would be assigned a taxonomic rank defined by that temporal band. The boundaries of the temporal windows are arbitrary at the outset and must be ratified by convention, but a proposal that I favor in principle would link each taxonomic rank to a specific geological episode. Serendipitously, there are 17 supraspecific ranks in modern versions of the Linnaean hierarchy (Mayr and Ashlock, 1991) and also 17 primary subdivisions in the traditional geological timescale (Futuyma, 1998), thus affording the possibility of a perfect one-to-one allocation of taxonomic rank to geological episode (Fig. 15.2).

If the field of systematics from its outset had been able to implement a temporal-banding strategy for erecting biological classifications, many of the inconsistencies and ambiguities in current taxonomies could have

geological episode

taxonomic rank

mya

timeclip

Pleistocene

subgenus

0-2

[B:pl]

Pliocene

genus

2-5

[C:pc]

Miocene

tribe

5-24

[D:mi]

Oligocene

subfamily

24-33

[E:ol]

Eocene

family

33-56

[F:eo]

Paleocene

superfamily

56-65

[G:pa]

Cretaceous

suborder

65-145

[H:cr]

Jurassic

order

145-205

[IJu]

Triassic

superorder

205-250

[J:tr]

Permian

cohort

250-290

[K:pe]

Carboniferous

subclass

290-350

[L:cb]

Devonian

class

350-410

[M:dv]

Silurian

superclass

410-440

[N:si]

Ordovician

subphylum

440-500

[O:od]

Cambrian

phylum

500-550

[P:ca]

Proterozoic

kingdom

550-2500

[Q:pr]

Archaean

domain

2500-3600

[R:ar]

FIGURE 15.2 Examples of how a strategy of temporal banding might be used to standardize biological classifications for extant species (see text). Shown is the one-to-one correspondence possible between 17 standard taxonomic ranks in some modern versions of the Linnaean hierarchy [see Mayr and Ashlock (1991)] and the temporal bands (Mya) for 17 traditionally recognized geological episodes [see Futuyma (1998)]. In one temporal-banding proposal, current classifications and nomenclatures could be revised (perhaps drastically), such that each clade would be ranked and named strictly according to the temporal window in which it arose. Under a less drastic proposal (which I favor), current classifications and nomenclatures would be retained, but each existing taxonomic name would simply be appended with a time clip signifying the approximate date of that taxon's origination. Note that these temporal-banding proposals do not extend to species-level taxonomic assignments, where biological criteria, including reproductive isolation (regardless of a species' date of evolutionary origin), would continue to apply.

288 / John C. Avise 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 Dro-sophila 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 timeclip 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.

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