Evolution__3rd_Edition

484 PART 4 / Evolution and Diversity
Figure 16.6
Possible classification of the
Drosophila adiostola group of
species of fruitflies. (Taken
from the top branch in the
phylogeny of Hawaiian
Drosophila in Figure 15.27,
p. 465.) These 14 species are
only a small part of the
Hawaiian Drosophila, which
in turn are only a (large) part
of the worldwide Drosophila
fauna. And yet what we know
about their phylogeny would
require at least five new levels
between the genus and species
level.
Phylocode would abandon
Linnaean ranks ...
. . . but may not catch on
Genus 1 2 3 4 5 Species
Drosophila
ornata
hamifera
paenehamifera
truncipenna
varipennis
neogrimshawi
clavisetae
touchardiae
cilifera
adiostola
peniculipedis
spectabilis
setosimentum
ochrobasis
How can we fit the phylogenetic hierarchy into the Linnaean hierarchy? Hennig
(1981) suggested using a numerical scheme in addition to the Linnaean terms. Alternatively,
a number of levels of the phylogeny can be collapsed into one Linnaean
level, to provide convenient and memorable categories. The classification is then
phylogenetic and cladistic, but not all the phylogenetic knowledge is represented in
the classification.
Another possibility is being considered by systematists at present: we might simply
abandon the Linnaean ranks of genus, family, order, and so on. Taxonomic groups
could consist simply of unranked clades (that is, monophyletic groups consisting of all
the descendants of a common ancestor). When a clade was discovered by phylogenetic
research, the clade could be named but not given a rank. Birds (or Aves) would be
a clade, and passerine birds would be another clade. They would both continue to be
used as taxonomic terms, just as they are now, but would lack a rank such as class
or order. The idea of naming clades but not giving them ranks is one component of a
proposed taxonomic system called phylocode. The proponents of phylocode have also
suggested other changes to Linnaean classificatory methods. The suggestions are controversial,
and the topic of active discussion. It is too early to say whether phylocode
will catch on.
The phylogenetic hierarchy of life in principle provides a good basis for the Linnaean
classification of life. Most biologists now accept that classification should be phylogenetic.
However, our knowledge of biodiversity and of phylogeny is expanding at a huge
rate. Phylocode may or may not come to replace the Linnaean classification, but it is a
..