Evolution__3rd_Edition

472 PART 4 / Evolution and Diversity
Biological species are classified
hierarchically ...
. . . but what kind of hierarchy?
And why a hierarchy at all?
16.1 Biologists classify species into a hierarchy of groups
Biologists have so far described approximately 1.75 million species of living plants and
animals, and perhaps a further 0.25 million extinct fossil species. Estimates vary for the
number of species that exist but have not yet been described: there may be between 10
and 100 million of them. Describing a species is a formalized activity, in which the
taxonomist has to compare specimens from the new species and other, similar species,
and then explain how the new species can be distinguished; the description also has to
be published. Describing species is the most important task of taxonomists, but it has
no particular connection with evolutionary biology.
The evolutionary interest of classification begins at the next stage. Biologists do not
think of their million or so described species simply as a long list, beginning with the
aardvark, working through buttercup, honeybee, and starfish, to end with zebra. Since
Linnaeus, species have been arranged in a hierarchy; Figure 3.5 (p. 49) used the wolf as
an example. Species are grouped in genera: the gray wolf species Canis lupus and the
golden jackal Canis aureus, for example, are grouped in the genus Canis; genera are
grouped into families: the genus containing dogs and wolves combines with several
other genera, such as the fox genus Vulpes, to make up the family Canidae; several
families combine to make up an order (Carnivora, in this example), several orders to
make a class (Mammalia), classes to make a phylum (Chordata), and phyla to make
a kingdom (Animalia).
Each species, therefore, is a member of a genus, a family, an order, and so on. The
problem of biological classification above the species level is how to group the species
into these higher categories. The problem has both a practical and a theoretical side.
Any number of practical problems can arise in deciding which genus to put a species
into and what level particular groups should have (genus or family?). But before these
questions, there is the logically prior question of what procedures should be used, and
what sort of hierarchy we should be trying to classify the species into.
If we take a million species and seek to arrange them into a classification, the
arrangement could be made in a large number of ways. A classification does not even
have to be hierarchical. Chemists, for example, classify elements by the periodic table,
which is not hierarchical. Why biological classification should be hierarchical is an
interesting question in itself (Section 16.8). However, we begin by assuming that
classification is hierarchical, and ask what the exact form of the hierarchy should be.
This chapter is about theoretical questions a of the relation between evolutionary trees
and biological classification a rather than practical questions of how to classify species
at the museum workbench.
16.2 There are phenetic and phylogenetic principles of
classification
In biology, two main methods are used to classify species into groups: the phenetic
and the phylogenetic methods. (Some would prefer to substitute “phenotypic” for
..