Evolution, 4th Edition

32 CHAPTER 2

prize goes to the single insect order Coleoptera: there are about 350,000 known

species of beetles (P). Among the families of flowering plants (angiosperms), the

sunflowers (23,000 species) and orchids (19,50 0 species) are fantastically diverse.

At the other end of the diversity spectrum, the most ancient lineage of all angio-

sperms is now represented by just a single species, Amborella trichopoda, which

survives only in remote rainforests on the South Pacific island of New Caledonia.

(Notice that here and elsewhere we use terms from taxonomy, the classification of

organisms. BOX 2A provides a review of some important aspects of classification.)

These are only a few of the fascinating glimpses into the history of life that phy-

logenetic studies have revealed. We must ask, though, how the relationships among

diverse species can be determined, how events in evolutionary history, such as the

echinoderms’ losing their heads, can be inferred from these relationships, and what

The scheme of classification that is used today was devel-

oped by the Swedish botanist Carolus Linnaeus (1707–

1778). Linnaeus introduced binomial nomenclature, a

system of two-part names consisting of a genus name and

a specific epithet (such as Homo sapiens). He proposed a

system of grouping species in a hierarchical classification of

groups nested within larger groups (such as genera nested

within families). The levels of classification—such as king-

dom, phylum, class, order, family, genus, and species—are

referred to as taxonomic categories, whereas a particular

group of organisms assigned to any of these levels is a

taxon (plural: taxa). Higher taxa are those above the species

level. Thus the species rhesus monkey (Macaca mulatta) is

placed in the genus Macaca, in the family Cercopithecidae;

Macaca and Cercopithecidae are higher taxa that exemplify

the taxonomic categories genus (plural: genera) and family,

respectively. Several intermediate taxonomic categories,

such as superfamily and subspecies, are sometimes used in

addition to the more familiar and universal ones.

To ensure that names are standardized, taxonomy has de-

veloped rules of procedure. For example, the genus name

and specific epithet ordinarily agree in gender: Rattus

norvegicus, not Rattus norvegica, for the brown rat. No two

species of animals, or of plants, can bear the same name.

The valid name of a taxon is the oldest available name that

has been applied to it. Thus it sometimes happens that two

authors independently describe the same species under

different names; in this case, the valid name is the older one.

Sometimes two or more species have masqueraded under

one name; in this case, the name is applied to the species

that the author used in his or her description. To prevent the

obvious ambiguity that could arise in this way, it is standard

practice for the author to designate a single specimen (the

type specimen, or holotype) as the “name bearer” so that

later workers can determine which of several similar species

rightfully bears the name. The holotype, usually accompa-

nied by other specimens (paratypes) that exemplify the

range of variation, is deposited and carefully preserved in a

museum or herbarium.

The rules for naming higher taxa are not all as strict as

those for species and genera. In zoology (and increasingly

in botany), names of subfamilies, families, and sometimes

orders are formed from the name of the type genus (the

first genus described). Most family names of plants end in

-aceae. In zoology, subfamily names end in -inae and fam-

ily names in -idae. Thus Columba (Latin for “pigeon”), the

genus of the familiar pigeon, is the type genus of the family

Columbidae and the subfamily Columbinae; Rosa (rose) is

the type genus of the family Rosaceae. Names of genera

and species are always italicized; taxa above the genus level

are not italicized, but are always capitalized.

Systematists today rely on phylogeny when classifying

organisms. A monophyletic taxon is one that includes all the

named descendants of a particular common ancestor (for

example, the traditional class Aves, which includes all birds,

is monophyletic). Most systematists today hold the opinion

that classifications should consist of monophyletic taxa only

and thus reflect phylogenetic relationships. A paraphyletic

taxon includes some, but not all, of the descendants from

a particular ancestor. (The traditional class Reptilia is para-

phyletic because it did not include the birds, which share a

common ancestor with dinosaurs and crocodiles. Reptilia is

monophyletic if we abolish the class Aves and include the

birds in the class Reptilia.) A polyphyletic taxon includes

species that do not exclusively share a common ancestor.

(The falcons, hawks, and eagles were included in the order

Falconiformes, but DNA evidence indicates that falcons are

more closely related to parrots and songbirds. They are

now recognized as a distinct order from hawks and eagles,

which are now called Accipitriformes.)

BOX 2A

Classification, Taxonomic Practice, and Nomenclature

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