Evolution, 4th Edition

424 CHAPTER 16

Classification

It is helpful, and often necessary, to name and classify any great variety of

objects—be they vehicles, books, or rocks—if we are to think and talk about them.

A simple classification of plants, for example, might include trees, shrubs, vines,

and herbs; but close examination will show that each of these categories includes

wildly different organisms that have in common only their overall form. Per-

haps classification by different criteria would summarize more information about

them. Carolus Linnaeus, who devised the scheme of classification that is still used

today (i.e., a hierarchical classification of groups within groups), used features

that he imagined represented propinquity in God’s creative scheme. For example,

he defined the order Primates by the features “four parallel upper front [incisor]

teeth; two pectoral nipples,” and on this basis included bats among the Primates.

But without an evolutionary framework, naturalists had no objective basis for clas-

sifying mammals by their teeth rather than by, say, their color or size. Saying that

some species were more closely “related” than others had a metaphorical, not a

genealogical, meaning.

Darwin gave classification an entirely different significance. In On the Origin

of Species, he wrote that when his views on the origin of species are adopted, the

term “relationship” among species “will cease to be merely metaphorical” and “our

classifications will come to be, as far as they can be so made, genealogies; and will

then truly give what may be called the plan of creation.”

Many systematists today are fulfilling Darwin’s prophecy, by using new data

to classify species phylogenetically. This usually means arranging them into

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Iguanidae

(iguanas)

Gekkonidae

(geckos)

“Lacertilia”

(lizards)

Epicrates

Vipera (adder)

Natrix

(grass snake)

Thamnophis

(garter snake)

Micrurus

(coral snake)

Naja (cobra)

Crotalus atrox

(w. diamondback

rattlesnake)

Crotalus horridus

(timber rattlesnake)

Boidae

Viperidae

Elapidae

Colubridae

Colubroidea

Crotalus

Serpentes

Squamata

FIGURE 16.23 Classification of

some snakes and lizards, based on

phylogeny. Squamata includes many

lineages of lizards and the snakes

(Serpentes). Lizards (sometimes

called the “Lacertilia”) do not form a

monophyletic group, but snakes do.

Among the snakes, the families Boi-

dae, Viperidae, Elapidae, and Col-

ubridae are monophyletic groups,

each with many genera (only a few

of which are shown here). Viperidae,

for example, includes the European

adder (Vipera) and the American

rattlesnakes. The timber and western

diamondback rattlesnakes are in a

group of closely related species that

form the genus Crotalus. (After [31].)

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