Evolution, 4th Edition

PHylogENy: THE UNITy ANd dIvERSITy of lIfE 425

monophyletic taxa, which include an ancestor and all of its descendants (FIGURE

16.23). The Serpentes is a monophyletic group that includes all snakes; the Viperi-

dae is a monophyletic group that includes all those venomous snakes with mov-

able fangs that can be erected, such as vipers and rattlesnakes; the genus Crotalus

includes most species of rattlesnakes. Some taxa in old classifications have proven

not to be monophyletic, but instead were paraphyletic or polyphyletic (FIGURE

16.24). A paraphyletic taxon includes an ancestor and some, but not all, of its

descendants. For example, the class Reptilia would be paraphyletic if it excluded

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Snakes

Crocodiles

Birds

Snakes

Crocodiles

Birds

Falcons

Eagles

Woodpeckers

Songbirds

Reptilia

(A)

Aves

Falconiformes

Piciformes

Passeriformes

Eagles

Falcons

Woodpeckers

Songbirds

Falconiformes

Piciformes

Accipitriformes

Passeriformes

(C) (D)

Squamata

(B)

Archosaurs

The most common

ancestor of falcons

and eagles was

also the ancestor

or woodpeckers

and other groups

Reptilia is paraphyletic, because it

does not include all the species that

descended from the most recent

common ancestor (red circle)

Archosaurs is monophyletic,

because it includes all the

species descended from the

most recent common

ancestor (red circle)

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Please replace with new images as these are very roughly silhouetted and not the nal images.

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FIGURE 16.24 Modern evolutionary biologists prefer to rec-

ognize only monophyletic clades as higher taxa. (A) Traditionally,

the class Reptilia included snakes and crocodiles but not birds,

which were put in the class Aves. This made Reptilia paraphyletic,

because it did not include all the species that descended from

the MRCA of snakes and crocodiles (indicated by the red circle).

(B) The names used now are for monophyletic groups: Squamata

includes snakes and lizards, and archosaurs include crocodiles

and birds. (C) Falcons and eagles were previously thought to

be each other’s closest relatives and were grouped together in

the order Falconiformes. (D) DNA-based studies now show that

falcons are more closely related to songbirds than to eagles.

The MRCA of falcons and eagles (red circle) was also the ances-

tor of woodpeckers and other groups. The earlier classification

that grouped falcons and eagles together made Falconiformes a

polyphyletic group because it included species that descended

from two distinct ancestors. In current classifications, the orders

Falconiformes (falcons) and Accipitriformes (eagles and relatives)

are monophyletic.

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