Evolution, 4th Edition

T HE TREE of LifE 29

tree. Closely adjacent twigs represent living species derived only recently from

their common ancestors (shared ancestors). Twigs on more distant branches repre-

sent species derived from more ancient common ancestors. Darwin expressed this

metaphor in some of his most poetic (and very Victorian) language:

The affinities of all the beings of the same class have sometimes been

represented by a great tree. I believe this simile largely speaks the truth. The

green and budding twigs may represent existing species; and those produced

during former years may represent the long succession of extinct species. At

each period of growth all the growing twigs have tried to branch out on all

sides, and to overtop and kill the surrounding twigs and branches, in the

same manner as species and groups of species have at all times overmastered

other species in the great battle for life. The limbs divided into great branches,

and these into lesser and lesser branches, were themselves once, when the

tree was young, budding twigs; and this connection of the former and present

buds by ramifying branches may well represent the classification of all extinct

and living species in groups subordinate to groups. Of the many twigs which

flourished when the tree was a mere bush, only two or three, now grown into

Futuyma Kirkpatrick Sinauer Associatesgreat branches, yet survive and bear the other branches; so with the species Evolution, 4e

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Evolution4e_02.02.ai Date 11-02-2016

Note: All nch art is from 3e except for Certhidea fusca which is new.

Certhidea olivacea

Certhidea fusca

Outgroups

Platyspiza crassirostris

Pinaroloxias inornata

Cactospiza heliobates

Cactospiza pallida

Warbler nches

Vegetarian nch

Ground nch

Cocos Island nch

Mangrove nch

Woodpecker nch

Camarhynchus parvulus

Camarhynchus pauper

Camarhynchus psittacula

Geospiza scandens

Geospiza conirostris

Geospiza conirostris

Geospiza magnirostris

Geospiza fortis

Geospiza fuliginosa

Geospiza difcilis

Geospiza difcilis

Tree nches

Cactus nches

Ground nches

FIGURE 2.2 The finches in the Galápagos

Islands and Cocos Island that have become

known as Galápagos finches (also sometimes

referred to as Darwin’s finches). The bills of

these species are adapted to their diverse

feeding habits. Some hybridization among

species may have affected apparent relation-

ships, as well as anomalies such as the oc-

currence of G. difficilis on two branches. The

outgroups are genera of finches distributed

in South America and the West Indies. (After

[10, 20].)

FIGURE 2.3 Darwin’s first speculative

diagram of a phylogenetic tree, in an 1837

notebook. The numeral 1 represented the

ancestor of groups A–D.

02_EVOL4E_CH02.indd 29 3/23/17 8:59 AM