Evolution, 4th Edition

THE TREE of LifE 35

be rotated without any change in the diagram’s meaning. Relationships among

the taxa are defined by the order of branching, not by the linear order of the tips of the

tree. However, the trees in FIGURE 2.8C represent relationships different from the

relationships portrayed in Figures 2.8A and B.

The lengths of the branches in a phylogenetic tree may or may not have any

meaning, depending on what information a researcher means to convey. If the

tree conveys only branching order, the relative lengths of branches have no sig-

nificance. If the tree is accompanied by an absolute time scale, however, the posi-

tions of branch points indicate when those events occurred. In some phylogenies,

the length of a branch indicates the number of evolutionary changes (e.g., DNA

nucleotide substitutions) that occurred on that branch.

Inferring phylogenies: An introduction

It can be difficult to determine phylogenetic relationships, and so evolutionary biolo-

gists are developing increasingly sophisticated methods. We will touch on some of

the difficulties and methods in Chapter 16. At this point we consider one simplified

approach, in order to convey the basic ideas.

Our estimate of how taxa are related to one another is based on characteristics

that are homologous among the taxa, such as the forelimb bones of tetrapod

(four-legged) vertebrates (FIGURE 2.9). Features are homologous among species if

they have been inherited from common ancestors. Homology describes not only

morphological and other phenotypic features, but also DNA sequences.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_02.09.ai Date 11-02-2016

Radius

Ulna

Carpals

Metacarpals

1

2

3 45

Humerus

Early tetrapod

3

1

(^12)
2
3
3
4
4
5
5
1
1
(^23)
1
2
2
3
3
4
5
Seal Horse Human
Adapted for swimming Adapted for running Adapted for grasping
Adapted for ying
S
Phalanges
Pterosaur (extinct) Bird Bat
4
Note: Pterosaur (extinct) drawing is the least like the others. I’ve tried to standardize the bone colors but this image would be ideal to redraw
in the style of the bird and bat to the right. Also Early tetrapod and Horse are illustrator drawings and vary slightly in style to the Seal, Human,
Bird, and Bat. The differences aren’t so drastic there but if we have time, perhaps we standardize all to the same level.
FIGURE 2.9 Forelimb skeletons of some tetrapod vertebrates. Compared with the
“ground plan,” as seen in the early tetrapod, bones have been lost or fused (e.g., horse,
bird) or modified in relative size and shape. Modifications for swimming evolved in the
seal, for running in the horse, for grasping in the human, and for flight in the bird, bat, and
pterosaur. All the bones shown are homologous among these organisms except for the
sesamoid bone (S) in the pterosaur; this bone has a different developmental origin from
the rest of the limb skeleton. (After [7].)
02_EVOL4E_CH02.indd 35 3/23/17 8:59 AM