Evolution, 4th Edition

EvoluTion And dEvEloPmEnT 389

that only two Hox genes are expressed in the developing thumb, whereas these

and two other Hox genes are expressed in digits 2–5.

Constraints on Adaptive Evolution

It is easy to imagine organisms that do not exist (FIGURE 15.24). Science-fiction

writers do so all the time, but biologists can cite many more realistic examples.

In no tetrapod vertebrate is the central (third) digit the shortest. No animals can

make their own food by photosynthesis (although corals incorporate photosyn-

thetic algae into their bodies). There appear to be constraints—restrictions—on

what can evolve.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_15.23.ai Date 01-04-2017

(A)

(B)

Module

correlations

H F

R 0.68

0.77

0.81

T

MC MT

0.61

0.82

H F

R 0.81

0.78

0.72

T

MC MT

0.65 0.47

0.81

H F

R 0.78

0.78

0.79

T

MC MT

0.63

0.73 0.72

H F

R 0.90

0.91

0.86

T

MC MT

0.81

0.87

H F

R 0.90

0.92

0.89

T

MC MT

0.81 0.84

0.93

H F

R 0.75

0.82

0.92

T

MC MT

0.81

0.70 0.90

H F

R 0.94

0.80

0.88

T

MC MT

0.95

H F

R 0.79

0.73

0.80

T

MC MT

0.73 0.85

0.58

Human Chimpanzee Gorilla Gibbon Macaque Langur Squirrel monkey Owl monkey

Overall modularity

3

1

2

4 Humans and apes Old World monkeys New World monkeys

Forelimb Hindlimb

H F

R T

MC MT

Functional modules

Developmental modules

FIGURE 15.23 Developmental and functional modules in the limbs of humans and their

relatives. (A) Features that are strongly correlated are considered parts of a module. Both

forelimbs (arms) and hindlimbs (legs) have three major sections, marking three develop-

mental modules. Bones in the corresponding sections are labeled H, R, and MC for humer-

us, radius, and metacarpals in the forelimb, and F, T, and MT for femur, tibia, and metatarsals

in the hindlimb. Both limbs have similar patterns of expression of Hox genes (see Figure

15.12B). (B) For each species in the phylogeny, the diagram at the top shows correlations

(numbers) between lettered boxes that represent bones in the forelimb (left boxes) and the

hindlimb (right boxes). Only statistically significant correlations are shown. For each species,

the correlations were combined into an index of overall modularity (shown by the colored

bars on the graph), which is greater if the set of correlations among the limb elements is

greater. The overall modularity is lower in apes (the four species at left) than in monkeys

(the four species at right), chiefly because of lower correlations between elements of the

forelimb and hindlimb. (From [87].)

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