Evolution, 4th Edition

528 CHAPTER 20

characters with distinct identities, distinguishable from other mammalian teeth or

bones [112]. Variation in some features may be hard to classify as one or the other—

but this is true of many distinctions in biology, such as cell types or species.

This distinction bears on one of the fundamental concepts of evolution: homol-

ogy. The forelimbs of various tetrapods, including the one-toed legs of horses, the

flippers of whales, and the wings of birds and bats, have long been recognized as

homologous, despite their many differences in form and function (see Fig ure 2.9).

Their structure is shared among tetrapods and is attributed to common ancestry. But

the same fundamental structure is seen in hindlimbs (where the femur corresponds

to the humerus, the tibia to the radius, and so on). The similarity between these seri-

ally repeated structures (which has long been called serial homology) suggests that

a similar genetic-developmental program is expressed in different parts of the body,

just as it is expressed in different species that have inherited this program from their

common ancestor. Wagner referred to this similarity, based on a common genetic-

developmental program, as biological homology [79, 111]. This is a broader concept

than the more commonly used concept of homology in systematics, where charac-

ters are defined as homologous if and only if they have been inherited from common

ancestors. In fact, a character might be biologically homologous in two species even

if their common ancestor did not express the feature. Recall (from Chapters 2 and 16)

that characters that have been “lost” in a lineage sometimes have been regained (e.g.,

the aquatic larval stage in salamanders). These cases may represent the reexpression

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_20.10.ai Date 12-28-2016

(A) (B)

Late 3rd instar larva - pupa transition

Adult

hth, n-Exd

dac

DII

Hth, n-Exd

dac

al

DII

Late 3rd instar (late prepupa)

Pupa

Leg development Horn development

FIGURE 20.10 The genetic toolkit that is shared by many animal

phyla includes regulatory gene circuits that underlie the devel-

opment of diverse features. A “limb program” that includes the

Distal-less (Dll) gene contributes to the development of diverse

outgrowths from the body wall. (A) Arrows point to developing

tube feet in a larval sea urchin. Each dark spot is the expression of

Dll in a tube foot. The lower photograph shows the developed

tube feet of a mature echinoderm, here a sea star. (B) Dll is one

of several genes that regulate the development of both legs and

horns in scarabaeid beetles. The upper diagrams show that be-

neath the body wall of the pupa (black line), the adult body wall

(blue line) develops, and forms outgrowths (in multiple colors)

that become a leg or a horn. The lower diagrams show the corre-

sponding parts of the fully developed leg in the adult and horn

in the pupa. The horizontal black bars indicate where various

genes are expressed. Some of the genes that are expressed in

leg development are also expressed in the horn. (A upper photo

courtesy of G. Boekhoff-Falk; B drawings from [78].)

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