Evolution, 4th Edition

MACROEvOLuTiON: EvOLuTiON AbOvE THE SPECiES LEvEL 527

photoreceptors and cups are most common in slowly moving or burrowing animals;

highly elaborate structures are typical of more mobile animals. The molecular basis

of vision has also evolved by comprehensible steps. All the major molecular com-

ponents, such as phototransduction cascades, the screening pigments needed for

directional photoreception, the cell membrane elaborations that enable low-resolu-

tion spatial vision, and lens crystallin proteins, were present in the common ances-

tors of animals and even fungi, and most of these were independently recruited for

vision in several animal lineages [82]. Neither at the morphological nor the molecu-

lar level is the notion of “irreducible complexity” a barrier to evolution.

The antievolutionary argument also fails to recognize that a component of a func-

tional complex that was initially merely superior can become indispensable because

other characters evolve to become functionally integrated with it. Although the eyes

of many annelid worms and other animals do not have a lens, those animals do quite

well without the visual acuity that a lens can provide. But a lens is indispensable for

eagles, since their way of hunting prey has been made possible only by such acuity,

and evolved after lenses did. Eagles and monkeys have acquired dependence on the

elements of a complex eye. Such dependence is often lost: many burrowing and cave-

dwelling vertebrates have degenerate eyes (see Figures 3.8 and 6.20).

Homology and the emergence of novel characters

“Novel” characteristics, in a broad sense, include both new modifications of ances-

tral structures, such as the elongated incisors that are the tusks of an elephant, and

what may be considered truly new structures. For example, sesamoids are bones that

develop in connective tissue. Such bones are the origin of novel skeletal elements,

such as the extra “finger” of giant pandas and moles (FIGURE 20.9) and the patella

(kneecap) in mammals, which is lacking in reptiles [79]. Günter Wagner distinguishes

character states from character identity: the tusks of elephants are one of many states

that the character “incisor” displays among mammals, but “incisor” and “patella” are

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_20.09.ai Date 12-16-2016

Q: Msp. queries if the mole photo should be replaced.

There is a mole photo from HWA 4e CH15 opener that is referenced.

(B) Brown bear (Ursus arctos)

Sesamoid

Sesamoid

(“thumb”)

(A) Giant panda

(Ailuropoda melanoleuca)

(C) Mole (Condylura cristata) (D) Shrew (Cryptotis parva)

Sesamoid

FIGURE 20.9 False fingers (“thumbs”)

have evolved from sesamoid bones in

the giant panda (A) and in moles (C). The

unmodified hands of these animals’ close

relatives, a bear (B) and a shrew (D), are

shown for comparison. Developmental

constraints probably prevented the evolu-

tion of a sixth true digit. These structures

exemplify what has been called “tinkering”

by natural selection: evolution of adapta-

tions from whatever variable characters a

lineage happens to already have. (From

[15, 77]; shrew and mole hand photos cour-

tesy of Marcelo Sánchez-Villagra.)

20_EVOL4E_CH20.indd 527 3/22/17 1:44 PM