Evolution, 4th Edition

526 CHAPTER 20

Complex characteristics

A common argument against Darwinian evolution is based on so-called irreducible

complexity: the proposition that a complex organismal feature cannot function effec-

tively except by the coordinated action of all its components, so that the feature must

have required all of its components from the beginning. Since they could not have all

arisen in a single mutational step, the feature (it is claimed) could not have evolved.

Needless to say, the first person to recognize this potential problem was Darwin

himself, in On the Origin of Species: “That the eye, with all its inimitable contriv-

ances for adjusting the focus to different distances, for admitting different amounts

of light, and for the correction of spherical and chromatic aberration, could have

been formed by natural selection seems, I freely confess, absurd in the highest

possible degree.” But he then proceeded to supply examples of animals’ eyes as

evidence that “if numerous gradations from a perfect and complex eye to one very

imperfect and simple, each grade being useful to its possessor, can be shown to

exist; if further, the eye does vary ever so slightly, and the variations be inher-

ited, which is certainly the case; and if any variation or modification in the organ

be ever useful to an animal under changing conditions of life, then the difficulty

of believing that a perfect and complex eye could be formed by natural selection,

though insuperable by our imagination, can hardly be considered real.”

Darwin’s claim has been fully supported by later research [81, 82, 85]. The eyes

of various animals range from small groups of merely light-sensitive cells (in some

flatworms, annelid worms, and others), to cuplike or “pinhole camera” eyes (in

cnidarians, molluscs, and others), to the “closed” eyes, capable of registering pre-

cise images, that have evolved independently in cnidarians, snails, bivalves, poly-

chaete worms, arthropods, and vertebrates (FIGURE 20.8). The evolution of eyes

is apparently not so improbable! Each of the many grades of photoreceptors, from

the simplest to the most complex, serves an adaptive function. Simple epidermal

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(B) Gastropod examples

Fishes; squids; some

gastropods, annelids,

and crustaceans

Some

gastropods

Most phyla; many Molluscs

annelids and some

gastropods

Some

gastropods

(A) Stages of eye complexity

Patella Pleuromaria Haliotis Turbo Murex

Lens

Nerves

Pigmented

cells

Photosensitive

cells

Eye cup

Lens

Iris

Pigmented cells

Photosensitive cells

Some annelids

and atworms

1 2 3 4 5 6

FIGURE 20.8 Intermediate stages in the evolution of complex

eyes. (A) Schematic diagrams of stages of eye complexity in vari-

ous animals, from a simple photosensitive epithelium, through

the deepening of the eye cup (providing progressively more

information on the direction of the light source), through gradual

evolution toward a “pinhole camera” eye, eventually including a

refractive lens and a pigmented iris for sharper focusing. (B) Most

of these stages can be found among various gastropod species

(snails and relatives), as shown in these drawings. (A after [85]; B

after [94].)

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