Evolution, 4th Edition

T HE TREE of LifE 47

HoMoPLASY iS CoMMoN Homoplasy—the independent evolution of a char-

acter or character state in different taxa—includes convergent evolution (conver-

gence), parallel evolution (parallelism), and evolutionary reversal. The eyes of

vertebrates and cephalopod molluscs (such as squids and octopuses) are a spec-

tacular example of convergence. Both have a lens and a retina, but their many

profound differences indicate that they evolved independently from ancestors

without eyes. For example, the axons arise from the back of the retinal cells in

cephalopods, but from the front in vertebrates (FIGURE 2.20).

Parallel evolution is a term that has been used to describe cases in which

independent evolution of a character state is thought to have similar genetic and

developmental bases, especially in closely related species. For example, muta-

tional change in a specific gene, Pitx1, is the basis of independent loss of the

pelvic girdle and fins in many freshwater populations of a small fish, the three-

spined stickleback (see Chapter 15). But the distinction between parallel evolu-

tion and convergent evolution may not be very meaningful because, as we will

see, the same gene often contributes to similar evolutionary changes in distantly

related organisms.

Evolutionary reversals constitute a return from a derived character state to

a more ancestral state [29]. For example, winged insects evolved from wingless

ancestors, but many lineages of insects have lost their wings in the course of

subsequent evolution. It was long assumed that complex characters, once lost,

are unlikely to be regained, a principle known as Dollo’s law. However, there are

Futuyma Kirkpatrick Evolution, 4e

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Evolution4e_02.20.ai Date 11-02-2016

(A) Octopus (B) Fish

Muscle

Lens

Cornea

Iris

Light

Lens

Cornea

Iris

Light

Retina Muscle Retina

Photoreceptor

To

brain

To

brain

Light Light

Integrating

neurons Photoreceptor

The photoreceptors point away

from the incoming light,

so the light must pass through the

retinal tissue to stimulate the

photoreceptors.

Retinal nerve cells form networks

that extensively process visual

information before signals go

to the brain.

The nerve cells that convey

visual signals from the retinal

receptors to the brain leave

the eye directly in multiple

optic nerves.

The photoreceptors point

toward the incoming light.

The nerve cells leaving

the retina gather into a

single optic nerve.

FIGURE 2.20 The eyes of (A) octopus

(cephalopod mollusc) and (B) a vertebrate

are an extraordinary example of conver-

gent evolution. Despite the many simi-

larities in the two eyes, note the several

differences, including interruption of the

retina by the optic nerve in the vertebrate,

but not in the cephalopod. (A after [34, 35];

B after [33].)

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