Evolution, 4th Edition

THE TREE of LifE 49

niche is often the best initial evidence of the feature’s adaptive significance. For

example, a long, thin beak has evolved independently in at least six different lin-

eages of nectar-feeding birds. Such a beak enables these birds to reach nectar in

the bottom of the long tubular flowers in which they often feed (FIGURE 2.22A,B).

Likewise, long tubular flowers have evolved independently in many lineages of

bird-pollinated plants (FIGURE 2.22C,D).

Convergence is also observed at the molecular level. Cardiac glycosides (CGs)

are toxic compounds that are synthesized and used for defense by several lin-

eages of plants (e.g., milkweeds, family Apocynaceae) and animals (e.g., toads,

family Bufonidae). They inhibit the sodium-potassium pump protein, and so

upset cell membrane potentials by disrupting ion transport. Many insects that

feed on plants with CGs are resistant to them and actually achieve protection

by storing them in their own tissues. Toads are resistant to their own toxin, and

resistance has also evolved independently in some rodents, hedgehogs, and four

lineages of snakes and lizards that eat toads or CG-containing insects (FIGURE

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_02.02.ai Date 11-02-2016

(A)

(B)

(C)

(D)

FIGURE 2.22 Examples of convergent evolution. Many groups of birds have indepen-

dently evolved long, slender bills for feeding on nectar produced at the base of long

tubular flowers. (A)^ Hummingbirds, family Trochilidae. This violet sabrewing (Campy-

lopterus hemileucurus) is from Costa Rica. (B) Sunbirds, family Nectariniidae. The greater

double-collared sunbird (Nectarinia afra) is native to South Africa. Bird-pollinated

plants also have converged, in flower characteristics. A long tubular flower, often red

or orange, has evolved independently in many groups of bird-pollinated plants.

(C) Erythrina, a member of the pea family, Fabaceae. (D) Many species of Aloe (Aspho-

delaceae) are visited by sunbirds in Africa and the Middle East.

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