Evolution, 4th Edition

Species selection

Selection among groups of organisms is called species selection when the groups

involved are species and there is a correlation between some characteristic and the

rate of speciation or extinction [19, 28, 42]. Species selection does not shape adapta-

tions of organisms, but it does affect the disparity—the diversity of biological charac-

teristics—of the world’s organisms. The consequence of species selection is that the

proportion of species that have one character state rather than another changes over

time (FIGURE 3.11). A likely example of the effects of species selection is the preva-

lence of sexual species compared with closely related asexual forms. Many groups of

plants and animals have given rise to asexually reproducing lineages, but with some

interesting exceptions, asexual lineages tend to be young, as indicated by their close

genetic similarity to sexual forms. This observation implies that asexual forms have a

higher rate of extinction than sexual populations, since few asexual forms that arose

long ago have persisted (see Chapter 10) [35].

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X

X

X

X

(A)

1

Time

2 3 4

(B)

1 2 3 4

Altruistic genotype

Selsh genotype

Population Population

Wynne-Edwards: Altruistic behavior

will evolve because group selection

favors it (i.e., more “selsh”

populations go extinct).

Williams: Within-population selection favors

the “selsh” allele and increases it more

rapidly than whole-population selection can

act, so the “selsh” allele will become xed.

FIGURE 3.10 Conflict between group and individual selection.

The rectangles represent four populations of a species (1–4), traced

through four time intervals; each circle is an individual organism in a

population: open if the individual is an altruistic genotype, filled if it is

a selfish genotype. Some new populations are founded by colonists

from established populations (shown by diagonal arrows), and some

populations become extinct (marked by X). Individuals with the selfish

genotype are assumed to have higher fitness than altruistic individu-

als. (A) An altruistic trait may evolve by group selection if the rate of

extinction of populations of the selfish genotype is very high. (B) Wil-

liams’s argument: Because individual selection operates so much more

rapidly than group selection, the selfish genotype increases rapidly

within populations and may spread by gene flow into populations

of altruists, and replaces them. Thus the selfish genotype becomes

fixed, even if it increases the chance of population extinction.

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Body size (degree of difference

from ancestral form)

Time

t 2

t 1

FIGURE 3.11 Species selection caused by a correlation between speciation rate and a

morphological character, such as body size (x-axis). Larger-bodied species persist longer

before becoming extinct, and so give rise to large-bodied species more often than

small-bodied species produce other small-bodied species. The lower extinction rate of

lineages with large body sizes is analogous to a lower mortality rate of individual organ-

isms in individual selection. The character value, averaged across species (red dots), is

greater at time t 2 (upper dashed line) than at time t 1 (lower dashed line). (After [18].)

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