Evolution, 4th Edition

■■A feature is an adaptation for a particular func-

tion if it has evolved by natural selection for

that function by enhancing the relative rate of

increase—the fitness—of biological entities with

that feature.

■■Because many characteristics are genetically

variable in natural populations, they may evolve

rapidly if selection pressures change. Especially

because humans drastically alter environments

and move species into new environments, many

historical examples of rapid adaptive evolution

have been documented, often within much less

than a century.

■■Natural selection is a consistent difference in

fitness among phenotypically different biologi-

cal entities. it is the antithesis of chance. Natural

selection may occur at different levels, such as

genes, individual organisms, populations, and

species.

■■Selection at the level of genes or organisms

is likely to be the most important because the

numbers and turnover rates of these entities

are greater than those of populations or spe-

cies. Therefore most features are unlikely to

have evolved by group selection, the one form

of selection that could in theory promote the

evolution of features that benefit the species

even though they are disadvantageous to the

individual organism. Both genic and individual

selection can be viewed as fitness differences

among genes, with “selfish genes” being those

that prevail.

■■Species selection is a correlation between a trait

and the rate of speciation or extinction. it can

result in variation among clades in diversity of

species.

■■Not all features are adaptations. Methods for

identifying and elucidating adaptations include

studies of function and design, experimental

studies of the correspondence between fitness

and variation within species, and correlations

between the traits of species and environmen-

tal or other features (the comparative method).

Phylogenetic information may be necessary for

proper use of the comparative method.

■■o rganisms may not have perfect adaptations be-

cause of functional compromises or trade-offs, or

because mutations enabling perfect adaptation

have not been available.

■■As a consequence of adaptation of species to

different environments and ways of life, natural

selection is the basis of adaptive radiations and

adaptive diversity. Competition for resources

is one of many factors that can select for differ-

ences among species.

■■Natural selection need not promote harmony or

balance in nature, and utterly lacking any moral

content, it provides no foundation for morality or

ethics in human behavior.

TERMS ANd CoNCEPTS

adaptation

altruistic trait

character

displacement

comparative

method

exaptation

fitness

frequency

function (vs. effect)

genic selection

group selection

(= interdemic

selection)

hypothetico-

deductive method

individual selection

kin selection

levels of selection

meiotic drive

natural selection

neutral allele

preadaptation

reproductive

success

segregation

distortion

selfish genetic

element

sexual selection

species selection

trade-off

SuMMARY

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