Evolution, 4th Edition

■■Steps by which some higher taxa have evolved

(e.g., birds, mammals) have been well docu-

mented in the fossil record. intermediate forms

give evidence of both mosaic evolution of dif-

ferent characters and of changes in the form and

function of specific characteristics.

■■Major changes in characteristics evolve not by

large jumps (saltations), but generally evolve

gradually, through intermediate stages. The

evolution of some characters does include ef-

fects of mutations with moderately large effects.

Complex structures such as eyes evolve by rather

small, individually advantageous steps. They may

acquire functional integration with other features

so that they become indispensable.

■■Homologous characters may be based on similar

or the same networks of regulatory gene inter-

actions. Novel features have arisen, in at least

some cases, by the recruitment of integrated

genetic and developmental pathways in new

contexts or combinations.

■■Some fundamental characteristics of develop-

mental processes and organismal integration

may enhance evolvability, the capacity of a

genome to produce variants that are potentially

adaptive.

■■The fossil record provides examples of both

gradual change and the pattern called punc-

tuated equilibria a rapid shift from one static

phenotype to another. The hypothesis that such

shifts require speciation is not widely accepted

because responses to selection do not depend

on speciation.

■■The long-term average rate of evolution of most

characters is very low because long periods of

little change (stasis) are averaged with short peri-

ods of rapid evolution, or because the character

mean fluctuates without long-term directional

change. The highest rates of character evolution

in the fossil record are comparable to rates ob-

served in current populations and can readily be

explained by known processes such as mutation,

genetic drift, and natural selection.

■■Stasis and low rates of character evolution can

be explained by genetic constraints, stabilizing

selection (owing largely to habitat tracking), or

gene flow among divergently selected popula-

tions that may prevent or reverse the evolution

of divergent phenotypes. in some cases, specia-

tion appears to be correlated with higher rates

of phenotypic and molecular evolution.

■■Long-term trends may result from individual

selection, species selection, or constraints that

bias the direction of evolution between char-

acter states. Driven trends, whereby the entire

frequency distribution of a character among

species in a clade shifts in a consistent direction

over time, are distinguished from passive trends,

in which variation among species (and therefore

the mean of the clade) expands from an ances-

tral state that is located near a boundary (such as

a minimal body size).

■■Probably no feature exhibits a trend common

to all clades in the tree of life. Features such as

genome size and structural complexity display

passive trends, in that the maximum has in-

creased since very early in evolutionary history,

but such changes have been inconsistent among

lineages. There is no clear evidence of trends in

measures of adaptedness, such as the longevity

of species or higher taxa, in geological time.

■■Certain aspects of evolution are predictable,

especially in the short term, and may be mani-

fested by convergent evolution. However, long

evolutionary histories are probably contingent:

that is, particular evolutionary events would have

differed, or would not have occurred, if any of a

great many previous events had been different.

unique events such as the emergence of human

intelligence may have been highly contingent

and improbable.

■■if “progress” implies movement toward a goal,

then there can be no progress in evolution. if

“progress” implies betterment or improvement,

improvement can be seen only relative to a spe-

cies’ environment or way of life.

TERMS AND CONCEPTS

biological

homology

driven trend

(= active trend)

habitat tracking

macroevolution

microevolution

passive trend

phyletic gradualism

phylogenetic niche

conservatism

punctuated

equilibria

saltation

stasis

SuMMARY

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