MACROEvOLuTiON: EvOLuTiON AbOvE THE SPECiES LEvEL 535
events [47, 48]. The rate of evolution of a lineage, and therefore the amount of
evolutionary change from the root of a phylogenetic tree to any extant species
(path length), is expected to increase with the number of speciation events in the
punctuated equilibria model, but not in the phyletic gradualism model (FIGURE
20.19). Mark Pagel and colleagues found that in many phylogenies of animals,
fungi, and plants, the numbers of nucleotide substitutions between taxa were sig-
nificantly correlated with the number of branch points (speciation events) between
them [86], as predicted if speciation accelerates evolution. Molecular evolution in
Foraminifera is strongly correlated with the origin of new morphologically rec-
ognizable species [23]. Similarly, speciation, rather than gradual evolution within
lineages, accounts for more than two-thirds of the variance in body mass among
species of mammals [68].
What might cause these patterns? Perhaps the additional evolutionary change,
over and above change within lineages, reflects simply the adaptive changes asso-
ciated with the evolution of reproductive isolation and ecological divergence that
generally occurs during speciation. A different possibility is that speciation may
enable differences between populations to persist in the long term. Geographic
populations of a broadly distributed species commonly inhabit a mosaic of differ-
ent environments but are connected by gene flow (see Chapter 8). Any one popula-
tion may adapt to a change in its local environment, but an adaptive change will
sweep through the species as a whole only if an environmental change affects all
the populations, and this may rarely occur [102, 109]. Moreover, different mutations
or genes often provide adaptation in different populations of a species that experi-
ence similar selection; thus, the species adapts as a mosaic of different, convergent
adaptations (see Chapter 6) [88]. Hence, a species will seldom evolve as a unified
whole [61]: change of an entire species may be rather rare.
The interplay between gene flow and spatially variable selection led Futuyma to
suggest that speciation may enhance adaptive evolutionary change by stabilizing
local adaptations that would otherwise be short-lived [27]. Different populations of
a species are adapted to local environments: populations of a plant to wetter versus
drier soil, or of an insect to different host plants. But the geographic location of
these kinds of environments shifts as the climate changes. Then divergent popula-
tions move about (by colonization and extinction), and come into contact sooner or
later. Much of the divergence that has occurred between them may then be lost by
interbreeding—unless reproductive isolation has evolved (FIGURE 20.20). Repro-
ductive isolation captures and stabilizes an adaptive set of genes that can track a
geographically moving habitat, or that can disperse from one patch to another of
such habitat, without being broken down by interbreeding. A succession of specia-
tion events, each “capturing” further change in a character, may result in a long-
term trend. Speciation might act like a piton for a climber who scales an adaptiveFutuyma Kirkpatrick Evolution, 4e
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Troutt Visual Services
Evolution4e_20.19.ai Date 12-16-2016TimeTimeCharacter value1 2(A) Phyletic gradualismCharacter valueNumber of branch points1 2(B) Punctuated equilibriaPath length(C)
GradualismPunctuatedFIGURE 20.19 Models of phyletic gradualism (A) and punctuated equilibria (B) suggest
how phylogenetic data might be used to determine whether speciation is associated with
enhanced evolution of molecular or morphological characters. In both models, lineages 1
and 2 differ in the number of speciation events (branch points), but not in the time elapsed
since the common ancestor of all the species in the clade. The total number of DNA nucleo-
tide substitutions along all the segments from the common ancestor to a tip (living species)
is the path length of the branch lineage. (C) With phyletic gradualism, the path length to
any living species is proportional to elapsed time, and is not affected by the number of
speciation events. The correlation between the total path length and number of branch
points in the phylogeny is expected to be zero, as indicated by the red horizontal line. In
the punctuated equilibria model, evolution is accelerated (or occurs only) at speciation, so
the path length, shown as the green line, is expected to be correlated with the number of
branch points. (C after [86].)20_EVOL4E_CH20.indd 535 3/22/17 1:44 PM