MACROEvOLuTiON: EvOLuTiON AbOvE THE SPECiES LEvEL 537
Both driven and passive trends could have several causes [48]. Individual selec-
tion within lineages could produce both kinds of trends (FIGURE 20.22A), or the
mean character of species in a clade could change as a result of species selection: a
correlation with speciation or extinction rates (FIGURE 20.22B,C). A trend could also
occur if changes are easier in one direction than the other; for example, genetic
or developmental pathways may act as ratchets—mechanisms that make rever-
sal unlikely. Losses of complex features may be irreversible in some cases [9].
Paleontologists noticed long ago that the maximal body size in many ani-
mal groups has tended to increase over time, a trend dubbed Cope’s rule [41].
The same pattern has been found in phylogenetic analyses of living mammals
[2]. An analysis of 1534 diverse species of late Cretaceous and Cenozoic mam-
mals shows a passive trend: mammals were small before the end-Cretaceous
mass extinction, and the lower size limit has remained nearly the same ever
since (FIGURE 20.23) [1]. However, mean and maximal sizes have increased,
especially since the end-Cretaceous mass extinction, when the explosive
diversification of mammals began. Matched pairs of older and younger fossil-
ized species in the same genera (likely ancestor-descendant pairs) increased in
body size more often than they decreased, which suggests that the trend was
caused by individual selection within species, rather than by species selection.
Species selection has been identified as the cause of a trend in the mode of
larval development in several clades of Cenozoic gastropods (FIGURE 20.24).
Species that lack a planktotrophic dispersal stage are more susceptible to
extinction than are planktotrophic species (species that feed as planktonic lar-
vae). However, the nonplanktotrophic species more than compensate by their
higher rate of speciation, probably because their lower rate of dispersal reduces
the rate of gene flow among populations [39, 49]. Individual selection, spe-
cies selection, and irreversibility all affect the proportions of self-compatible
(SC) versus self-incompatible (SI) species in the family Solanaceae (tomato,
tobacco, nightshade, and others) [33]. Self-fertilization has a long-term dis-
advantage because inbreeding reduces genetic variation. But individual selec-
tion often favors selfing for several reasons; for example, reproduction occursFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_20.22.ai Date 12-27-2016Body sizeTime(A)Body sizeTime
t 1 t 2 t 1 t 2(B)Body sizeTimeSelective extinction Selective speciation(C)FIGURE 20.22 Three of the possible causes of driven trends.
(A) Individual selection within all three species in this clade favors
larger size, on average. There are short-term fluctuations in size
around the overall trend. (B) Smaller species have a higher extinc-
tion rate, so species selection causes a trend toward larger size.
(C) Recent speciation events (shown by broken branches) havebeen more frequent in the upper clades (with larger body size)
than in the lower clades, so species selection results in a trend
toward larger size. In (B) and (C), blue arrows indicate the average
body size before selective extinction or speciation (t 1 ) and red
arrows indicate the average body size afterward (t 2 ). (After [48].)Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_20.23.ai Date 03-08-17Body mass (g)80 70 60 50 40 30 20 10 Present
Time (Mya)1010010001041051061071FIGURE 20.23 A passive trend: Cope’s rule in
late Cretaceous and Cenozoic North Ameri-
can mammals. Each of 1534 species is plotted
as a line showing its temporal extension and
its body mass (estimated from tooth size).
Although small mammals persist throughout
the Cenozoic, there is an increasing num-
ber of large species over the course of time.
Although the trend is called “passive,” the
evolutionary changes of individual lineages
may have been caused by natural selection.
(After [1].)20_EVOL4E_CH20.indd 537 3/22/17 1:44 PM