MACROEvOLuTiON: EvOLuTiON AbOvE THE SPECiES LEvEL 531
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
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Evolution4e_20.13.ai Date 12-26-201650100150200250300350400450500550Body mass (kg)60 55 50 45 40 35 30 25 20 15 10 5 Present
Time (Mya)Hyracotherium
(browser) Mesohippus
(browser)Merychippus
(grazer)Equus
(grazer)FIGURE 20.13 Estimated body masses of 40 species
in the horse family, Equidae, plotted against geological
time. Although some lineages decreased in size, more
lineages increased, so the average body size in the family
increased over time. (After [66].)explained simply by mutation and random genetic drift, without a
push from natural selection [65]. So the question that requires an
answer is not “Can rates of mutation and natural selection explain
the rate of long-term evolution?” The problem, rather, is to explain
why evolution is often so slow. One major reason (as explained in
the caption to Figure 20.12) is that the direction of selection may
fluctuate, so that a character mean changes rapidly in the short
term but averages little change over a longer time. But other expla-
nations may also be important.
In the extreme, biologists are challenged to understand the
existence of “living fossils”—organisms such as the ginkgo (see
F i g u r e 17. 2 2), the horseshoe crab (Limulus), and the coelacanth
that have changed so little over many millions of years that they
closely resemble their Mesozoic or even Paleozoic relatives (FIG-
URE 20.14). The synapomorphies (shared derived characters) of
large clades also represent conservatism: almost all mammals, no
matter how long or short their necks, have seven neck vertebrae
(see Figure 3.21), and almost no tetrapods have had more than five
digits per limb. (The earliest tetrapods had more, but soon settled
on five.) The hypotheses proposed to explain phylogenetic conser-
vatism include stabilizing selection and internal constraints.
One important reason why the optimal condition of a charac-
ter may remain unchanged is phylogenetic niche conservatism: long-continued
dependence of related species on much the same resources and environmental
conditions [43, 115]. This is manifested by habitat tracking: a shift in the geographic
distribution of species along with changes in the geographic distribution of habitat
to which the species are adapted. For example, diverse molluscs experienced theFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
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Evolution4e_2014.ai Date 11-02-2016(A) (B)(C) (D)FIGURE 20.14 Two “living fossils” and their
extinct relatives. (A) The horseshoe crab Limulus
polyphemus, found today along the Atlantic
coast of North America, closely resembles fossils
(B) as far back as the Triassic. (C) Coelacanths are
lobe-finned fishes that originated in the Devo-
nian and were thought to have become extinct in
the Cretaceous, until the living species (Latimeria
chalumnae) was discovered in 1938. (D) An ex-
tinct coelacanth from the Jurassic period.20_EVOL4E_CH20.indd 531 3/22/17 1:44 PM