MACROEvOLuTiON: EvOLuTiON AbOvE THE SPECiES LEvEL 539
of a shark and a falcon, or even of a bird-hunting falcon and a rodent-hunting
hawk, since they are as adapted to different tasks as are flat-head and Phillips-
head screwdrivers.
If efficiency of design has increased, does that mean that organisms are more
highly adapted than in the past? Darwin imagined that if long-extinct species were
revived, they would lose in competition with today’s species. If he were right, we
might expect the fossil record to document many examples of competitive displace-
ment of early by later taxa—but this pattern is less common than replacement by later
taxa, well after the earlier ones became extinct (see Chapter 19).
We might suppose that species longevity would be a measure of increase in
adaptedness, but environments are almost ceaselessly changing, and natural
selection does not imbue a species with insurance against future environmental
change. We have seen that in many clades, the age of a genus or family does not
influence its probability of extinction, implying that a lineage does not become
more extinction-resistant over time (see Fig ure 19.9).COMPLEXiTY John Maynard Smith and Eörs Szathmáry have pro-
posed a list of major transitions in the history of life, most of them
marked by increasing hierarchical organization (see Ta b l e 17. 2) [69].
That is, entities have emerged that consist of functionally integrated
associations of lower-level individuals [75]. The first cells arose from
compartments of replicating molecules; the eukaryotic cell evolved
from an association of prokaryotic cells; multicellular organisms
with different cell types evolved from unicellular ancestors that
formed clonal aggregations of undifferentiated cells. The highly
integrated colonies of a few kinds of multicellular organisms, among
them the social insects, certain social mammals, and humans (FIG-
URE 20.26), have been called the “pinnacles of social evolution”
[116]. The difficulty to be overcome in all these transitions was that
selection at the level of the component units (e.g., individual cells)
could threaten the integrity of the larger unit (e.g., multicellular
organism). In general, such conflict has been suppressed by develop-
ment through a stage (e.g., the unicellular egg) that establishes high
relatedness (and thus the power of kin selection) among the com-
ponent units (e.g., the genetic identity of the cells of a multicellular
organism) [69, 76]. Lineages of clonal multicellular organisms (such
as animals and brown algae) consistently are more complex—they
have more cell types—than nonclonal multicellular lineages (such
as cellular slime molds) [25]. Multicellular organisms have many
defenses against disruption by rogue cells [37].
The major changes in hierarchical organization represent only a
few evolutionary events, in which the great majority of lineages did
not participate, so this is not a universal trend. It is difficult to define,
measure, or compare complexity among very different organisms. The
anatomical complexity of Cambrian animals was arguably as great
as that of living forms. Certainly, complexity has increased in some
clades; for example, the number of types of appendages has increased
in many lineages of crustaceans. However, many characteristics have
evolved toward simplification or loss in innumerable clades (FIGURE
20.27) [67]. This is true of both morphology and behavior. For example,
eusociality has evolved many times in the Hymenoptera but has also
been frequently lost [14]. The advantage of eusociality, or probably the
advantage of any complex behavior, must depend on the environment,
and there is no guarantee that it will always increase.FIGURE 20.26 Two “pinnacles of social evolution” and
their technology. African termites of the genus Macro-
termes cooperate to build mounds in which a constant
temperature is maintained by air conditioning: cool air
flows into the base of the mound, passes through verti-
cal tunnels as it warms, and flows out at the top. Author
Futuyma, shown for scale, has been transported from
New York to Ethiopia by, and in every way depends on,
the technological products that social cooperation has
made possible. (Photo by D. J. Futuyma’s camera.)20_EVOL4E_CH20.indd 539 3/22/17 1:44 PM