Evolution__3rd_Edition

552 PART 5 / Macroevolution
Figure 18.14
The origin of higher taxa
could theoretically be by (a)
extrapolated microevolution
over long time periods, or (b)
a distinct process that does not
operate in microevolution. The
two ideas are here illustrated for
the evolution of mammals from
reptiles. (a) and (b) are not the
only two possible relations
between microevolution and
macroevolution. See also
Figure 1.7 (p. 14) for another
way of imagining (b).
But some features of
macroevolution are not predictable
from microevolution
Form
(a) Extrapolation
Reptiles
Time
Mammals
(b) Distinct process
occur in taxa living at the poles than in taxa living at the equator. Kemp (1999) argued
that, in the origin of mammals, it was usually a small carnivorous taxa that gave rise to
the next major radiation. Thus, at each stage (Figure 18.10) a variety of forms evolved a
large and small herbivores, small carnivores, and others. We might expect that sometimes
the next major radiation would begin with a large herbivore, and sometimes with
a small carnivore. But in fact small carnivores are disproportionately represented.
If Kemp, Jablonski, and Bottjer, are right, macroevolution is not simply extrapolated
microevolution. At any one time, natural selection will be favoring a variety of adaptations
in different lineages a tropical adaptations in tropical species, polar adaptations
in polar species. Something about the polar adaptation makes them more likely to contribute
to macroevolutionary change. That something, whatever it is, cannot be seen
simply by studying microevolution.
The theory of macroevolution in Figure 18.14b is controversial. If correct, it would
challenge some deep tenets of neo-Darwinism. But the general idea that macroevolution
is not simply predictable from microevolution need not be controversial. Kemp,
Jablonski, and Bottjer’s arguments are orthodox enough. In the other chapters of Part 5
we shall look at several macroevolutionary phenomena, and reflect on their conceptual
relation with microevolution. In some cases, macroevolution will likely be extrapolatable
from microevolution; in other cases macroevolution will likely not be extrapolatable
from microevolution. In this chapter, we have seen that the origin of higher taxa
can at least be mainly understood as the evolution of adaptation by natural selection
over the long term.
Form
Reptiles
Time
Mammals
..