Evolution__3rd_Edition

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Macroevolution may be due to
extrapolated microevolution
The origin of mammals is an
example
CHAPTER 18 / The History of Life 551
largest scale. However, some biologists have argued that micro- and macroevolution
proceed by distinct processes. Then the terms are not arbitrary: microevolution would
refer to evolutionary phenomena driven by one set of processes and macroevolution to
the evolutionary phenomena driven by a different set of processes.
We can ask, for any macroevolutionary phenomenon, whether it can be explained by
microevolutionary processes that persist for a long time. That is, we can ask whether
macroevolution is due to “extrapolated” microevolution. In this chapter we have
looked at two major transitions in detail: the origin of mammals and the origin of
humans. Some might question whether the past 4 million years of human evolution
really amount to a macroevolutionary event. The origin of mammals, however, is
unambiguously an example of macroevolution.
Two important points can be made about the origin of mammals. First, the changes
from reptilian to mammalian characters evolved in gradual stages. Sidor & Hopson
(1998) looked at the rate of evolution in mammal-like reptiles quantitatively. They
measured the number of character changes per time unit and found not only that
mammals evolved in many stages but also that the rate of morphological evolution was
approximately constant over the 100 million-year period. The second is that the largescale
differences between mammals and reptiles concern adaptations. The mammals
have a high-energy, high metabolic rate kind of physiology, with locomotory adaptations
for rapid movements (upright rather than sprawling gait) and adaptations for
powerful and efficient feeding (the mammalian teeth and jaw articulation). These are
surely adaptive changes, which would have been brought about by natural selection.
The general evolutionary model suggested by the mammal-like reptiles, therefore,
is one of cumulative action of natural selection over a long (100 million year) period.
The accumulation of many small-scale changes resulted in the large-scale change from
reptile to mammal. The theory of mammalian origins is therefore an extrapolative
theory. A similar conclusion could be drawn about the origin of humans, and of terrestrial
plants and vertebrates. In these example, macroevolution proceeds by the same
process a natural selection and adaptive improvement a as has been observed within
species and at speciation; but the process is operating over a much longer period.
The extrapolative model is not the only model for the evolution of major groups, but
it is the most important one and the only one that can be illustrated with detailed
fossil evidence.
Figure 18.14 illustrates, as a theoretical alternative, how the origin of higher taxa
might not be extrapolative. Higher taxa might originate when some rare process of
large-scale change came into operation. Then the macroevolutionary event would not
be extrapolatable from the normal processes of microevolution. No evidence exists
for the process of Figure 18.14b, and it is unlikely in theory (Section 10.5.1, p. 266).
However, the origins of many higher taxa have been little studied, and some biologists
argue that some higher taxa may have originated by exceptional, revolutionary
processes.
The two views in Figure 18.14 are only two of the possible relations between
microevolution and macroevolution. Macroevolution might also be unextrapolatable
from microevolution, not because their driving processes differ, but because the
species that evolve into higher taxa are a non-random subset in some way. For example,
Jablonski & Bottjer (1990) argued that major evolutionary break-throughs more often