Evolution__3rd_Edition

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Figure 23.1
(a) The number of families of
mammals in North America
increased abruptly in the
Paleocene and Eocene, after
which it has remained constant.
(Alroy 1999 shows a similar
pattern for an updated,
taxonomically finer, dataset.)
(b) The number of families of
bivalves has increased steadily
through time. Redrawn, by
permission of the publisher,
from Stanley (1979). © 1979
WH Freeman & Company.
The Cambrian explosion may have
followed an adaptive breakthrough
Number of families
(a) Mammalia (b) Bivalvia
120
120
100
80
60
40
20
0
L. Cret. Pal. Eoc. Olig. Mio.
Pli. P.
100
80
60
40
20
0
CHAPTER 23 / Extinction and Radiation 645
Ord. Sil. Dev. Miss. Pen. Perm. Tri. Jur. Cret. Pal. Neog.
100 80 60 40 20 0 500 400 300 200 100 0
Time (Myr BP)
Time (Myr BP)
3. Replacement of competitors. One taxon may radiate if it is adaptively superior to
its competitors. The superior taxon will take over from the inferior one, which
will be driven extinct. The superior taxon may have become superior because of
environmental change, or because it evolved a new, superior adaptation. We look
at replacements in Section 23.7.
4. Adaptive breakthroughs. A taxon may evolve a new adaptation that allows it to outcompete
another taxon (see point 3 above), or may allow it to exploit a previously
unexploited resource. For instance, we saw in Sections 18.5 and 18.6 (pp. 538–42)
how plants and animals colonized the land as they evolved a set of appropriate
adaptations.
The colonization of land was somewhat analogous to the colonization of a new
island chain (point 1 above). However, we can distinguish between colonization of
an existing area, made possible by the evolution of a new adaptation, and colonization
of a new area that requires no new adaptation. The colonization of land required new
adaptations for support, respiration, and water retention. By contrast, the finch that
first colonized the Galápagos probably did not have to evolve a new adaptation before
it could colonize the island. After the colonization of land, plants and animals both
underwent adaptive radiations. These radiations were made possible by an adaptive
breakthrough.
The proliferation of animals with hard skeletons in the “Cambrian explosion”
(Section 18.4, p. 535) is one of the most important adaptive radiations in the history
of life. But the reason why it occurred remains uncertain. One hypothesis proposes
that predators evolved escalated skills around this time, making hard skeletons advantageous.
If so, the radiation may be an example of a replacement following changed
conditions (factor 3 in the list above). The radiation of animals with hard parts may
have occurred as they replaced soft-bodied predecessors.
Adaptive radiations can be understood in terms of Darwin’s principle of divergence
(Section 16.8, p. 487). Darwin was interested in why evolution usually shows a diverging,
tree-like pattern. He explained the pattern by competition. More similar forms
will compete more strongly than less similar forms, which tends to “push” species
apart during evolution. Species diverge to escape competition. Darwin’s principle of
divergence likely needs to be slightly modified to incorporate the modern theory of