Evolution__3rd_Edition

..
Time
Punctuated
equilibrium
extreme
the evidence supports a pluralistic
view...
Character
Phyletic
gradualism
extreme
CHAPTER 21 / Rates of Evolution 603
Figure 21.6
Punctuated equilibrium and phyletic gradualism are extremes of
a continuum. Even here, the theories are simplified, for instance
phyletic gradualism may not proceed in a straight line, but may
contain reversals, as in Figure 21.4 (see Sheldon 1996).
of the patterns in different lineages, can occupy any point between these extremes.
Thus, according to the theory of punctuated equilibrium, the majority (perhaps more
than 90%) of evolutionary lineages should show a punctuated pattern whereas a
phyletic gradualist might claim the opposite. Nature itself could lie anywhere between
the two. Likewise there are plenty of patterns between the punctuated and gradual types
of change (Figure 21.6) and nature could show any of them. Punctuated equilibrium
and phyletic gradualism are not the only alternatives to be tested between. Research
aims to find out what the frequencies of the different patterns are.
Moreover, the two extreme positions are not schools of thought that are being advocated
by two opposed camps of evolutionary biologists. They are disembodied theories,
not positions that large numbers of people are committed to. Some individual paleobiologists
do think that the majority of cases fit the punctuational pattern; but the same
cannot be said for phyletic gradualism. It is even possible that no phyletic gradualists
exist (for reasons explained in Box 21.1).
However, the question of what the relative frequencies are of sudden and gradual
evolution during speciation merits an answer in itself. Eldredge & Gould (1972) posed
this question, and they have stimulated a major research program in the past 25 years.
They have also inspired paleontologists to collect data to new standards. The number of
biometrical studies, using relatively complete stratigraphic sequences, is growing, but is
not large. A review by Erwin & Anstey (1995) found some examples of gradual evolution,
some of punctuated equilibrium, and some with a mix of the two. Jackson &
Cheetham (1999) found that the majority (29 of 31 studies) of the evidence that they
surveyed fitted punctuated equilibrium. Here we look at only two examples, to illustrate
two of the patterns and the kinds of evidence that are available.
21.4.2 Caribbean bryozoans from the Upper Miocene and Lower
Pliocene show a punctuated equilibrial pattern of evolution
Cheetham (1986) studied in detail the evolution of a group of the sessile aquatic
invertebrates called Bryozoa (also called Polyzoa). His study included species in the
genus Metrarabdotus. Some members of the genus are alive in the seas today and they
also have an extensive fossil record. He worked on fossils dug up in the Dominican
Republic, which are the remains of animals that once lived in the Caribbean seas. The
main samples of fossils in the study date from the Upper Miocene and Lower Pliocene
(8–3.5 Myr BP), but some other species extend the age range.