Evolution__3rd_Edition

..
. . . disappear after the
measurement intervals are
corrected for
Phyletic gradualism ...
. . . and punctuated equilibrium are
contrasting theories
CHAPTER 21 / Rates of Evolution 599
vertebrate than the invertebrate measurements (compare the mean time intervals for
vertebrates and invertebrates in Table 21.1). When Gingerich corrected for the difference
in intervals (by extrapolation from Figure 21.3) he deduced that invertebrates
actually evolve faster than vertebrates. That particular correction may or may not
be appropriate, but it is advisable to look at the time intervals when comparing the
evolutionary rates of different lineages.
Rates of evolution can still be compared between different taxa, or between different
kinds of taxa, despite the problem of time intervals. However, the problem does need to
be taken into account. We shall concentrate on a question that should not be much
influenced by the difficulties implied by Gingerich’s result. It is also outstandingly the
most lively modern controversy about evolutionary rates: the theory of punctuated
equilibrium.
21.3 The theory of punctuated equilibrium applies the
theory of allopatric speciation to predict the pattern
of change in the fossil record
Eldredge & Gould (1972), in a famous essay, argued that paleontologists had misinterpreted
neo-Darwinism. The fossil record had posed an apparent problem for
Darwin because it does not show smooth evolutionary transitions. A common pattern
is for a species to appear suddenly, to persist for a period, and then to go extinct. A
related species may then arise, but with little sign of any transitional forms between the
putative ancestor and descendant. Since Darwin, many paleontologists have explained
this pattern by the incompleteness of the fossil record. If evolution was really gradual,
but most of the record has been lost, the result would be the jerky pattern that we observe.
Eldredge and Gould distinguished two extreme hypotheses about the pattern of
evolution (Figure 21.5). They named one of them phyletic gradualism, which states that
evolution has a fairly constant rate, that new species arise by the gradual transformation
of ancestral species, and that the rate of evolution during the origin of a new species is
much like that at any other time (Figure 21.5b).
They contrasted phyletic gradualism with their own preferred hypothesis, punctuated
equilibrium (Figure 21.5a). They used the standard theory of speciation a
allopatric speciation, which we looked at in Chapter 14 a to argue that the fossil record
should show a pattern different from phyletic gradualism. If new species arise allopatrically
and in small isolated populations then the fossil record at any one site may not
reveal the speciation event. If the site preserves the record of the ancestral species, the
descendant species will be evolving elsewhere. The newly evolving species will not be
preserved at the same site as its ancestor. The new species will only leave fossils at the
same site as its ancestor if it reinvades the same area. Reinvasion could happen if the
descendant either was outcompeting its ancestor or was sufficiently different and could
coexist ecologically. Either way, the new species would be fully formed by the time it
turned up as fossils in the same place as its ancestor. The transitional forms would be
unrecorded not because of the incompleteness of the fossil record at that site but
because the interesting evolution took place elsewhere. The reason why the transitional