Evolution__3rd_Edition

602 PART 5 / Macroevolution
Apparent punctuations may be due
to...
. . . taxonomic artifacts ...
. . . or to ecophenotypic switches
methods discussed in Chapter 14 have been used for that kind of research. Instead we
can concentrate here on the empirical question of what pattern of evolution is observed
during speciation. Does the fossil record show new species evolving suddenly, or gradually
with many intermediate stages?
21.4 What is the evidence for punctuated equilibrium and
for phyletic gradualism?
21.4.1 A satisfactory test requires a complete stratigraphic record
and biometrical evidence
In the fossil record, one species is often observed to be suddenly replaced by another.
New species are rarely observed to evolve smoothly from their ancestors. However,
these observations do not strongly count in favor of the theory of punctuated equilibrium.
The fossil record is incomplete and the punctuated pattern will therefore appear
in most fossil samples whether the underlying evolutionary pattern is gradual or punctuated.
Two crucial conditions must be met by any test of the ideas. One is that the
stratigraphic sequence should be relatively complete (that is, sediments should
have been laid down fairly continuously). The other is that the evidence should be
biometrical, not taxonomic.
Taxonomic evidence alone is inconclusive, because taxonomic categories such as
species are discrete entities. The forms in a lineage will necessarily jump from being
members of species A at one point to being members of species B at another point
whether evolution in the lineage is sudden or gradual. Taxonomists, quite rightly,
include a range of forms within a single species, and the observation that a single
species persists for a certain amount of time tells us nothing about whether its morphology
is changing gradually, or is constant in form. Thus, we need measurements
of the forms in a population over time, to see whether the average changes suddenly
or gradually.
It also helps to know whether any changes in a population are genetic. In some
species, individuals can grow up with distinct forms, depending on the environmental
conditions in which they develop. These changes in development are called “ecophenotypic
switches.” The phenotype switches from one form to another, depending on the
environment; these switches are not genetic, evolutionary events. The theory of punctuated
equilibrium is an evolutionary theory, and needs to be tested with evolutionary
data. With fossils, we cannot be sure that any observed change in a population is not an
ecophenotypic switch. However, we can at least avoid evidence in which the morphological
change looks like the kind of change that can be induced in modern species by
changes in the environmental conditions. Fryer et al. (1985) discuss ecophenotypic
switches in snails and how they may have contaminated some research on punctuated
equilibrium.
The question is not simply whether “either” punctuated equilibrium “or” phyletic
gradualism is right. The two theories represent extreme points in two continuous
dimensions: the pattern of evolution itself in any one lineage, and the relative frequency
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