126 CHAPTER 6 | Macroevolution and the Early PrimatesAll working evolutionary scientists—including Gould,
the champion of the punctuated equilibrium model—
recognize the importance of both rapid change and grad-
ual Darwinian processes. Gould describes Darwinian
evolution as variational change that occurs
by the twofold process of producing copious and
undirected variation within a population and then
passing along only a biased (selected) portion of
this variation to the next generation. In this man-
ner, the variation within a population at any mo-
ment can be converted into differences in mean
values (average size, average braininess) among
successive populations through time.^1
He states that this kind of change is unsettling because
it is not predictable and does not proceed according to
simple natural laws such as gravity. Instead,the sensible and explainable but quite unpredictable
nature of the outcome (dependent upon complex
and contingent changes in local environments), the
nonprogressive character of the alteration (adaptive
only to these unpredictable local circumstances and
not inevitably building a “better” organism in any
cosmic or general sense)—flow from the variational
basis of natural selection.^2Genetic mechanisms underlie both rapid and gradual
changes because mutations can have small or large effects.
It is particularly interesting to see how molecular genet-
ics supports Darwinian evolutionary change. For example,
the tailoring of beak size and shape to diet among finches
on the Galapagos Islands, in the Pacific Ocean west of
Ecuador, constituted Darwin’s classic example of natural
selection (Figure 6.2). Recently scientists identified two
proteins along with the underlying genes that control beak
shape and size in birds. It is all the more impressive that
Darwin was able to make his inferences about natural se-
lection without the benefit of molecular genetics.
A fundamental puzzle in the fossil record is that scientists
have not been able to pinpoint the precise moment when
variational change leads to the formation of a new species.
More recent populations may appear sufficiently changed
from ancestral populations to be called different species. The
difficulty arises because, given a reasonably good fossil re-
cord, one species will appear to grade into the other with-
out a clear break. This gradual directional change over time,
called anagenesis, can occur within a single line, without
any evident branching (see Figure 6.1). Speciation is inferred
as organisms take on a different appearance over time.humans and chimps. Scientists have discovered certain
key genes called homeobox genes that are responsible for
large-scale effects on the growth and development of the
organism. If a new body plan happens to be adaptive, nat-
ural selection will maintain this new form for long periods
of time rather than promoting change.
Paleontologists Stephen Jay Gould and Niles Eldred
proposed that speciation occurs in a pattern of punctuated
equilibria—the alternation between periods of rapid specia-
tion and times of stability. Often this conception of evolution-
ary change is contrasted with speciation through adaptation,
sometimes known as Darwinian gradualism. A close look at
genetic mechanisms and the fossil record indicates that both
models of evolutionary change are important.Sometimes mutations in a single gene can cause reorganization of
an organism’s body plan. Here the “bithorax” homeobox gene has
caused this fruit fly to have two thoraxes and two sets of wings. An-
other homeobox gene, “antennepedia,” caused legs to develop in the
place of antennae on the heads of fruit flies.© David Scharf/Photo Researchers, Inc.
homeobox gene A gene responsible for large-scale effects
on growth and development that are frequently responsible for
major reorganization of body plans in organisms.
punctuated equilibria A model of macroevolutionary
change that suggests evolution occurs via long periods of stabil-
ity or stasis punctuated by periods of rapid change.
anagenesis A sustained directional shift in a population’s av-
erage characteristics.(^1) Gould, S. J. (2000). What does the dreaded “E” word mean anyway?
Natural History 109 (1), 34–36.
(^2) Ibid.