Speciation by Punctuated Equilibrium
The Short Summary
A group of creatures gets isolated from the rest of their
species. They can evolve easily, because they are a small group.
Later, they spread and replace their parent species. Examples are
Charles Darwin wrote in 1859:
Only a small portion of the world has been geologically explored. Only
organic beings of certain classes can be preserved in a fossil
condition, at least in any great number. Widely ranging species vary
most, and varieties are often at first local, -- both causes rendering
the discovery of intermediate links less likely. Local varieties will
not spread into other and distant regions until they are considerably
modified and improved; and when they do spread, if discovered in a
geological formation, they will appear as if suddenly created there,
and will be simply classed as new species.
In 1972, Niles Eldredge and Stephen Jay Gould revived this idea, under
the name Punctuated Equilibrium. They agreed that transitional
fossils are plentiful, and that smooth
transitional sequences are sometimes found. However, they argued
that these are not as common as theory predicted. Instead, we often
see a species go on unchanged for a long time. And then the species
is replaced, without any transition, by a new species that looks like a
variation of the old one.
The Origin of Species, Chapter 14, p.439
Their explanation is that a group of creatures was cut off from the
rest of their species. Since the group probably lived in a small
inhospitable fringe area, they would be under selection
pressure. Being a small group, they were able to evolve fairly
quickly. Then, later, they spread, and replaced their parent species.
Now, imagine the fossil record. In the small fringe area, the
complete history might be recorded. In the much larger main region, we
would see the parent species, and then suddenly see a slightly
different species. The chances are very good that we will never happen
to dig for fossils in the small region. So, there's a reduced chance
of finding transitional fossils.
Is There Any Evidence For Punctuated Equilibrium?
Yes. Several examples of this exact scenario are known. For example,
there's a marine microfossil, a trilobite, a brachiopod, and some dinosaurs (including a Tyrannosaurus).
How Quickly Does This Happen?
There's no theoretical reason why it needs to take more than hundreds
or thousands of years. In practice, 500,000 years is still "suddenly",
when it comes to dinosaurs.
What Is The Mechanism Of Evolution In These Cases?
The theory of Punctuated Equilibrium does not say, and it shouldn't.
There are a number of known evolutionary mechanisms, such as the
Founder Effect, Natural
Selection, neutral drift, sexual selection, and so on.
Other mechanisms may be discovered in the future. There is no
particular reason to expect that cases of Punctuated Equilibrium must
all use the same mechanism. The point of the theory is only that
evolution is more likely to happen to small groups, isolated from the
homogenizing effect of the larger main group.
What Happens To The Parent Species?
That varies. If there was a mass extinction (or even a small one),
they may be gone, and the descendant species moves into the resulting
vacuum. Or, in the dinosaurs example, the
parents had all migrated elsewhere. In the microfossil example, the parent species
co-exists to this day with the new species. Or, the new species might
fight it out with the parent species. It depends on why this happened,
and on how different the child species is.
Is This Common? Why?
It seems to have happened a lot. For example, we have been learning
recently that the ocean has risen and fallen a great many times. Each
time it happened, it would fragment any wide-ranging species into a
bunch of little geographic areas. Later, when the ocean level changed
back, the fragments would try to spread back into the main area. This
would leave "punctuated equilibrium" in the fossil record.
Last modified: 1 October 1997
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