The Fossil Record 87By the 1940s, studies of breeding and gene flow had shown that large populations are
resistant to evolutionary change, because the unusual genes of any new variant are quickly
diluted by interbreeding with the normal members of the rest of the large population. But
small populations can change dramatically in relatively short periods of time. For example,
when a population (or just a single pregnant female) reaches an isolated place like an island,
all of their rare genetic mutations will soon become dominant, because all of the island’s
inhabitants will be their descendants. This effect is not restricted to islands. Small popula-
tions on the mainland can be genetically distinct if they do not interbreed with other popula-
tions. Religious sects such as the Amish in Pennsylvania, for example, have high frequencies
of unusual genes, because they are highly inbred and very few non-Amish convert and
marry into the Amish population to replenish the gene pool.
From these studies, Mayr (1942) concluded that small isolated populations on the fringe
of the main population (peripheral isolates) would be the likeliest source of new species. Their
ranges are usually separated from the main population (allopatric), usually by water or
mountain barriers. Once these populations have become distinct, they can then come into
contact with the main population (becoming sympatric), but they will no longer be able to
interbreed. This, in a nutshell, is the allopatric speciation model. By the mid-1950s, it was
widely accepted among nearly all biologists (with modifications as more exceptions were
encountered), and soon they began to modify their definition of species to incorporate the
interbreeding criterion.
Surprisingly, paleontologists seemed to be unaware of the implications of the allo-
patric speciation model, perhaps because apparently very few of them thought about
fossils in a biological way or kept up with the literature on modern speciation theory.
It was not until almost 30 years later in 1972 that two young scientists, Niles Eldredge
of the American Museum of Natural History in New York and Stephen Jay Gould of
Harvard University, first integrated modern concepts of biological species with the fossil
record. If the allopatric speciation model applied to the fossil record, then we should
not expect to see speciation in the fossils from the main population. Instead, speciation
should occur in small, peripherally isolated populations that have little chance of being
fossilized. In addition, all of the data from biology showed that this process of speciation
typically takes place in tens to hundreds to thousands of years, which is a geologic instant
as far as paleontologists are concerned. The age difference between two bedding planes
is often many thousands of years. Thus we would not expect to see the gradual transi-
tions between species preserved very often; instead, we expect to see new species when
they immigrate back into the main population after their isolation and speciation events.
In other words, they would suddenly appear in the fossil record. Once they were estab-
lished, speciation theory would predict that the main population would remain stable and
not change gradually through time but that new species would continually arise on the
periphery and migrate back to the homeland. Eldredge and Gould (1972) called their idea
punctuated equilibrium, because the fossil record seem to show species stability without
change (equilibrium or stasis) except when it is punctuated by the arrival of new species
from elsewhere (fig. 3.11B).
When the original Eldredge and Gould (1972) paper appeared, it caused a storm of
controversy in paleontology. Gradualism was a deeply embedded concept, and many pale-
ontologists had been trying to study it for their entire careers. Example after example of
possible gradualism was raised, but they all suffered from some problem in the analysis or