88 Evolution and the Fossil Record
the data, which Gould and Eldredge (1977) quickly pointed out. Eventually it became clear
that gradualism was extremely rare among multicellular animal fossils. (Microfossils, on
the other hand, show a lot of gradualism, but they are not strictly sexual but largely clonal
or asexual and so are not bound by the interbreeding criterion of Mayr’s allopatric specia-
tion theory.) Fossil species do show an incredible stability over many millions of years of
strata, which Gould and Eldredge (1977) called stasis. Some biologists tried to explain away
this stasis with mechanisms such as stabilizing selection (selection against the extremes of a
population, reinforcing the mean tendency), but this does not explain how some fossil popu-
lations persist unchanged through millions of years of well-documented climatic change
(surely, a strong selection pressure), as documented by Prothero and Heaton (1996), Prothero
(1999), and Prothero et al. (2012). As Gould (1980a, 2002) pointed out, the persistence of fos-
sil species through millions of years of intense selection pressure suggests that they are not
infinitely malleable by selection but instead have an integrity or some sort of internal homeo-
static mechanism that resists most external selection. This is a radical notion for evolution-
ary biology and is still hotly controversial. Most paleontologists argue that the fossil record
shows things that can’t be seen in fruit flies or living populations, but many biologists are
unconvinced that the fossil record can’t be explained by some neo-Darwinian mechanism
(see chapter 4).
But this was not as startling for paleontologists as it was for biologists. Even though
paleontologists had been trying to find Darwinian gradualism in the fossil record for a cen-
tury, it was the stasis and abrupt appearance of fossil species that made biostratigraphy
work so well. If everything evolved gradually, there would be a major problem of how to
break up a gradually transforming lineage into nonarbitrary species segments. As Gould
wrote, “All paleontologists knew that the practical world of fossil collecting rarely imposed
such a dilemma. The oldest truth of paleontology proclaimed that the vast majority of spe-
cies appear fully formed in the fossil record and do not change substantially during the long
period of their later existence (average durations for marine invertebrate species may be as
high as 5 to 10 million years). In other words, geologically abrupt appearance followed by
subsequent stability.” But instead of being an embarrassment of absence of evidence for evo-
lution, the prevalence of stasis is a powerful message for evolutionary biologists that there is
something to be explained that still has not been explained by experiments or observations
of living species. Gould (1993) put it this way:
Stasis, or nonchange, of most fossil species during their lengthy geological lifespan
was tacitly acknowledged by all paleontologists, but almost never studied explicitly
because prevailing theory treated stasis as uninteresting nonevidence for nonevolu-
tion. . . . The overwhelming prevalence of stasis became an embarrassing feature of
the fossil record, best left ignored as a manifestation of nothing (that is, nonevolution).We have seen that punctuated equilibrium is simply an application of modern biological
speciation theory to the fossil record, an application that happened to explain and highlight
the long-known fact of stasis in fossil species. This stasis, in turn, is now causing discomfort
among many evolutionary biologists, because there is not yet any good mechanism in neo-
Darwinian theory for it, suggesting we still have a lot to learn about evolution and specia-
tion. But this is a good thing! If we had all the answers, and paleontology provided no new or
interesting facts and ideas, science would be very boring.