86 Evolution and the Fossil Record
Darwin’s friend and supporter Thomas Henry Huxley warned him that, “You have loaded
yourself with an unnecessary difficulty in adopting Natura non facit saltum [Nature does not
make leaps] so unreservedly.” To Huxley, gradualism was not a necessary part of evolution-
ary theory.
As the century after Darwin progressed, paleontologists set out to try to document what
Darwin expected. A few examples of apparently gradually evolving fossil sequences were
documented (fig. 3.11A) but generally not a lot were found. As we shall discuss in chapter 4,
however, evolutionary theory quickly became the domain of geneticists by the early and
middle twentieth century, and paleontologists were pushed into the background.
In the 1940s and 1950s, another development occurred in evolutionary biology: the
development of modern speciation theory. Darwin had assumed that all one needed to
explain the origin of species (as in his title) was the transformation of lineages. But by the
middle twentieth century, it was clear that the real issue in speciation was the splitting of
lineages into two or more new species. Field biologists who observed species interacting in
nature noticed that species are defined by reproductive isolation; that is, different species
cannot successfully interbreed. In particular, ornithologist and evolutionary biologist Ernst
Mayr (1942) found from studying birds in New Guinea that most species had distinct non-
overlapping geographical ranges, and islands often harbored their own species. From this,
he proposed the allopatric speciation model.
FrequencyPhyletic gradualismStructureTimePunctuated equilibriumFrequency StructureTime(A) (B)FIGURE 3.11. (A) The classic notion (inherited from Darwin) was that evolution should show gradual transfor-
mations of species (shown by bell-shaped frequency distributions) through time, or “phyletic gradualism.”
(B) Eldredge and Gould (1972) pointed out that according to Mayr’s allopatric speciation model, most
speciation should happen too rapidly to be seen in the fossil record. Instead, fossil samples will show
apparent abrupt speciation from populations outside the sampled area, followed by long periods of stability
or stasis.