Page 137 - Introduction to Paleobiology and The Fossil Record

124 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

returned more or less to their pre-fl ooding showed stasis, associated either with gradual-
condition. So, they argue, this classic example ism (15 cases; 37%) or with punctuated pat-
of punctuated equilibrium might be better terns (26 cases; 63%). It seems clear then that
interpreted as an example of repeated climate stasis is common, and that had not been pre-
change and migration. The new study casts dicted from modern genetic studies.
serious doubt on a classic case of supposed Microfossil groups such as the single-celled
punctuated equilibrium, but does not, of foraminifera, radiolaria and diatoms (see pp.
course, reject the whole concept. 209, 211 and 229, respectively) commonly
show gradual patterns of evolution and
Consensus on speciation in the fossil record speciation. The microscopic skeletons of
pelagic (open ocean) plankton can often be
This debate might have led paleontologists to recovered in large numbers from sedimen-

despair of ever finding a convincing case to tary deposits that can be shown to have
assess the two models of species evolution. accumulated continuously over vast periods
Now, after more than 30 years of debate, and of time. A study by Sorhannus and collea-
hundreds of case studies, there seems to be a gues in 1998 on the diatom Rhizosolenia
consensus (Benton & Pearson 2001). Both (Box 5.3) is probably the most detailed
modes of evolution happen in different situa- recent work on speciation in planktonic
tions, punctuated equilibrium particularly organisms.
among sexually reproducing species that live In this case, speciation is evidently sympat-
in ever-changing environments where barriers ric (happening on the same spot), because the
may be established, and phyletic gradualism same splitting event is seen in most of the rock
is seen among asexual organisms, such as cores from around the equatorial belt of the
microorganisms that live in the surface waters Pacifi c. There is no evidence of an invasion of
of the oceans, where evolution is slow and one species from an isolated population else-
barriers non-existent. So, it seems that where; indeed, it is difficult to imagine where

Williamson (1981) mistook migrations for that population might have hidden and yet
punctuations, but doubtless his snails were remained viable. Second, it is clear that most
evolving by punctuational speciation, had the morphological evolution was not associated
evidence been clearer. with speciation, but occurred afterwards, over
The fossil record demonstrates the wide- about 500,000 years after the morphological
spread occurrence of stasis. In a review by distinction first becomes visible. Third, one of

Erwin and Anstey (1995) of 58 published the new biological species evolved more
studies on speciation patterns in the fossil rapidly than the other, becoming gradually
record, with organisms ranging from radio- smaller and evolving a markedly diminished
laria and foraminifera to ammonites and hyaline area, whereas the other retained a
mammals, and stratigraphic ages ranging morphology more like the ancestral species.
from the Cambrian to the Neogene, 41 (71%) Finally, the two species must have evolved

Box 5.3 Gradual speciation in radiolarians

Rhizosolenia is a planktonic diatom that occurs today in huge abundance in the highly productive

waters of the equatorial Pacific. The siliceous valves of this genus rain on to the seafl oor, where they
accumulate in thick piles, mixed with other types of sediment. The morphological evolution of Rhi-
zosolenia can be traced by sampling cores of this sediment, which have been taken in several places
in the equatorial current system. Relative depths within each core provide a relative chronology, and
this chronology can be tied to an absolute age scale using magnetic field reversals in the sediment.

Ulf Sorhannus and colleagues from the University of Pennsylvania used this technique to study several
million years’ worth of evolution of Rhizosolenia, which encompasses a well-marked speciation event
(Fig. 5.5).