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126 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
slightly different environmental tolerances, a process as species selection, then the species-
for although their geographic ranges overlap level characters must be distinct from the
for all their evolution, one of the two daugh- individual-level characters involved in natural
ter species is entirely absent in one of the selection. It is not enough to say, for example,
cores. that among African large cats, lions might
Sympatric speciation and gradual evolution survive certain kinds of competitive situations
are probably rarer among marine inver- because they are larger than the other hunters.
tebrates and continental vertebrates, where Being large is an individual-level character,
there are many more possibilities for the and selection for size is through natural selec-
establishment of physical barriers to inter- tion. Species-level characters must be irreduc-
breeding. Studies of lineage evolution among ible to the individual level.
marine invertebrates from shallow waters Possible species-level characters include the
suggest punctuated patterns of speciation. size of the geographic range of a species, the
Such studies are much harder to make than pattern of populations within the overall
those of deep-sea microfossils because conti- species’ range, characteristic levels of gene
nental shelf sediments accumulate sporadi- flow among the populations of a species, and
cally, and this makes it harder to acquire average species’ durations. Some studies have
information with high sampling precision. suggested that species-level characters of these
Nonetheless, immensely detailed studies have kinds may play a part in evolution. Geograph-
been carried out. For example, in long-term ically widespread species of gastropods, for
studies Alan Cheetham and Jeremy Jackson example, tend to have longer durations than
of the Smithsonian Institution have sampled more localized species, and hence can be said
various genera of bryozoans in the past 10 to survive longer because of a species-level
million years of sediments in the Caribbean, character. If species selection is a real force in
and their studies suggest punctuational pat- evolution, then Darwinian evolution would
terns of speciation (Box 5.4). have to be expanded to incorporate a hierar-
Current evidence suggests that Eldredge chy, or multilevel array, of processes.
and Gould (1972) were right to challenge the A possible resolution of this issue is the
assumption that evolution always had to be effect hypothesis of Vrba (1984). She argued
slow and gradual; in some cases it seems clear that some species-level characters may be
that species can split off rather rapidly, and reducible indirectly to the individual level.
that is entirely consistent with Darwinian evo- That is to say, a broadly based feature of the
lution. Paleontological studies have shown species actually depends on some other char-
that species often remain unchanged for long acter that is under the infl uence of natural
periods – the new phenomenon of stasis that selection. She gave an example from her own
had not been predicted from genetics. Asexual work on the evolution of antelope over the
planktonic microorganisms appear to speciate past 6 myr (Fig. 5.7). About 5 Ma antelopes
slowly, perhaps over intervals of 0.5–1 myr in branched into two groups, one consisting of
a gradualistic way, and sexually reproducing long-lived species that never became diverse,
animals that occupy divided and complex and the other of shorter-lived species that
habitats perhaps tend to speciate rapidly, in a radiated widely. Species’ duration in the fi rst
punctuated manner. group was 2–3 myr and total species diversity
through the Plio-Pleistocene was two; in the
second group species’ duration was 0.25–
Species selection
3 myr and 32 species evolved. Surely here, she
Steven Stanley (1975) argued that a punctua- argued, species selection was taking place: the
tional model of evolution could imply a dif- character of short species’ duration in the
ferent kind of process, termed by him species second group permitted great success, as mea-
selection, that occurred at the same time as, sured by overall species diversity. Vrba noted,
but separate from, natural selection. Stanley however, that the long-lived antelope had
envisaged a process that sorted species, and wide ecological preferences, while those in the
ensured that some parts of the tree of life second group were specialists. Hence, the
might diversify rapidly and others more whole pattern could be explained by natural
slowly. He emphasized that if there was such selection at the level of individual antelope,
