Evolution__3rd_Edition

666 PART 5 / Macroevolution
Taxa with certain attributes tend to
proliferate over evolutionary time
The relation between extinction
rate and mode of development ...
good of the group (or species) they belong to, and we have seen that it is difficult for
adaptations of this sort to arise. In species selection, there is no question of individuals
using a disadvantageous developmental mode in order to boost the speciation rate of
their taxonomic groups. Direct and planktonic development are favored by natural
selection in different taxonomic groups for good ecological reasons within each
species: but they can then have different long-term consequences for radiation and
extinction. We have no reason to suppose that what is favored by the short-term process
of natural selection will always be the same as that which allows species to last a long
time or split at a high rate. Natural selection may favor adaptations within some species
that result in reduced long-term survival and adaptations that increase it in others.
Species selection is another example of a reason why macroevolution cannot simply
be extrapolated from microevolution (Section 18.8, p. 550). Within a species natural
selection favors one character in one species and another in a different species; but
species selection over long periods may cause the species with one of the characters to
proliferate, because of the character’s consequences for speciation or extinction rates.
This does not mean that the long-term process contradicts, or is incompatible with, the
short-term process, only that we cannot understand the long-term evolutionary pattern
by studying natural selection in the short-term alone and extrapolating it.
A similar conclusion can be drawn from the argument about niches. Again,
macroevolution cannot simply be predicted from microevolution. A microevolutionary
study would reveal how natural selection was favoring various characters in
the stickleback populations, according to the aquatic environments they were occupying.
The key to macroevolution is the persistency of the niches over time, and that is
irrelevant to the short-term process of natural selection and to investigations of it.
(Natural selection does not favor one adaptation over another because it allows the
organisms to occupy a longer lasting niche.) Thus additional factors beside those
studied in the short term come to matter when we try to understand evolutionary
phenomena on the grand scale.
23.6.4 Forms of species selection may change during mass extinctions
We saw in Section 23.6.1 that, in the normal times of the late Cretaceous before the mass
extinction, the extinction rate was higher in species with direct than with planktonic
development. Jablonski (1986) found similar relations for the other two variables: taxa
that contained more species and that had broader geographic ranges had lower extinction
rates than taxa with smaller ranges or with less species. He compared these results
with those from the Cretaceous–Tertiary mass extinction and found that at that time
two of the three correlations disappeared (Table 23.1). Species-rich taxa had the same
chance of extinction as species-poor ones, and planktonic species had the same chance
of extinction as directly developing ones. Only broad geographic range continued to be
associated with a lower extinction rate. The extinction seems to have been so massive as
to have taken out groups almost at random.
At any rate, the relations between the characters of a taxon and its extinction
probability were significantly altered. In normal times, planktonically developing
and species-rich taxa have lower probabilities of extinction than directly developing
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