ing disparities. Some branches of the evolutionary tree contain many species
either because new species form easily or because they are unusually resistant
to extinction once they arise. Jablonski calls these "species-rich
clades" as opposed to "species-poor clades," or branches
that never contain many species.
During normal times, species-rich clades tend to increase their numbers
of species continually--and to win increasing numerical advantage
over species-poor clades. The environments of normal times must encourage
either rapid speciation or persistence thereafter. But why, then, don't
species-rich clades take over the biosphere entirely? Jablonski finds
that these same species-rich clades fare worse than species-poor clades
during mass extinctions. The individual species in species-poor clades
have wider geographic ranges and broader ecological tolerances than the
narrow-niched taxa of species-rich clades. This geographic and ecological
breadth probably protects these species in extreme environments that mass
extinction must generate. These same features of breadth may cut down
their rate of speciation in normal times (fewer opportunities for isolation
and exploitation of new environments), thus rendering their groups species-poor.
This contrary behavior of species-rich clades in normal and catastrophic
times preserves a balance that permits both species-rich and species-poor
clades to flourish throughout life's history. More important in our context,
it emphasizes the qualitative difference between normal times and catastrophic
zaps. Mass extinctions are not simply more of the same. They affect various
elements of the biosphere in a distinctive manner, quite different from
the patterns of normal times.
As we survey the history of life since the inception of multicellular
complexity in Ediacaran times, one feature stands out as most puzzling--the
lack of clear order and progress through time among marine invertebrate
faunas. We can tell tales of improvement for some groups, but in honest
moments we must admit that the history of complex life is more a story
of multifarious variation about a set of basic designs than a saga of
accumulating excellence. The eyes of early trilobites, for example, have
never been exceeded for complexity or acuity by later arthropods. Why
do we fail to find this expected order?
Perhaps the expectation itself is faulty, a product of a pervasive,
progressivist bias in Western thought and never a prediction of evolutionary
theory. Yet if natural selection rules the world of life, we should see
some fitful accumulation of better and
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