Evolution__3rd_Edition

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8 If natural selection favors one form of a character in
one species and another form in another, and if the different
forms of the character cause different speciation
or extinction rates, then there may be a trend toward
more of the kind of species with higher speciation, or
lower extinction, rates. The process is called species
selection.
9 If the niches of some species last longer than others,
those species will have lower extinction rates. If some
niches are so positioned that new species can easily
evolve from them, then the species occupying them
will be more likely than average to give rise to new taxa.
10 Different kinds of species may suffer differentially
in mass extinctions. The relation between the characters
of taxa and their extinction rates changed between
the late Cretaceous and the Cretaceous–Tertiary mass
extinction.
11 Large-scale evolutionary replacements of one
taxon by another occur by either competitive or
Further reading
CHAPTER 23 / Extinction and Radiation 679
independent replacement. The frequencies of the taxa
through time provide a partial test between the two
explanations.
12 The global number of species since the Cambrian
may show a logistic increase up to the Permian followed
by a steady increase; or a persistent exponential
increase; or may have been constant. The different
results depend mainly on differing statistical corrections
to the observed number of fossil species through
time.
13 Various views are possible about the importance of
mass extinctions in the history of life. At one extreme,
mass extinctions may be a distinct kind of event and a
creative historic force, responsible for shaping many of
the observed changes in the fossil record. At the other
extreme, mass extinctions may differ little from extinctions
at other times, or may even be artifacts, and have
made little difference to the course of evolutionary
history.
Wilson (1992) is a book for a broad audience on biological diversity. It covers many
themes not covered in this chapter, such as the number of extinctions that modern
humans are causing. Gould’s Natural History column (1977b, 1980, 1983, 1985, 1991,
1993, 1996, 1998, 2000, 2002a) included a number of essays on extinction, particularly
mass extinction. Jablonski (1986, 2000) looks at the macroevolutionary significance of
mass extinctions. See also Gould (2002b). Magurran & May (1999) contains researchlevel
papers. Givnish & Sytsma (1997) is a multiauthor research-level book using
molecules to study adaptive ratiation.
Sereno (1999) compares the radiation of dinosaurs at the end of the Triassic with the
Tertiary radiation of mammals (see his fig. 1). The dinosaur radiation was much
slower, perhaps reflecting the less catastrophic extinction at the end of the Triassic than
the end of the Cretaceous a but comparative analysis of radiation rates is largely a
research problem for the future. See also Chapter 13 on character displacement and
Chapter 16 on divergence.
A further factor that causes extinction, perhaps particularly in plants, is that a species
increasingly hybridizes with other species as it becomes rare. Its genes then become
diluted out of existence. See Levin (2000) and his references.
For Sepkoski’s work, Sepkoski (1992) is a standard source and cites earlier papers.
But his database continued to be updated until his death in 1999, and Peters & Foote