Evolution__3rd_Edition

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Figure 23.10
Species selection by
differences among lineages
in: (a) extinction rates, and
(b) speciation rates. There is a
trend over time toward more
species with large body size.
(a) Species with large body sizes
have lower extiction rates
(last longer) than species with
smaller body size. Speciation is
equally likely to produce a new
species with a smaller or a larger
body size than its ancestor; the
speciation rate is also constant
over time. (b) Species with
larger body sizes have higher
speciation rates than species
with smaller body sizes.
Speciation is equally likely to
produce a descendant species
with smaller or larger body
size than its ancestor. Each
species has the same longevity
(extinction rate). In both cases,
natural selection within the
lineage does not favor
individuals with a larger
body size. This is shown by
the “blown up” insets in the
center: within a species,
selection maybe stabilizing
or inoperative. The insets
are attached to (a) but they
implictly apply to (b) too.
(The figures have a punctuated
pattern of evolution, but
whether evolution is really
gradual or punctuated is
irrelevant in the theory of
species selection.)
CHAPTER 23 / Extinction and Radiation 665
23.6.3 When species selection operates, the factors that control
macroevolution differ from the factors that control
microevolution
The trend toward increasing numbers of snail species with direct development is an
example of what is sometimes called species selection. Species selection is a higher level
analog of normal natural selection within a population. Species selection means, other
things being equal, that those kinds of species that have lower extinction and higher
speciation rates will tend to increase in frequency over evolutionary time.
The key question, for determining whether a trend is caused by species selection, is
whether natural selection within a species is driving evolution in the direction of the
trend. Consider a trend toward increasing body size (Figure 23.10). If natural selection
within each species is stabilizing, but species in which body size is larger have lower
extinction rates, then the trend to larger body size is driven by species selection. If
natural selection within each species favors larger body size, then the trend is probably
driven by conventional natural selection. The question is difficult to study.
However, Alroy (1998) studied it for a trend to increasing body size in North American
fossil land mammals. He found that the trend, on average, could be accounted for by
increases within each lineage suggesting that species selection is at most a minor factor
in this case.
Species selection should not be confused with group selection (Section 11.2.5,
p. 303). Group selection aims to explain why individuals sacrifice themselves for the
Time
(a) Differential extinction rate
Character (e.g., body size)
Neutral
Stabilizing
selection
Number of offspring (fitness)
Number of offspring (fitness)
Character
Character
Time (b) Differential speciation rate
Character (e.g., body size)