Evolution__3rd_Edition

..
Taxonomic survivorship curves are
approximately log linear, ...
. . . implying that evolution is not
progressive
CHAPTER 22 / Coevolution 637
22.7 The probability that a species will go extinct is
approximately independent of how long it has existed
Now we are going to stay with the question of how influential coevolution has been in
the history of life, but shift the scale of the evidence up to a more abstract, and general,
level. We shall look at Van Valen’s (1973) work. Van Valen inferred, from the shape of
taxonomic survivorship curves, that macroevolution is shaped not only by coevolution,
but by a particular mode of coevolution, called the “Red Queen” mode. The kind
of escalatory coevolution we looked at in the previous section is (or can be) an example
of Red Queen coevolution. But before we come to the coevolutionary interpretation,
we must first look at the evidence from taxonomic survivorship curves.
We met taxonomic survivorship curves in Chapter 21 as a means of studying evolutionary
rates (Section 21.6, p. 609). Here we use them to study extinction rates. To plot
a taxonomic survivorship curve, take a higher taxonomic group, such as a family or
order, measure the duration in the fossil record of its member species, and plot the
number (or percent) of species that survive for each duration (Figure 21.12, p. 610). In
1973, Van Valen published a large study, based on measurements of the durations
of 24,000 taxa, but with one crucial difference from earlier studies a he plotted the
graphs after taking logarithms of the numbers surviving. He found that survivorship
tends to be approximately linear on the log scale; that is, survivorship is log linear
(Figure 22.15).
Van Valen’s result gave a new interest to survivorship curves in evolutionary theory.
With survivorship curves like Figure 21.12 on an arithmetic scale, it could be seen that
different taxa evolved (taxonomically) at different rates, and it was possible to argue
about why that should be so. But when it was noticed that the curves were linear on a
log scale, more interesting questions could be asked. It has been questioned just how
linear his results really are, and many of them are not for species, but for genera or even
families within a higher taxon. Moreover, some of the extinctions will almost certainly
have been pseudoextinctions, due to taxonomic division of a continuous lineage, rather
than the true extinction of a lineage (Box 23.1, p. 647). However, Van Valen’s results
provide strongly suggestive evidence of logarithmic linearity in taxonomic survivorship
curves. But what does the log linearity mean?
The log linearity of taxonomic survivorship curves means that species do not evolve
to become any better (or worse) at avoiding extinction and that the chance a species
will go extinct is independent of its age. Of the species that survived to a time t million
years after their origins, a certain proportion go extinct by time t + 1 million years; then,
of the survivors to time t + 1, the same proportion go extinct by time t + 2, and so on.
Species decay at an exponential rate, with a constant proportion of the survivors going
extinct in the next age unit. It could have been otherwise. If, for example, evolution is
progressive, the probability of extinction might decrease with time, as the level of adaptation
improves, so later species would last longer. For this reason, Van Valen’s result is an
important piece of evidence that evolution is not in general progressive.
Another theoretical possibility is that extinction rates might increase over time. For
instance, if evolution is escalatory, the level of individual adaptation in a species relative
to competitors may not change in any particular direction. However, all the species