Evolution__3rd_Edition

..
. . . or a static equilibrium ...
. . . or a dynamic equilibrium,
known as the Red Queen
The Red Queen may explain the
log-linear survivorship curves
But uncertainties remain
CHAPTER 22 / Coevolution 639
evolved to a set of optimal states and then simply stay there. This may be a common
kind of coevolution. If there is one optimum form for an organism to have in order to
compete with members of other species, its species will evolve to it and evolution will
come to a stop.
The other kind of equilibrium is dynamic. This is the Red Queen equilibrium.
Instead of evolving to an optimal state and then staying there, this result arises when
adaptive improvement is always possible, and the species continually evolves to attain
that improvement. Van Valen originally suggested that this mode of convolution
would explain the log linear survivorship curves he had documented. His name for it a
the Red Queen hypothesis a alludes to the Red Queen’s remark in Lewis Carroll’s Alice
Through the Looking Glass: “here, you see, it takes all the running you can do, to keep in
the same place.” The analogy for running is coevolutionary change. In the Red Queen
mode of coevolution, natural selection continually operates on each species to keep up
with improvements made by competing species; each species’ environment deteriorates
as its competitors evolve new adaptations. This deterioration is the cause of
extinctions in the model. On average, a group of competing species have balanced
levels of adaptation, and they all lag behind their best possible states. At any one time,
one species may experience some random run of bad reproductive luck, and go extinct.
Coevolution will result in a log linear survivorship curve if the rate of environmental
deterioration is roughly constant through time. If the species’ competitive environments
deteriorate in fits and starts, the survivorship curve will be non-linear.
Why should the rate of environmental deterioration be approximately constant?
Van Valen reasoned that it would follow from the zero sum nature of competitive ecological
interactions. The total resources available, he thought, will stay approximately
constant. If one species adaptively improves, it will temporarily at least be able to take
more of the resources and its population will expand. This increase will be experienced
by its competitors as an equivalent decrease in the resources available to them. The
selection pressure on them to improve will increase, by an amount proportional to the
loss in resources caused by the competitor’s improvement. They will then tend to
improve their competitive abilities, and make up the ground lost to the competitor.
The justification may be correct, but it is debatable. Resource levels, for example, have
probably changed through evolutionary time and competitors may not always compete
for a constant-sized pie. If resource levels increase, Van Valen’s argument might
predict that extinction rates would decrease, and the change in the resource level would
cause a change in the extinction rate.
In summary, the Red Queen mode is not the only possible form of evolution among
antagonistically coevolving species. Many species may in fact coevolve in Red Queen
mode, but it is not an automatic theoretical consequence of antagonistic coevolution.
An additional problem is that log linear survivorship curves like Figure 22.15 can arise
by processes other than Red Queen coevolution. They can even arise if extinction rates
vary in absolute time (McCune 1982).
Van Valen identified an important factual generalization in the log linearity of
taxonomic survivorship curves. He also put forward a plausible explanation for it, in
his Red Queen hypothesis. However, biologists remain uncertain both about how often
extinction rates are constant, and about how good an explanation Red Queen coevolution
provides for constant extinction rates. The Red Queen hypothesis continues to