Evolution__3rd_Edition

638 PART 5 / Macroevolution
Number surviving
1,000
100
10
Osteichthyes (extinct genera)
All
Chondrostei + Holostei
Sarcopterygii
1
0 50 100
Time (Myr)
150
Number surviving
1,000
Figure 22.15
Three taxonomic survivorship curves that plot the number
of genera (for mammals and Osteichthyes) or families (for
reptiles) surviving for various durations in the fossil record.
Antagonistic coevolution can lead
to...
. . . the extinction of one or other
party ...
100
10
Reptilia (families)
1 0 100 200
Time (Myr)
Total extinct
Extinct except
Late Cretaceous
extinctions
Living
would be evolving to invest more heavily in armaments and defenses. Those armaments
and defenses would probably develop via some trade-off with other adaptations
(Section 10.7.5, p. 284). Heavily armed descendant species might then be more vulnerable
to environmental stresses than their lightly armed ancestors. Extinction rates might
increase over time. In fact that does not seem to happen. Van Valen’s result suggest
that no process leading to an increase or decrease in the chance of extinction generally
operates.
22.8 Antagonistic coevolution can have various forms,
including the Red Queen mode
Chapter 23 will look in detail at the factors causing extinctions. Here we concentrate on
only one factor: antagonistic coevolution. As ecological competitors, or parasites and
hosts, evolve against each other, if one competitor fails to evolve an adaptive improvement
to keep up with its antagonists, it may go extinct. If a host species evolves a new
kind of immunity, then if the parasite does not soon evolve a way of penetrating the
defense, it will go extinct. What is the pattern of extinction rates likely to be for this
process? Here is an analysis of the question, simplified from Stenseth & Maynard Smith
(1984).
Two species (A and B) that are undergoing antagonistic coevolution will have a
certain state of relative adaptation at any one time. One possibility is that one of the
species, such as A, has superior adaptations. In this case, species B is heading for extinction
unless it can soon evolve a better level of adaptation. Relations of this kind are
unstable. They cannot exist for long, as the adaptively inferior species will go extinct.
Alternatively, species A and B may be at some kind of equilibrium. We can distinguish
two kinds of equilibrium. One of them is static. The competing species have
Number surviving
1,000
100
10
1
Mammalia (major therian orders) (genera)
Extinct
Living
0 10 20 30
Time (Myr)
Note that the lines, with this logarithmic y-axis, are
approximately straight. Osteichthyes are bony fish and the
two groups drawn on that graph are subgroups of bony fish.
Redrawn, by permission of the publisher, from Van Valen (1973).
..