Evolution__3rd_Edition

..
22Coevolution
C oevolution happens when two or more species influence
each other’s evolution. It is often invoked to explain
coadaptations between species, and we begin by considering
whether coadaptation provides evidence of coevolution.
Coevolution strictly speaking requires reciprocal influences
between species, but there is a related phenomenon, called
sequential evolution, in which changes in one species
influence the other but not the reverse. The chapter then
looks in turn at coevolution between flowering plants
and insects, between parasites and hosts, at antagonistic
coevolution in general, and the phenomenon of evolutionary
escalation. Finally, we look at the “Red Queen” mode of
coevolution. For plant–insect and parasite–host coevolution
we look at cophylogenies a in which the phylogenetic trees of
the two interacting taxa form mirror images. We consider the
evolution of virulence in parasitic diseases, including diseases
of humans. The Red Queen hypothesis suggests that species
continually evolve to maintain a level of adaptation against
competing species. Van Valen invented the hypothesis to
explain a general result he discovered in the fossil record:
the chance of extinction of the species in a taxonomic group
is independent of the age of the species. The status of the
Red Queen hypothesis is uncertain, not least because it is
difficult to test.