Evolution__3rd_Edition

632 PART 5 / Macroevolution
Figure 22.9
Phylogenies of primate hosts
and primate lentiviruses (the
group of viruses that includes
HIV, though HIV and humans
are not shown here). They are
approximate, but not perfect,
mirror images. The timescale
of the phylogenies is shown,
based on molecular clock
inferences. Note the different
timing of the splits in the two
taxa: the cophylogenetic
relations are not (if the
molecular clock is reliable)
due to cospeciation.
(Figure courtesy of
Dr D.L. Robertson.)
Some parasite phylogenies do not
match their hosts
Encephalization quotients ...
Primate host Lentivirus
Chimpanzee
Colobus
Mandrill
Drill
Sooty mangabey
Red-capped mangabey
Talapoin
Vervet
Grivet
L’Hoest
Sun-tailed
Sykes
25 20 15 10 5 0 0 5 10
Time (Myr) Time (kyr)
to exploit baboons. The splits in the lentivirus phylogeny then all represent host
switches. The match between phylogenies would not be due to cospeciation but to phylogenetically
constrained host switching. The influence of the host’s physiology, and
particularly its immune system, on host switching, could be analogous to the influence
of plant chemistry on insect evolution (Section 22.3.3 above).
However, the reason for the cophylogenies in Figure 22.9 is an unsolved research
problem. The main point of the example here is to show that cophylogenies alone
are not complete evidence for cospeciation. Evidence about the timing of the splits is
also needed, for instance from a molecular clock. For the gophers and lice, both phylogenetic
and molecular clock evidence support cospeciation. For the primates and
lentiviruses, the phylogenetic evidence is consistent with cospeciation but the molecular
clock evidence counts strongly against it.
Finally, some taxa of parasites and their hosts do not show cophylogenies. For
example, we looked at the phylogenies of pocket gophers and one group of parasitic
lice. These showed cophylogenies because the lice have limited powers of dispersal
independent of their hosts. Other taxa of lice that can move independently do not show
mirror-image phylogenies with their hosts (Timm 1983).
In summary, we have looked at three possible relations between the phylogenies
of parasites and hosts. One is that they have cophylogenies caused by cospeciation. A
second is that they have cophylogenies, but for some reason other than cospeciation.
A third is that they do not show cophylogenies. All three patterns can be found in different
examples.
22.6 Coevolution can proceed in an “arms race”
SIVcol
SIVrcm
SIVmnd-2
SIVdri
SIVcpz
SIVagmVER
SIVagmGRI
SIVsm
SIVlhoesti
SIVsun
SIVtal
SIVsyk
A graph of brain size against body size for many vertebrate species reveals that larger
vertebrates have larger brains (Figure 22.10). The brain size of a species can be expressed
relative to this general trend, as an encephalization quotient. The encephalization quotient
of a species is the ratio of its actual brain size to the brain size it would be expected
to have given its body size and the general trend in Figure 22.10. If its actual brain size is
..