Evolution__3rd_Edition

..
. . . is probably due to cospeciation
...
. . . as can be tested by molecular
clocks
Molecular clocks refute
cospeciation in primates and
lentiviruses
CHAPTER 22 / Coevolution 631
The main mirror-image pattern between the pocket gopher and lice phylogenies is
probably due to cospeciation. That is, a host species and its parasite species tend to split
at the same time. For example, if we look at the top of the figure on both sides there is a
split that produced the branches labeled E and F. The branch E on the host side probably
represents an ancestral pocket gopher that was parasitized by the lousy ancestor of
the four species in the branch E of the parasites. The ancestral gopher, and its ancestral
louse, species then split twice. The events down the E → C → A + B branch (moving up
the figure) look very like cospeciation.
Why should host and parasite speciate synchronously? Probably because the
same circumstances favor speciation in both groups. For instance, the ranges of the
two could be fragmented by some biogeographic factor, and the normal process of
allopatric speciation occur in both parasites and hosts. (If any factor divides the range
of the gophers, it will also divide the range of these lice, because the lice have limited
independent powers of dispersal.) The same conditions drive speciation in both parasite
and host and the result is cospeciation.
Figure 22.8b provides a stronger test of cospeciation. Hafner et al. used the estimated
number of changes in each branch as a molecular clock to estimate the time when the
branch originated. The clock runs faster in the lice, likely because their generation
times are shorter than their hosts (Section 7.4, p. 169). If there was real cospeciation,
the speciation events in host and parasite should have occurred simultaneously. Hafner
et al. used two molecular clocks, one for all the nucleotide substitutions and the other
for only the synonymous nucleotide changes. We know from Chapter 7 that synonymous
changes are more likely to be neutral and therefore probably provide a more
accurate clock. In both cases (Figure 22.8b) the points for the branch lengths cluster
around the line for simultaneity, but the fit is better for the synonymous change clock.
Figure 22.8b is good evidence that the host and parasite species tended to speciate at the
same time.
The importance of the molecular clock test is shown in the next example (Figure
22.9). The phylogenies of primates and the primate lentiviruses are near mirror
images. (The primate lentiviruses are the group that includes HIV. HIV and human
beings are excluded from Figure 22.9 for technical reasons, but HIV-1 came from
SIVcpz in chimpanzees and HIV-2 came from SIVsm in sooty mangabeys: see Section
15.10.2, p. 451.) In Figure 22.9, eight of the 11 splits are mirror images of each other in
the two phylogenies, suggesting only three host switches. We might, naively, deduce
that primate lentiviruses cospeciate with their primate hosts. However, a look at the
timescales at the foot of Figure 22.9 suggests that deduction is false. The viruses evolve
much faster than their hosts, and the split times for the viruses are only a few thousand
years, against a few million years in the host monkeys. Unless the molecular clock is
massively misleading, by three orders of magnitude, the cophylogenies here are not
evidence of cospeciation.
Why do the primate lentiviruses have a similar phylogeny to their hosts, despite the
huge difference in splitting times? The answer is uncertain. One possibility is that viruses
tend to switch between hosts that are phylogenetically closely related (Charleston &
Robertson 2002). The immune systems of chimpanzees and humans are probably
more similar than those of baboons and humans. A virus that is adapted to live in chimpanzees
can probably more easily switch to exploit humans than a virus that is adapted