Evolution__3rd_Edition

..
Virulence depends on ...
. . . the relatedness between
parasites within a host, ...
. . . and whether transmission is
vertical or horizontal
CHAPTER 22 / Coevolution 627
Natural selection can favor higher or lower virulence according to the transmission mode
of the parasite, and other factors
One idea about how natural selection will work on virulence is that it will usually act to
reduce it. Parasites depend on their hosts, and if they kill their hosts they will soon be
dead too. For this reason, parasites (arguably) might evolve to keep their hosts alive.
The objection to this argument, and the reason why it is almost universally rejected by
evolutionary biologists, is that it is group selectionist (Section 11.2.5, p. 301). Although
a parasite species has a long-term interest in not destroying the resource it lives off,
natural selection on individual parasites will favor those parasites that reproduce themselves
in the greatest numbers over those that restrain themselves in the interest of preserving
their hosts. The short-term individual advantage of greater reproduction will
usually outweigh any long-term group or species advantage of reproductive restraint.
The modern theory of virulence looks at other factors. One is the number of parasites
that infect a host. If the host is infected by one parasite, all the parasitic individuals
will be the offspring of the original colonizer and they will all be genetically related
brothers and sisters. Kin selection (Section 11.2.4, p. 298) will then operate to reduce
any selfish proliferation within the host. If the host has suffered multiple infections, by
contrast, the parasites will be unrelated. Natural selection will favor individual parasites
that can consume as much of the host as possible, as fast as possible, before any of the
other parasites take advantage of the resource. Virulence will increase. If an individual
restrains itself to preserve the host, other parasites will step in to take it over.
With multiple infections, evolution towards more virulent parasites can occur even
within a single host, if the generation time of the parasites is short relative to the host’s.
There is abundant evidence that more virulent strains can evolve by competition
between parasites within the host (Ebert 1998). In all, we can predict that diseases arising
from single infections will have lower virulence than diseases arising from multiple
infections.
A second factor is whether there is vertical or horizontal transmission of the parasites
between hosts. In an external parasite, transmission may mean the movement of an
adult parasite that has been living off one individual host on to another host. In internal
parasites it typically means the movement of the offspring of parasites living inside one
host on to another host. In vertical transmission a parasite transfers from its host to the
offspring of that host; this can be done by a variety of mechanisms a by the mother’s
milk, or simply by jumping from host parent to host offspring when the two are near
each other, or inside the gamete. In horizontal transmission, the parasite transfers
between unrelated hosts, not particularly from parent to offspring, and this may be
done through breathing, or by a vector such as a biting insect, or by copulation of one
host with another. Some parasites are transmitted vertically, others horizontally: what
consequence does this have for the evolution of virulence? A vertically transmitted
parasite requires its host to reproduce to provide resources for itself or its immediate
offspring, whereas horizontally transmitted parasites have no such requirement.
Consider the success of a more and a less virulent strain of parasite in the two cases. A
vertically transmitted parasite experiences a trade-off between making more offspring
and the success of those offspring. A parasite that reproduces more will be more
virulent as it uses up more of the host; but it will reduce the host’s reproduction. The