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Evolution__3rd_Edition

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..<br />

Virulence depends on ...<br />

. . . the relatedness between<br />

parasites within a host, ...<br />

. . . and whether transmission is<br />

vertical or horizontal<br />

CHAPTER 22 / Coevolution 627<br />

Natural selection can favor higher or lower virulence according to the transmission mode<br />

of the parasite, and other factors<br />

One idea about how natural selection will work on virulence is that it will usually act to<br />

reduce it. Parasites depend on their hosts, and if they kill their hosts they will soon be<br />

dead too. For this reason, parasites (arguably) might evolve to keep their hosts alive.<br />

The objection to this argument, and the reason why it is almost universally rejected by<br />

evolutionary biologists, is that it is group selectionist (Section 11.2.5, p. 301). Although<br />

a parasite species has a long-term interest in not destroying the resource it lives off,<br />

natural selection on individual parasites will favor those parasites that reproduce themselves<br />

in the greatest numbers over those that restrain themselves in the interest of preserving<br />

their hosts. The short-term individual advantage of greater reproduction will<br />

usually outweigh any long-term group or species advantage of reproductive restraint.<br />

The modern theory of virulence looks at other factors. One is the number of parasites<br />

that infect a host. If the host is infected by one parasite, all the parasitic individuals<br />

will be the offspring of the original colonizer and they will all be genetically related<br />

brothers and sisters. Kin selection (Section 11.2.4, p. 298) will then operate to reduce<br />

any selfish proliferation within the host. If the host has suffered multiple infections, by<br />

contrast, the parasites will be unrelated. Natural selection will favor individual parasites<br />

that can consume as much of the host as possible, as fast as possible, before any of the<br />

other parasites take advantage of the resource. Virulence will increase. If an individual<br />

restrains itself to preserve the host, other parasites will step in to take it over.<br />

With multiple infections, evolution towards more virulent parasites can occur even<br />

within a single host, if the generation time of the parasites is short relative to the host’s.<br />

There is abundant evidence that more virulent strains can evolve by competition<br />

between parasites within the host (Ebert 1998). In all, we can predict that diseases arising<br />

from single infections will have lower virulence than diseases arising from multiple<br />

infections.<br />

A second factor is whether there is vertical or horizontal transmission of the parasites<br />

between hosts. In an external parasite, transmission may mean the movement of an<br />

adult parasite that has been living off one individual host on to another host. In internal<br />

parasites it typically means the movement of the offspring of parasites living inside one<br />

host on to another host. In vertical transmission a parasite transfers from its host to the<br />

offspring of that host; this can be done by a variety of mechanisms a by the mother’s<br />

milk, or simply by jumping from host parent to host offspring when the two are near<br />

each other, or inside the gamete. In horizontal transmission, the parasite transfers<br />

between unrelated hosts, not particularly from parent to offspring, and this may be<br />

done through breathing, or by a vector such as a biting insect, or by copulation of one<br />

host with another. Some parasites are transmitted vertically, others horizontally: what<br />

consequence does this have for the evolution of virulence? A vertically transmitted<br />

parasite requires its host to reproduce to provide resources for itself or its immediate<br />

offspring, whereas horizontally transmitted parasites have no such requirement.<br />

Consider the success of a more and a less virulent strain of parasite in the two cases. A<br />

vertically transmitted parasite experiences a trade-off between making more offspring<br />

and the success of those offspring. A parasite that reproduces more will be more<br />

virulent as it uses up more of the host; but it will reduce the host’s reproduction. The

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