Lecture VIII - Evolution of virulence continued


Back to the rabbit / myxoma virus from Lecture VII

What factors lead to the increase in the rabbit population following the population crash around 1951?

1) Rabbits evolved resistance

Test: Original 1950 strain reinjected into 1957 Aussie rabbits

Result: Rabbits more resistant.

2) Virus evolved lower virulence

Test: 1957 virus in Aussie rabbits injected into 1957 European rabbits

Result: Fewer rabbits perished --> virus less virulent


BUT: Parasites don't always evolve to be less virulent. Consider the sheep liver fluke.

Differential expression among host types

This process could lead to the extinction of ants, followed by the extinction of the pathogen. Selection acts on individuals.

Virulence depends on Transmission


Evolution of Virulence Graph (from: Ewald, P. (1987) Ann. NY Acad. Sci 503: 295-306)

Consider the case of vertical transmission in species with two sexes. If we assume the pathogen is maternally transmitted, clearly there would be selection on the pathogen to skew the sex ratio of its host. Those pathogens that eliminate male offspring in the early stages of development would leave more descendents than those that do not because the maternal resources formally tied up in making sons can be reallocated into making daughters. In the long run, ths may cause the extinction of the pathogen (i.e., if no males are produced, there can be no host offspring that carry the pathogen; both hosts and pathogens go extinct). Selection acts on individuals in the present time.


Key Concepts under development

a) Trade-off

example: increasing reproductive rate for parasite potentially leads to more propagules available for transmission, but also reduces host survival time

b) Stabilizing selection

trade-offs can lead to stabilizing selection.

To contrast stabilizing selection from directional selection, first revisit what directional selection is.

Assuming the presence of additive genetic variation within a population, directional selection will move the mean of the trait value distribution to the right.

Contrast the figure above with one depicting stabilizing selection

Stabilizing selection does not affect the mean of the trait value distribution (assuming completely symmetrical stabilizing selection).

Stabilizing selection can be thought of as a "selection-selection" balance. Like in mutation-selection balance, there is some intermediate point that is maintained by the complete balance of 2 opposing forces.


Another way to think about these two modes of selection is to consider a plot of trait value vs. relative fitness. Pure directional selection is non-linear. Pure, symmetric stabilizing selection is non-linear with an intermediate optimum.

For completeness, let's consider disruptive selection (the inverse of stabilizing selection)


c) Evolutionarily Stable Strategy (ESS)

An ESS is a point (or strategy) that when common cannot be invaded any other strategy in the population. No alternative strategy can increase in frequency when rare against an ESS

If we consider a figure of parasite growth rate vs. parasite relative fitness, the ESS the point where the first derivative is 0 and the second derivative is negative (peak of curve).

Consider the following figure. The green strategy cannot spread when rare against the common red strategy.

Conversely, the red strategy is not stable to invasion by the green strategy in this figure.


This approach (strategy thinking) assumes mutations are occasionally spun off (creating addtivie genetic variation within the population).


d) Correlated response to selection

We discussed this topic in the context of virulence and parasites. Consult figures and text in the Ebert (1998) paper.

Main lecture points of correlated evolution in SPEs:

1) there must be additive genetic variation within the population, otherwise there would be no response to selection.

2) Selection in the new host environment results in adaptation to that particular host environment (increase in parasite survival ability) and reduces the ability of the parasite to infect the old host (decrease in parasite survival ability). Here, the two traits are a) survival in new host and b) survival in the old host. Selection on one trait (survival in the new host) causes an indirect reponse in the other trait (survival in the old host).