6.13 - The evolutionary race between hosts and pathogens
Hosts have defences, yet pathogens can sometimes avoid or survive them. You will connect these evasion mechanisms to natural selection, explain why selection can act in both populations, and distinguish evidence consistent with an evolutionary race from proof that every stage of one has occurred.
Selection acts on both populations
A host and its pathogen can each change the conditions under which the other survives and reproduces.
Coevolution
Evolutionary change in interacting populations in response to selection that they impose on one another.
Recall that natural selection requires variation, an inherited component of that variation, and differences in reproductive success. Mutations do not occur because an organism knows what it needs. Selection changes the relative contribution of existing variants to later generations; an individual does not deliberately redesign itself.
In a host population, some inherited variants may make infection less likely or reduce its harmful effects. If affected hosts leave fewer surviving offspring, a protective inherited variant can become more common. Survival matters through its effect on reproduction: a difference that never changes contribution to later generations does not by itself establish natural selection.
The changing host population creates a different environment for the pathogen. Among naturally occurring pathogen variants, those better able to persist despite a common host defence may leave more descendants. Their inherited features can consequently become more frequent. That change may then favour a different protective host variant. This reciprocal selection is the basis of the evolutionary-race explanation.
The word race is a metaphor. Neither population has a goal, and neither must continually become more complex, more harmful or universally successful. An advantage can depend on which host variants are common. Conditions and costs also matter: a protective response that damages host tissues, for example, need not be advantageous in every setting. Diversity can persist rather than one side winning permanently.
Evasion explains why a defence can be incomplete
An evasion mechanism changes how effectively a host defence recognises, reaches or destroys a pathogen.
The preceding lessons supply two useful examples. Normally, a macrophage encloses a bacterium in a phagosome, and lysosome fusion supplies destructive conditions and enzymes. M. tuberculosis can reduce normal phagosome maturation and fusion, allowing some engulfed bacteria to persist. Engulfment is therefore evidence of uptake, not a guarantee of destruction. This is one reason a functioning defence can be incomplete, rather than evidence that macrophages have no protective effect.
HIV provides a different persistence example. Viral DNA can remain integrated in a resting host cell without active particle production. Such a latent reservoir can persist even while immune responses and treatment control active replication. Persistence in a host cell differs from the whole bacterial cell remaining inside a phagosome. It does not mean every symptom-free person has no viral replication elsewhere.
A further general principle is antigenic variation: naturally occurring differences or changes in exposed antigens can affect recognition by antibodies or antigen-specific cells. A memory response selected against one antigenic form need not recognise another form equally well. This does not imply complete loss of every immune defence; different antibodies, T cells and innate responses may recognise other features. Nor does it mean an antibody instructs a pathogen to produce the required change.
| Defence relationship | How evasion can make it incomplete | What the observation alone does not establish |
|---|---|---|
| Engulfment followed by intracellular destruction | Survival when normal destructive processing is reduced | That every engulfed bacterium survives, or the historical sequence of evolutionary changes |
| Recognition of an antigenic form | A different form is recognised less effectively by that particular response | Escape from all immune responses or a deliberate change |
| Recognition of actively producing infected cells | A resting infected cell expresses little of the activity being targeted | Elimination of all host control or absence of replication elsewhere |
For an evolutionary explanation, connect the evasion feature to inherited variation and relative reproductive success. The presence of a useful feature alone does not show when it arose or reconstruct every previous host response.
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