Evolution of parasite life history

Life history theory

In general, life history refers to the "choices" (not really choices, but evolved strategies) that an organism makes about the timing of growth, reproduction, and death. Individual life histories are interesting of themselves, and also underlie demography and population dynamics, which we will study later. The microevolutionary struggles of individuals for fitness, and possibly the selection between lineages for survival, determines the life-history patterns we see among taxa.

Applications: conservation biologists routinely use life history and demography to figure out the best ways to conserve species (sea turtles); to eliminate pest species; and to understand how species will evolve in response to human harvesting pressure (marine fisheries). People haven't actually done this for parasites (mostly they're just trying to find chemical targets that work, and hope that parasites don't evolve too much in response), but they could ...

As always in this class, we can look both at the life histories of parasites themselves, and how they have evolved to fit the constraints of a parasitic life cycle, and at how the life cycles of hosts are affected by parasites. First, however, we'll discuss life history in general. Classical life history traits are:

Obviously, these traits are all tied together, by physiological correlations (e.g. large size is correlated with high fecundity and longevity) and tradeoffs (fecundity vs longevity, size at maturity vs timing of maturity, etc.). The correlations and tradeoffs depend on the detailed physiology of organisms and on their environment (in a nutrient-rich environment, potential growth rates will be higher and so the tradeoff between early maturity and large size at maturity will be weaker). In any case, organisms will evolve to maximize their fitness in a particular environment.
RVx = mx + ¥
å
 t = x+1 
 mt Surv(x,t)
That is, total reproductive value is the current reproductive output, m, plus (potential) future reproductive output times the probability of surviving that long. Tradeoffs come in the tradeoff between current reproduction and future reproduction and survivorship, or indirectly between current reproduction and allocation to other traits (e.g. size) that will change future survivorship and reproduction.

r/K selection

One classical paradigm for how all those different life-history characters are connected is the r-K tradeoff.
Begon, Harper and Townshend give these connections between the various traits of (in this case) r-selected organisms, which are all ultimately driven by the fluctuating nature of the environment.

How would you think about controlling r vs K-selected organisms?

Parasites are often thought to be purely r-selected. In particular, it was originally thought that parasites were unusually small and fecund. The real story is, of course, more complicated.

Body size

Non-parasitic lineages typically obey Cope's Rule: lineages increase in body size through evolutionary time. (Q: why?) (But there is a tendency for species body-size distributions to be right-skewed (Fowler and MacMahon 1982, Blackburn and Gaston 1994a): how does this happen? Suggestion: within-lineage evolution for larger size, but between-lineage selection through higher speciation rates in smaller animals (smaller = quicker life cycle = faster evolutionary rates?) and possibly more evolutionary inertia (ditto).)

Do parasites obey Cope's Rule? The typical assumption is that parasites evolve to be smaller, but as Poulin's discussion suggests this is really caused by a confusion between the fact that parasites are small (they have to be, so they can live in or on their hosts) and the possibility that they have gotten smaller. Small size is a necessary pre-adaptation for parasitism (as long as we are sticking to the "close association" part of the definition of parasitism): we wouldn't expect to see elephants becoming a parasitic lineage ...

Changes in body size

Poulin looks at the body-size distributions of various families of parasites. He is comparing within families, which takes care of part of the problem with phylogenetic contrasts, but does not correct (e.g.) for the possibility that smaller species speciate more rapidly. Also, there is probably detection bias: smaller species are harder to discover. Nevertheless, just looking at body-size distributions for nematodes, copepods, and isopods, shows that free-living stages are smaller than or about the same size as parasites of invertebrates, and typically smaller than parasites of vertebrates.

We have to be careful interpreting these pictures:

Ecological correlates of body size

Fecundity

Correlations and suites of characters

Skorping et al 1991: Morand 1996:

Host life-history responses to parasites

Just as a final reminder, there's a strong possibility that parasites can affect the life histories of their hosts. Here's the abstract from a recent study of mosquitoes infected by microsporidian parasites:
Interactions between host and parasite life history traits : we showed that females of C. pipiens increase their developmental rate and thus pupate earlier when infected by the mirosporidian parasite V. culicis. This decrease of the developmental time, in agreement with theoretical predictions, comes at the cost of reduced adult size and hence reduced fecundity (Agnew et al. 1999). Because the reaction is manifested by the infected individuals themselves, this life-history adjustment reflects phenotypic plasticity.

Agnew, P., Bedhomme, S., Haussy, C. and Michalakis, Y. 1999. Age and size at maturity of the mosquito Culex pipiens infected by the microsporidian parasite Vavraia culicis. Proc. R. Soc. Lond. B (in press).

What does the existence of plasticity say about the evolutionary history and ecological prevalence of the mosquito-microsporidian association?