Coevolution: Host–Parasite


Hosts and their parasites have an intimate and antagonistic relationship; therefore, they are expected to play important roles in influencing each other's evolution. When a host evolves a defence against a parasite, then the parasite may evolve counterā€adaptations to its host's defence. Hostā€“parasite coevolution may have several possible outcomes, and, contrary to popular belief, does not necessarily lead to mutualism.

Keywords: infectious diseases; myxomatosis; virulence; immunology; HIV

Figure 1.

Gene‐for‐gene models of host–parasite coevolution. The haploid host has two alleles, A1 and A2, with frequencies of p and 1 − p, and the haploid parasite has two alleles, B1 and B2, with frequencies r and 1 − r. A1 hosts are resistant to B2, but susceptible to B1 parasites, whereas A2 hosts are resistant to B1 parasites and susceptible to B2 parasites. The arrows show the directions of evolutionary changes of host and parasite allelic frequencies. (a) When the parasite's genotypes are not highly specific, the result is a stable polymorphism. (b) When the parasite's genotypes are more specific, then the result is a cycle of fluctuating allelic frequencies. (c) When parasites do not evolve within their hosts, then this increases the amplitude of the fluctuations, evolving to a succession of states (moving from genetic fixation from one corner of the state space to the next). Adapted from Clarke and Maynard Smith .

Figure 2.

A computer simulation of host–parasite coevolution. (a) The diploid host has three resistance alleles at a locus (a1, a2, and a3), and (b) the haploid parasite has six alleles (b1b6). Each parasite strain is able to evade the defences of one of the host's genotypes (e.g. b1 can infect host a1a2). From Seger .

Figure 3.

The myxoma virus evolved from high to intermediate virulence over time (the range of virulence is the shaded area).

Figure 4.

Serial passage experiments typically show that virulence evolves and increases over time when a parasite is artificially transferred from one host to the next. From Ebert.

Figure 5.

Brood parasites, such as the cuckoo (Cuculus canorus), (a) eject one of their host's eggs from the nest and (b) replace it with an egg (bottom) that mimics the appearance of the host's eggs (top). (c) The parasitic young matures rapidly and ejects the host's eggs, and exploits its host's parental behaviour with vocal trickery using calls that sound remarkably like the whole brood of the host's chicks. (d) In a species in which the parasitic chick (left) does not eject its nestmates (right), it has evolved mouthparts that mimic the young of the host. From Krebs and Davies .

Figure 6.

The coevolutionary diversification of pocket gopher species and their parasitic chewing lice. Coexisting hosts and lice parasites (connected by dotted lines) are often congruent, but not always. The evolutionary congruencies indicate that cospeciation has occurred, whereas incongruencies indicate host switching. C, Cratogeomys; G, Geomys; O, Orthogeomys; P, Pappogeomys; T, Thomomys and Z, Zygogeomys.



Clarke B (1976) The ecological genetics of host–parasite relationship. In: Taylor A and Muller R (eds) Genetic Aspects of Host–Parasite Relationships, pp. 87–103. Oxford: Blackwell.

Dawkins R (1982) Host phenotype of parasite genes. In:The Extended Phenotype, p. 209. Oxford: Oxford University Press.

Ebert D (1998) Experimental evolution of parasites. Science 282: 1432–1435.

Ebert D and Hamilton WD (1996) Sex against virulence: the coevolution of parasitic diseases. Trends in Ecology and Evolution 11: 79–82.

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Frank SA (1996) Models of parasite virulence. Quarterly Review of Biology 71: 37–78.

Krebs JR and Davies NB (1987) An Introduction to Behavioral Ecology, 2nd edn. Oxford: Blackwell Sciences Publications.

Lively CM and Apanius V (1995) Genetic diversity in host–parasite interactions. In: Grenfell BT and Dobson AP (eds) Ecology of Infectious Diseases in Natural Populations, pp. 421–449. Cambridge: Cambridge University Press.

Maynard Smith J (1989) Evolutionary Genetics, p. 299. Oxford: Oxford University Press.

Page RDM and Hafner MS (1996) Molecular phylogenies and host–parasite cospeciation: gophers and lice as a model system. In: Harvey PH, Leigh Brown AJ, Maynard Smith J and Nee S (eds) New Uses for New Phylogenies. Oxford: Oxford University Press.

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Further Reading

Clayton DH and Moore J (1997) Host–Parasite Evolution: General Principles and Avian Models. Oxford: Oxford University Press.

Frank SA (1992) Models of plant–pathogen coevolution. Trends in Genetics 8: 213–219.

Krebs JR and Davies NB (1987) An Introduction to Behavioral Ecology, 2nd edn. Oxford: Blackwell Sciences Publications.

Nesse RM and Williams GC (1994) Why We Get Sick. The New Science of Darwinian Medicine. New York: Random House.

O'Neill SL, Hoffmann AA and Warren JH (eds) (1997) Influential Passengers – Inherited Microorganisms and Arthopod Evolution. Oxford: Oxford University Press.

Ridley M (1993) The Red Queen: Sex and the Evolution of Human Nature. New York: Macmillan.

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How to Cite close
Penn, Dustin J(Apr 2001) Coevolution: Host–Parasite. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001765]