Host defenses and parasite counterdefenses both have costs

Parasites and hosts have such a powerful effect on each other that we might expect an ever-escalating “arms race” in which host resistance and parasite counterdefenses both get stronger and stronger over time.

Such trade-offs have been documented in a number of host-parasite systems, including Drosophila fruit flies and the parasitoid wasps that attack them (described in Concept 13.2). Alex Kraaijeveld and colleagues (2001) have shown that selection can increase both the frequency with which fruit fly hosts encapsulate wasp eggs (from 5% to 60% in five generations) and the ability of wasp eggs to avoid encapsulation (from 8% to 37% in ten generations). But they have also shown that there are costs to these defenses and counterdefenses. For example, fruit flies from lineages that can mount an encapsulation defense have lower larval survival rates when they compete for food with flies of the same species that cannot. Similarly, wasp eggs that avoid encapsulation by becoming embedded in host tissues take longer to hatch than do other eggs.

The evolutionary changes in host and parasite populations that we've discussed in this section reflect the profound effects these organisms have on each other. Next, we'll focus on some of the population consequences of host-parasite interactions.