Co-evolution can be defined as reciprocal adaptations in the two interacting organisms that brings about the evolutionary change in both of them. In terms of the relation of host and parasite, it can be explained as follows:
A parasite is an organism that is totally dependent on the host organism for its survival, but in doing so, it also harms the host. The host evolves over a long period of time to protect itself from parasite, while parasite evolves so that it can find another way to derive nutrition from the host and hence, the cycle continues.
Four special adaptive features evolved in parasites for their parasitic mode of life are as follows:
- Parasites have organs for attachment such as suckers that help them to firmly attach to the host body and derive nutrition from them. For example, hooks and suckers in Taenia solium.
- Parasites are covered by protective body covering, that is, cuticle (in case of Ascaris lumbricoides) and tegumen (in case of Taenia solium) to protect them from harmful effects of digestive enzymes of the host.
- All parasitic organisms are usually devoid of locomotory structures; they do not require to move in search of food.
- Parasitic organisms have high reproductive capacity to ensure the continuation of parasitic race.