Explain co-evolution with reference to parasites and their hosts. Mention any four special adaptive features evolved in parasites for their parasitic mode of life.

Co-evolution is a type of mutual relationship between two interacting organisms. In this type of relationship both the species are unable to survive in the absence of the other species. In co-evolution many parasites evolves to be the host-specific such that both the host and parasite co-evolve, i.e. if the host evolves mechanism for rejecting or resisting the parasite the parasite evolve mechanism to neutralise them such that it survive with the same host species.

A parasite organism gets diet and protection from other living organism called host. Parasite in turn harms the host. To counteract this harm the host evolve for a long period to protect itself from the parasite. After the host evolve to protect itself from the parasite, now the parasite starts evolving to find other way to derive nutrition from the host. In this way the cycle continues. This is how the relationship between parasite and host exists.

Special adaptive features evolved in parasites for their parasitic mode of life are as follows:

- Loss of unnecessary sense organs

- Presence of adhesive organs or suckers to cling to the host

- Loss of digestive system

- High reproductive capacity

- Loss of chlorophyll and leaves/foliage

- Resemblance of egg in case of blood parasitism