So there is still plenty of room left for new parasite species. Another way of course that parasites can be acquired is by host capture, when the parasite transfers from one host to another one in the same ecosystem. Since parasites can effect the reproductive success of their hosts, either by removing resources or by ultimately killing the host, parasites can exert an influence on host evolution. We will talk about the influence of hosts on parasite evolution in a later lecture. So parasites can influence host evolution.
Animals show a high degree of protein polymorphism.
That is, if you take virtually any protein you find that there are a large number of different isoforms. These isoforms differ by perhaps just a few amino acid residues, different individuals have different isoforms, but the isoforms seem to be biologically identical. One suggestion is that most of this polymorphism represents neutral mutations. The proteins biological function is not altered by these mutations so they are neither selected for nor selected against. An alternative explanation is that protein polymorphism is a host response to parasitism.