magna_nz

As said as above, a case of redundancy. Same as asking "why do we have kidneys, instead of 1 when all we need is 1"? It's like a failsafe.

Depends by what you mean by "evolve". RNA Viruses, such as "HIV", are under selective pressures when in the human body (such as the immune system). When RNA Viruses enter a host cell, they use an enzyme called "reverse transcriptase" which turns their RNA into DNA to be inserted into the hosts genome, for later use and for trascription. Since, reverse transcriptase is an error-prone process, the virus, when replicated, has a slightly different genome to the original virus genome. This causes a lot of variants of HIV in the body, and since all of the variants STILL encode a pathogenic virus (rather than code something benign), I guess this could be classified as "evolving".