Many types of viruses[1] are like this by virtue of their high replication rate and RNA's poor error correction while copying (in comparison to DNA). It's one of the reasons why anti-viral therapy can be so tricky in viruses like HIV, as it's such a rapidly moving target.
As as analogy to computer viruses and other malware, the effect is very much like the polymorphic program code found in some of these, albeit not going as far as to derive new functionality as biological mutations can, through the combination of mutation and selection.
