Complex organisms can also drastically change from point mutations, although such changes are more likely to kill the organism as they grow more complex. Viruses are so incredibly simple and make so many copies that this doesn't matter.

Many organisms can hybridize, which can make drastic changes with much less chance of fatal errors. Plants especially like this; see farmed maize vs wild maize or the entire brassica genus.

Viruses also hybridize though, and can do so much more drastically. Most of the critical genes are in the host, so virus genomes are free to do whatever, and because they highjack other genomes a very small change can radically alter their behavior.