Viruses do so by cross breeding with other viruses, they laterally transfer genetic material. It's just as simple as viruses that survive are ones that got lucky in how it changes.

An animal species survived because it developed a long nose and that's what was needed in an upcoming environmental shift. All the other species of its kind died because they didn't develop a long nose. So the long nosed species lives on.

Viruses change so quickly due to environmental changes so they reproduce a lot. The virus species that can survive in its environment or spread faster, get cell entry quicker is survival of the fittest.

The other viral mutations die out.