Re: How do viruses change?

Any change in DNA is called a mutation. There are many different type of mutations. Single base changes such as an A (Adenosine) mutating to a G (Guanosine) are called "point mutations". Insertion or deletion of a few bases is another type of change. Duplication of a gene or gene region is also very common.

Mutations can be detrimental, in the most severe case the organism can't survive at all with the mutation and this is called a "lethal mutation". Lethal mutations are difficult to study, because the organism can't be grown to study it. There are other highly detrimental mutations that are only lethal in some condtions. For example, in fruit flies, there are mutations that are lethal at one temperature but not at another temperature.

Most mutations are "neutral" or close to neutral, providing neither any benefit or detriment to individuals carrying them. These are often called "variants" rather tham "mutations". For example we don't consider people with red hair to be "mutants", the various hair colors in humans are just neutral variations.

Mutations can be beneficial. They might allow an organism to grow faster, or in a new environment, or on a new food source.

Viruses, like any other biological life form, do mutate and evolve over time. Evolution is the end result of the mutations plus selection. Selection eliminates detrimental mutations and increases the number of individuals carrying the beneficial mutations, on average, over time.

Adaptation implies that an organism is facing a new environment. For example if an influenza A virus that has been spreading in pigs for the past 20 years infects a human, it will not survive and go on to infect more humans if it does not adapt to the human host. Influenza A viruses do switch hosts rather often. For example from infecting ducks to infecting chickens. Each individual virus has a very very low chance of doing this. But there are several billion viruses in each infected chicken, and several million infected chickens in the world each month. The population sizes are enormous. Likewise there are at least hundreds of millions of pigs, and over 6 billion humans, with hundreds of thousands to millions of infections each year.

When one individual chicken, pig or other animal gets infected with two different influenza A viruses at the same time, those two strains can trade genes with each other. This is probably what you have heard about "using existing genes in a different configuration". The flu viruses have 8 chromosomes, so when 2 viruses get together they can produce 64 different types of reassortment offspring.