“You must be joking. Evolutionary biology is a key for understanding a host of phenomena, with quite substantial practical significance. E.g. drug resistance.”
How does speciation explain ‘drug resistance’?
More specifically, natural selection. Populations of microorganisms are not uniform. There is always some spectrum of properties. For example, certain virions of HIV may have a mutated protease. A lucky mutation (lucky for from the point of view of the virus, of course) makes a drug molecule incompatible with the receptor. Other viruses do not proliferate, but the lucky one does and starts a new population, passing on the favorable mutation. The effect of this mutation may be then amplified by subsequent mutations/natural selection.
BenKenobi: "How does speciation explain ‘drug resistance’?"
"Speciation" does not explain drug resistance.
Evolutionary processes like random mutations and natural selection (or in this example, human activity) do explain drug resistance.
A "species" is an artificial scientific construct -- falling between a breed or race, and a genus -- to help us understand how different biological groups relate to each other.
Typically, scientists define a "new species" as one that has changed so much it can no longer interbreed with its parent populations. Thus even vastly different dogs fall into just one species, while similar seeming horses, zebras and donkeys have multiple species.
For example, the biological classifications for humans are:
Drug resistence in bacteria is not known to have risen to the level of speciation, since genetically mutated bacteria resistant to drugs are assumed capable of interbreeding with original strands.
But the processes which cause drug resistance are the same processes which over long enough times can lead to a new species, genus, family, order, etc.
Examples of speciation witnessed in historical times have been linked to by other posters.
Evidences for past speciations are abundant in both the fossil records and DNA comparisons of similar living species.
To cite just one example, it is said that human and chimp DNA is something like 98% identical.
The fossil record shows a common ancestor around five million years ago.