What Darwin did was recognize that this within population process due to human manipulation was a model of selection in the natural world. On average, if a population is not increasing in size, then each individual leaves one descendent. If some individuals have variations that better enable them to survive and reproduce, then they will leave more offspring. Contrarily, individuals that have less good variations will leave fewer or no offspring. This process he called Natural Selection, to distinguish it from domestication, or Artificial Selection.
Microevolution is evolution within a species.
But the organisms that fill these niches are related not to each other but to other organisms on the same continent.