Microevolution is evolution on a small scale within a single population. That means narrowing our focus to one branch of the tree of life.
If you could zoom in on one branch of the tree of life
scale the insects, for example you
would see another phylogeny relating all the different insect lineages.
If you continue to zoom in, selecting the branch representing beetles,
you would see another phylogeny relating different beetle species. You
could continue zooming in
until you saw the relationships between beetle populations. Click on
the button below to see this in action!
But how do you know when you've gotten to the population level?
For animals, it's fairly easy to decide what a population is. It is a group of organisms that interbreed with each other that is, they all share a gene pool. So for our species of beetle, that might be a group of individuals that all live on a particular mountaintop and are potential mates for one another.
The potential to interbreed in nature defines the boundaries of a population.
Biologists who study evolution at this level define evolution as a change in gene frequency within a population.