There were two assumptions that I highlighted in the last lecture that I said that I'd follow up on today. The other had to do with the size of the population. In particular, whether it was a small population, or not. We can all recognize different species, most of the time. Most of the common organisms that we see are different enough that we are not likely to be confused.
Dogs are different than cats, for example.
Miniature poodles and great Danes, for example. They are the same species. This is a biological species definition. What evolutionary biologists focus on is the process of species developing these differences. Ultimately, all the differences have to be traced to allele changes.