Book
1942
Systematics and the Origin of Species
Ernst Mayr
Defines species by reproductive isolation rather than morphology, and argues geographic isolation of populations is the ordinary route to a new one.
No free copy is linked yet — expect to buy or borrow this one elsewhere.
Read these first
Not a difficulty warning — a route. These are in the library too.
Groundwork for
Works in the library that name this one as a prerequisite.
Filed under Evolutionary Biology in Biology.