I won't make many comments here as the text is quite good. The intoduction includes a stunning apology. This is important because it is quite clear that in order to understand speciation, you need to have a good definition of species . Because of this, the concept of what a species is, is quite subjective and very confusing. Botanists are sorry, but that is the nature of the organisms called plants.
Speciation is an extension of evolution.
If a species has members that are evolving in response to different stimuli than other members, then it becomes likely that genetic diversity will at first increase but then mechanisms that improve the reproductive effort of the evolving members will ultimately select for reproductive isolation. By definition, then, the one species becomes two species and speciation has occurred. You should read pages 558-559 carefully, perhaps making an outline to understand some of the possible mechanisms that lead to speciation. Again, you should be familiar with these. The third species separated from common ancestors WAY WAY back in evolutionary time.