To me, the concept of species is very muddled and mixed up. As a plant breeder, I am all the time making, or attempting to make inter-species crosses, and even entirely new species. I use all sorts of tricks to overcome hybridization barriers. As examples: Using artificial lighting to change what time of year a species flowers, so that it will be flowering at the same time as a different species. Cutting off the stigma that would normally reject pollen from a different species. Pollinating with mixed species pollen. Plain old making tens of thousands of crossing attempts to find the one in a thousand that works.
I love looking at the genetics, say of plants growing along the shore of a huge lake. Each plant can breed successfully with it's nearby kin, but by the time the species has spread to circumnavigate the lake, they may be incapable of reproducing with others of their same kind. So they are two separate species at that point, with lots of intermediates spread along the lake shore.
I would feel very comfortable calling Chihuahua and Great Dane separate species.
Here's an interesting article I found that discusses the genetics of social behavior in humans. I've noticed this sort of thing in breeds of dogs... Sure love the personality of a blue heeler for example, and every blue heeler has approximately the same personality. http://time.com/91081/what-science-says-about-race-and-genetics/
"The economic historian Gregory Clark has provided one by daring to look at a plausible yet unexamined possibility: that productivity increased because the nature of the people had changed." In other words, that their genetics changed which ushered in the industrial revolution.