Specification

What makes a species a species?
Are dog breeds different species?

There are many definitions of what kind of unit a species is (or should be). A common definition is that of a group of organisms capable of interbreeding and producing fertile offspring, and separated from other such groups with which interbreeding does not (normally) happen. Other definitions may focus on similarity of DNA or morphology. Some species are further subdivided into subspecies, and here also there is no close agreement on the criteria to be used.Wiki

Even closely related populations are normally considered different species if there is some kind of reproductive barrier between them. This could be a genetic barrier - incompatible chromosomes preventing offspring. It could be a physical barrier like a ocean or mountain range.
You could, using artificial insemination, inseminate a Chihuahua with mastiff semen but you might end up with an embryo so large it couldn't be carried to term. A male Chihuahua would not be able to make much progress with a female mastiff without some crampons or other climbing gear. Does this make them different species? Your call
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