its about the horses..?

Question

what mechanisms and modes of speciation may have allowed for the changes accounting for the changes in the species.

Answer 1

The main modes of speciation are allopatric, peripatric, parapetric, and sympatric.

Allopatric speciation occurs when two populations of the same species are isolated from each other by major geographical boundaries. This is the most likely mechanism for the speciation of the different horses and zebras.

Peripatric speciation occurs when a small population on the periphery of the range gets isolated from the main population. This may have occured for some species of horse, but it's unlikely due to the wide-ranging nature of horses.

Parapatric speciation occurs when a a population of a species occupies a new niche within the original range of the main population. As horses are fairly specialized for the wade-ranging herbivore niche, it's highly unlikely that they underwent this type of speciation.

Finally, sympatric speciation occurs when certain individuals within a population suddenly become a new species. This type of speciation is still being debated and would be extremely rare.

Most likely the original horse species was divided by major geographic boundaries such as river shifts, continental drift (for example the zebra lineage from the horse lineage), etc, and new species arose via allopatric speciation.

Now, most if not all of the breeds of domestic horse (e.g. arabian, quarter horse, thoroughbred, etc) are races that are being propagated through controlled breeding programs and so are not considered separate species.

Answer 2

Adaptations to terrain and environment ... as it is with all species. With human interaction and the demands we place on the species is all relevant. We breed for the traits we desire for our purposes.

Answer 3

Here is a very good website about precisely that.
Horses are probably the best fossil record we have.