If speciation occurred solely because of changes in chromosome counts between the original parents (who were 2n) and the offspring (who were 4n), would this be an example of speciation by natural selection?

Yes...This is an example of speciation by natural selection

Explanation:

As we know that natural selection always promote those evolutionary changes that are necessary. Changes that happen in chromosome number always promote with a specie that is closely related to the parent specie and produce fertile off-springs. These new generations are most adaptable and can survive in the environment more efficiently than the previous ones.

Additionally, other than natural selection, speciation can also happen as a result of random mutations, genetic drift and by means of artificial selection as well. But the natural likelihood of all these is probably low as compared to that of natural selection.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.