At least in terms of a back-of-the-envelope calculation, what is a believable range for the number of gene changes between parent and child species to represent speciation between species of megafauna?
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To paraphrase an old saying, gene changes should be weighed not counted. There are a plethora of types of gene changes, and they vary by orders of magnitude in their impact. In particular, which genes are affected has a huge impact. Some point mutations (change in a single or a few nucleotides) can be lethal. On the other hand there are trisomies where an entire chromosome is duplicated. These may cause severe health problems but don't result in a different species.
As is discussed in many other questions on this site, the definition of a species is fuzzy and context specific.