> However, humanity lives in an increasingly global environment, and natural selection works slowly - so no two generations will be able to draw a line saying "they're a different species,"

True, but what makes a species a species remains the same. Eventually our descendants wouldn't be able to mate with us (or would want to), and that perspective will mark the end of the human species.

> I think there will be a wide range in what is considered "human."

See above.