In fact there are several different definitions used of species.
To start with the chimp-with-a-tail his DNA would NOT be the same as that governing his parents morphology. Amd in fact he would probably be different enough to be classed as a new species under the "morphological species concept". Which incidentally is the way fossil species are usually classified on the basis that morphology is all a fossil preserves (and not all of that).
By the definition more commonly used for living species capable of sexual reproduction (the "biolgical species concept") the chimpanzee-with-a-tail would be part of the same species. That concept is based on interfertility. Isolated populations can easily drift into becoming new species on this basis as in the example of the mosquitoes in the London Tube system.
Actually telling the differnece between a subspecies and a true species is quite difficult in the wild. It was only recently recognised that there were two species of African Elephant.
http://news.nationalgeographic.com/.../1217_leeelephant.html
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