How do we define a "new" species.

Yes, it has the same DNA "they passed on", but not the "same" DNA as it parents, otherwise there would be no genetic difference that would result in the major morphology difference - the tail. Inherited DNA contains some mistakes (mutations) not present in the parental DNA - if a chimp was born with a tail it would likely be due to a mutation not present in its parents' DNA, since chimps don't normally have tails. I'm not sure what you mean by "generation gap", but a distinction between two species is a failure to interbreed under normal (wild) conditions. Thus, if tailed and normal chimps interbreed, they are one species. If the tail presents some barrier to mating between tailed and normal chimps, even if just behavioral, then a "new species" has likely formed.

Really - you mistake wolves for dogs? But in any case, this emphasizes the issue of simply using "looks-like" or morphology-based species definitions. Many organisms have evolved to look exactly like another (mimicry), but an organism and its mimic could never produce offspring due to DNA-level reproductively incompatibilities. In other cases there are species that look quite different and but are still capable of mating and producing healthy offspring despite their overt morphological differences. Many crosses between two different species produce sterile offspring. I have seen some species definitions that reflect this by specifying the production of fertile offspring.

Generally it is a DNA-level issue. Offspring need to have a complete complement of all genes necessary for life; however, if you have two species where the genes are differently distributed across the chromosomes, or a different number of chromosomes, the necessary complement of genes is not going to make it into the offspring even if the mismatched parents manage to produce a fertilized egg, and it will likely die in utero.(Please let me know if that makes sense.)In many cases protein-level incompatibilities between sperm and egg also prevent fertilization.

As you state later, this is quite incorrect. There have been some cases where males and females of the same species were once identified as separate species because they looked so entirely different from one another - that's differences within a single species - until they were seen successfully mating.Here is one of my favorite examples of "don't judge a species by its cover": These are eight different cichlid fish species from the rift lakes of Africa. Using a morphology-based model to predict relatedness and reproductive success, you would likely guess that related/compatible species were organized by rows. However, the opposite is the case - the columns represent closely related species, even though they look different and have different modes of feeding. In captivity, species within the same column can mate with one another, even though they look quite different. However, the species are incapable of mating between columns, even with species that share their morphology (facing one another in rows in the figure).The image was taken from a nice paper on jaw evolution in these cichlids - the full text is free here.

I'm not disagreeing with you regarding dogs as ring species; but ring species are an interesting case because extinction of certain intermediate subspecies would result in a speciation event, without any change in the genetics or location of the two "new" species.In other words, if a worldwide cult killed off all dogs except St.Bernards and Chihuahuas, we would have two dog species instead of one, even though the genetic makeup of the St.Bernards and Chihuahuas would remain unchanged (except for future potential gene flow).(Perhaps this is more obvious and less interesting than I first thought...)

I think somewhere above in this thread (or perhaps another) someone was arguing that selective breeding by humans has "limits", and could never result in formation of a new species. With the dog ring species we have produced new species, in a way.If a mass extinction of dogs occurs as I described above, we would have created a new species by selective breeding, and I suppose passed the "limit" that the anti-evolution people sometimes claim exist.