Speciation;
http://www.madsci.org/posts/archives/1997-12/875114237.Ev.r.html
Speciation occurs when the adaptations of a subset of organisms within a species isolate that subset genetically from the rest of the species. In other words, there are many adaptations which occur within species that simply extend the gene pool - it isn't until there are enough different adaptations to prevent a small group from reproducing with the rest of the species that that group becomes a separate species of its own. The easiest way, that I can think of, to explain this is to give some examples.
The first example of adaptation on the brink of speciation comes from work done by Don McPhail and
his colleagues at the
University of British Columbia. They examined the fish life in several small isolated lakes on Vancouver Island, and found that the
Sticklebacks in many of the lakes had adapted to two separate habitats. Those fish that swam near the surface, picking up free-swimming food were categorized as
limnetic, and those fish that stayed near the bottom eating material in the muck were categorized as
benthic. Since becoming isolated to the lakes, the limnetic fish had become thinner, smaller, and faster - adaptations that improve their ability to exploit their niche. In contrast, the benthic fish become larger, fatter, and slower, adapting to their niche. Now for the speciation... the two forms of sticklebacks can be mated to produce a viable and fertile hybrid (arguing against their being separate species), however the hybrid cannot compete for food in either niche so few reach sexual maturity in the wild; furthermore, the mating seasons in the lake depend on water temperature such that the limnetic form mate about a month before the benthic form, so crossbreeding rarely, if ever, occurs in the wild. So it becomes a point of semantics whether the limnetic and benthic forms in a given lake can be considered separate species.
http://www.talkorigins.org/faqs/speciation.html
Here is a short list of referenced speciation events. I picked four relatively well-known examples, from about a dozen that I had documented in materials that I have around my home. These are all common knowledge, and by no means do they encompass all or most of the available examples.
Example one:
Two strains of
Drosophila paulistorum developed hybrid sterility of male offspring between 1958 and 1963. Artificial selection induced strong intra-strain mating preferences.
(Test for speciation: sterile offspring and lack of interbreeding affinity.)
Dobzhansky, Th., and O. Pavlovsky, 1971. "An experimentally created incipient species of Drosophila",
Nature 23:289-292.
Example two:
Evidence that a species of fireweed formed by doubling of the chromosome count, from the original stock. (Note that polyploids are generally considered to be a separate "race" of the same species as the original stock, but they do meet the criteria which you suggested.)
(Test for speciation: cannot produce offspring with the original stock.)
Mosquin, T., 1967. "Evidence for autopolyploidy in
Epilobium angustifolium (Onaagraceae)",
Evolution 21:713-719
Example three:
Rapid speciation of the Faeroe Island house mouse, which occurred in less than 250 years after man brought the creature to the island.
(Test for speciation in this case is based on morphology. It is unlikely that forced breeding experiments have been performed with the parent stock.)
Stanley, S., 1979.
Macroevolution: Pattern and Process, San Francisco, W.H. Freeman and Company. p. 41
Example four:
Formation of five new species of cichlid fishes which formed since they were isolated less than 4000 years ago from the parent stock, Lake Nagubago.
(Test for speciation in this case is by morphology and lack of natural interbreeding. These fish have complex mating rituals and different coloration. While it might be possible that different species are inter-fertile, they cannot be convinced to mate.)
Mayr, E., 1970.
Populations, Species, and Evolution, Massachusetts, Harvard University Press. p. 348
