I think some people are confusing the terms species,adaptation and evolution.
The theme which runs across the theory of evolution is that DNA is mutable through
(1) meiosis(crossing over of homologous chromosomes)
(2) fertilization (different responses to the combinations of DNA from different individuals ie n + n'= n (dominant character of the two))
The important point is this environment is so influential that the resulting good combinations of genes survive while other that is with poor or unfit genetic make-up become extinct. Such gradual mutations which with different speeds over certain geological and biological time scales result in what we call the ORIGIN OF SPECIES.
Darwin did name his book "on the origin of species" but quite amazingly the exact answer was not discussed with great authority in his theory. He too believed in ACQUIRED CHARACTERISTICS ie adaptations acquired after birth in somatic rather than genetic make up.
However unlike Lamarck he didnt commit the mistake of making it the central theme. he insisted on the wonderful fossil record and some special mechanism through which 'the mysterious group of organisms(species)' arise.he however didnt explained that "mechanism" too beautifully.
By virtue of Mendelism and mendel's calssical genetics and other developments in palaentology(fossil study) and genetics in 20th century we got the theory named NEO DARWINISM which is roughly the sum of darwinism and calssical genetics(mendelism) and is an important part of the modern gentic knowledge.
strictly are defined as
" a group of organisms which can interbreed in nature and at the same time produce fertile offsprings(offsprings with the ability to reproduce themselves)"
MULE is what we get from the cross of a male Language Removed and female horse.
Mule cant reproduce furthur . Language Removed and horse are not the same SPECIES. I am not sure about wolf and dog whether they can produce a fertile offspring or not. But my definition is quite clear now.
I hope this post would have helped some people.