Enter this newly born chimera: a cycloptic cat. Scientists were dubious, naturally, at first but apparently:
…medical authorities have a name for the bizarre condition.
"Holoprosencephaly" causes facial deformities, according to the US National Institute for Neurological Disorders and Stroke.
In the worst cases, a single eye is located where the nose should be, according to the institute's Web site.
Admittedly, I can't see any great advantages in having one eye, hence why most creatures have two or more. However, this crazy kitten shows us that it is indeed possible for a significant physical change to occur over one generation (unless, of course, both parents are cycloptic).
Now, if this cat did have an advantage over regular cats, and it lived in a relatively small population (which are historically quite common due to various extinctions and geographical isolations), you would see the evolution of a new species. All you would need is a little time.