microbes and viruses do this all the time, of course, but as their reproduction is asexual it is difficult to know if they are a species or not (one of the criteria being that a species can only breed with itself successfully). An example in nature is the london plane tree, which is a very succesful adaptation to city life. I believe that some of the species of galapagos tortoise have come into existence since records began too, but need to look this up.
Races of rat, such as the harvard rat, are probably now species due to their forced deviation from the norm for use as lab animals.