One word: "dogs".

(Are you skeptical that chihuahuas and huskies are the same species? Some dog breeds are incapable of mating successfully without artificial insemination, or reproducing effectively -- think of a great dane foetus gestating in a female chihuahua -- but within a small single-digit number of generations they can produce hybridised "mutts" that are interfertile. Similarly, the brassicae are pretty much the same thing.)

I noticed that your understanding of 'species' doesn't consider obligate parthenogenesis. There are, for example, some 50 species of lizard that only reproduce parthenogenetically. Under your definition, these are not a species because there is no mating. I submit that that is a much more critical shortcoming than a ring species.

Perhaps your objection is more based on a lack of knowledge than anything else?

Quoting now from Stephen J. Gould (1992):

> I do not say that these criteria are free from exceptions; nature is nothing if not a domain of exceptions, where an example against any clean generality can always be found. Some distinct populations of plants, for example, can and frequently do interbreed with others that ought to be separate species by all other standards. (This is why the classification of certain groups--the rhododendrons, for example--is such a mess.) But the criteria work in the vast majority of cases, including humans.

Since then we've learned a lot more about bacteria, which employ horizontal gene transfer. This completely breaks the traditional model.

Our own human cells can also pull off some odd tricks. See http://en.wikipedia.org/wiki/HeLa#New_species_proposal for one proposal for why HeLa cells should be considered its own species. (Note also the mention of special rules for an independent unicellular asexually reproducing species.)

Google is your friend.

FWIW, an awful lot of seed growing involves preventing unwanted crosses, but unless you were saving seeds from your garden you might overlook this.

As for overlooking - you're right, I did and should have thought of that before commenting. After a few attempts at saving seeds it became obvious to me that buying them each season was preferable.

But what I'll mention to you is: Have you not noticed how similar the taste is between some of these mentioned items?

An example of where this is useful is around Neanderthals. "Did they have language?", "Did they have burial with ceremony?" are questions where it's fine to have belief on the current evidence because there's not enough data to know the answer.