I agree with duga and would argue further that it actually holds up well for every monophyly, potentially every taxonomic level and some new ones we don't have yet. If you did a genetic analysis of different populations of a species and created a cladogram, you could find that each population forms a monophyletic group, even if there is gene flow. It's a sign that the populations are diverging, they are on different evolutionary paths - although that wouldn't mean they won't converge again some time in the future.
Similarly, you could do it on a larger scale and say that wasps have taken a different evolutionary path from beetles, which is basically what taxonomy does already.
Then you can have a new way of defining species not yet discussed in this thread based on genetic analysis. At the moment, results vary with markers used, but imagine a future where it's easy to do whole-genome sequencing and you had a sequencer where you could just put in some rat, cat and dog DNA. They should come out as separate monophylies for you to base species on which is actually a proper result and not just something theorized, so it has something going for it. It would require some equipment we don't have yet and a helluva lot of processing power if you are to compare whole genome sequences of many species, but it might still be just a matter of time.
It would still run into the problem of changes between many species being transitional though and where to draw the line is a problem. Rats, cats and dogs are easy, but what about the brassicas? Still sounds like it could be an effective way of defining species to me.
|