If you’re at all interested in topics of speciation, biodiversity, and the number of species on Earth, you should check out this blog post and discussion at Jerry Coyne’s blog Why Evolution is True on How many species are there?:
Since I work on speciation, I’m often asked how many species there are on Earth. You’d think that we have a pretty good idea of this, but we don’t. Most species of viruses and bacteria—indeed, if there are such things as viral and bacterial “species”—can’t be easily seen in the field or grown in the lab, and most other species are nematodes or insects hidden in the forests of the tropics.
Some of the comments are potentially very helpful for curiosity-seekers who might want a crash-course in these topics. I’ve quoted a few below the fold. In any event, it’s amazing just to see such a number. It’s truly a testament to just how vast biological diversity is. Even for a single small plot of land in your local park, far more vertebrates and angiosperms are there or pass through than 99% of people realize, and this pales in comparison next to the number of invertebrates, fungi, and microbes.
So maybe someone can explain this to me… I recognize that the term “species” has always been pretty arbitrary, but at least in the case of sexual organisms, there is an objective referent (can they/do they reproduce and make viable offspring). If one wanted, one could even conceivably come up with an objective metric, e.g. if two chosen populations of organisms were to reside in the same geographical area, in order to call them two separate species then the percentage of successful matings resulting in viable offspring that had one parent from each population would have to be below a certain threshold (ignore the implausibility of ever measuring this — my point is merely that an objective metric is possible in principle, and to use that as evidence that species classification in sexual organisms is at least partially objective)
But what the hell criteria do you use in asexual organisms? A certain amount of DNA divergence? A judgment call on morphological or behavioral differences?
I just don’t understand how one could even begin to say that this asexually reproducing bacteria is of species A, and this other similar one is of species B, though in the same genus. It doesn’t just seem partially arbitrary, it seems completely arbitrary.
“There’s a possibly apocryphal story about biologist J. B. S. Haldane, who was once asked what one could infer about the nature of the Creator from his creation. “An inordinate fondness for beetles,” Haldane supposedly replied.”