Ending my previous post with a quote from Alice in Wonderland was a trifle unfair on my taxonomist colleagues but it does help make the point that taxonomy – the classification of living organisms – is largely an edifice constructed by humans. Within this edifice, there is one level – the species – that should be unambiguous: members of the same species are able to share genetic information amongst themselves. If two organisms cannot reproduce and produce fertile offspring, they don’t belong to the same species. Full stop. You should, in other words, be able to define any species “beyond reasonable doubt”. The reality for the freshwater algae is, as we have seen in earlier posts, that this is not always practicable as many can thrive without needing to undergo sexual reproduction. Taxonomy then becomes a process of assembling information about properties that can be seen or measured leading to a species concept that is based on the “balance of evidence”. Of course, shared properties implies shared genes which, in turn, implies a shared heritage, so we should expect the “balance of evidence” approach to give us the right answer most of the time. My point is that we need to remember the limitations.
In the previous post, I discussed two examples where people generally agreed (on balance of evidence) that the organisms each represented distinct species but disagreed on the genus to which each belonged. The problem we face is that the “beyond reasonable doubt” criterion only applies to species and every higher level taxonomic category – genus, family, order, class etc – are defined entirely by “balance of evidence”. New approaches based on genetic differences between species may help us but the problem lies as much with the definition of terms such as “genus” as with our ability to allocate each species to the correct genus. Ironically, it is the higher levels of classifications (e.g. “class”) that may gain the most added clarity from genetic methods, as these can show distinct differences where other forms of evidence are confused by a mixture of convergent evolution and our own pre-conceptions, formed from the biases inherent in the older literature (see reference below).
Postscript: I had planned to make some analogies between the reluctance of many algae to have sex with the love life of giant pandas, who also seem to prefer celibacy to passion. However, as I was writing the piece, news broke that the female giant panda in Edinburgh Zoo might be pregnant. That this was achieved by artificial insemination rather than waiting for Yang Guang to do his stuff is, perhaps, a useful lesson for those of us who study algae…
Sims, P.A., Mann, D.G. & Medlin, L.K. (2006). Evolution of the diatoms: insights from fossil, biological and molecular data. Phycologia 45: 361-402.