Hang on, am I still on time with this? I made up my mind late about participating and have to start with a disclaimer: this is not going to be an in-depth discussion of the topic – just a collection of my rambling thoughts. Gaps are certain to occur. So…
…way back, when I started getting into Foraminifera, one of the things that really bugged me was how species were distinguished for ‘everyday analysis’. See, molecular systematics have only relatively recently helped shed some light on forams, with some surprising results. Before this, foraminiferal taxonomy was pretty much exclusively based on comparing external features of the tests, although some studies also considered test ultrastructure (this is still the case for palaeoceanographic analyses, for obvious reasons).
For most ecological studies, forams are still identified using more (electron microscopy) or less (drawings and light microscopic) sophisticated… pictures. Of course, hand-drawn pictures, prone to some degree of ‘interpretation’ by the scientist or artist – beautiful as they may be – have their own set of problems.. but that would be another post. On top of this, forams make life more difficult because their morphology (in this case the form of their tests) can vary with environmental conditions – for example, the tests of planktonic species can be left- or right-coiling depending on the characteristics of the surrounding seawater – if you’re into it, I was quite excited to find an introductory classroom activity explaining how to use this for palaeoceanographic reconstructions.
But apart from the day-to-day technicalities, how do you distinguish one species from the next? What criteria do you use, and at what point do you decide that it’s not one species anymore, but another?
Darwin’s ‘On the Origin of Species’ triggered, to quote the Wikipedia entry on the species problem shamelessly, “a crisis of uncertainty for some biologists over the objectivity of species, and some came to wonder whether individual species could be objectively real — i.e. have an existence that is independent of the human observer.” Darwin himself said “I was much struck how entirely vague and arbitrary is the distinction between species and varieties”. Wikipedia has a short discussion on the definition of species.
The first way of telling one species from another that probably comes to most people’s mind is reproduction: For sexually reproducing organisms, two individuals can be assigned to the same species if they can successfully interbreed and produce fertile offspring. Of course, this is not a criterion that can be used for asexually reproducing organisms such as bacteria. Here, the ‘cut-off’ for the distinction of a species is more or less arbitrary and underlies revisions: the level of genetic overlap between two bacteria for them to be called the same species today is 98.7%.
The decision of what makes a species may also depend on the context. Especially when looking at biodiversity (also thinking about conservation here) and the function of different organisms in an ecosystem, it’s also important to consider the ecology of two different organisms, i.e. how they ‘react’ to environmental parameters. What if they share the same genes to 98 or 99% – but there’s a gene in the remaining 1 or 2% that lets them do different things in their specific environment? For example, couldn’t there be a gene for an enzyme in one of the two that lets them utilize a different energy source or that makes a metabolic pathway more efficient? That could be the onset of speciation through natural selection…
So the most bullet-proof approach to defining a species would be to look at all of the above: morphology, genetic makeup, physiology and ecology. Of course, that’s hardly ever achievable – even just a combination of two of these is usually beyond the scope of most studies. Two weeks ago, I listened to a talk of someone who had done some preliminary studies (oh, the days of molecular analyses being too expensive for ecologists really do seem to be over for good) on a lagoon in Southern Texas. When asked whether they had checked their critters under the microscope following their molecular analyses to confirm identities, the PI said no. It’s not done routinely anymore.
The introduction of terms such as evolutionarily significant units (used in conservation) illustrate attempts to get around addressing the difficulties with the species concept, and I understand the term operational taxonomic unit (OTUs, commonplace in microbiology nowadays) as just a way to distinguish something that genetically seems to be a species until it’s further defined.
Ironically, a decline in ‘traditional’ taxonomy – based on morphology – has gone more or less hand in hand with the rise of molecular systematics. People usually specialize in one or the other (with more going for the ‘sexier’ molecular systematics), while not many routinely use both approaches. That’s a good way to miss information: for example, if you don’t look at an organism, you won’t notice morphological variability of a species – either based on genetic makeup or caused by environmental factors, as the left- and right-coiling forams mentioned above.
On the other hand, the foram genus Ammonia, when examined both morphologically and with molecular tools, is a good example of how dodgy it can be to rely solely on morphology to distinguish species. Of course, this problem of genetically different, but morphologically very similar ‘cryptic’ species can also lead to problems with the interpretation of the fossil record and palaeoceanographic reconstructions (I also found a wonderful lecture – really, go look! – on cryptic species of planktonic forams).
Right on time, there’s a brand new forum here on NN to discuss where we are with taxonomy today. The forum intro states pointedly that taxonomists are getting “thin on the ground and long in the tooth” – yes, most of those I’ve met invariably had long, white beards and/or fuzzy grey hair (no, I didn’t ask to see their teeth). Except for a few characters at the Natural History Museum in London, that is (hi Tim, hi Adrian!). Admittedly, it does take a special kind of person to count the setae on an amphipod’s leg.. but if you’re a budding taxonomist, don’t let that keep you from pursuing your dreams – we really do need you!
The bottom line is that the concept of ‘species’ is just that: a concept that helps us classify and organize the diversity of life on this planet somewhat, so we can assess it and work with it… and that we’ll continue to need taxonomists to do this.
Closing comment: if you ever find yourself describing a new species and you have to pick a name, try to beat these.
1 Darwin C (1859) On the origin of species by means of natural selection. Murray, London, p 48