Jeroen Speybroeck wrote:Forgive me for dwelling on this just a little more.
I have always felt uneasy with (parapatric) subspecies.
Lots of good questions there, Jeroen.
Personally, I am also very skeptical of subspecies, and this applies increasingly to the majority of systematists. Back in the first half of the 20th century, when morphological data was all we had access to, subspecies made sense. Today, with access to far more data, that is less and less the case, and more and more systematists tend to consider different sets of differentiated populations as either full species or as just differentiated populations of a single taxon.
The definition of species has also improved a lot over the years - basically any independent evolutionary lineage doing its own thing. Much of the often rather acrimonious discussion in the past has been more about how we should diagnose such entitities than about what the fundamental concept of a species is, but the two problems have often been confused in the past. This is less common today. Of course, the methods and criteria for diagnosing such lineages still vary between researchers, and different criteria can sometimes give different results - that is only to be expected when dealing with the products of an evolutionary process. The traditional criterion of reproductive isolation remains relevant where such lineages are in contact, but others, often based on genetic information, are becoming increasingly important as well.
How do we define "subspecies"? Originally perceived as nothing but a geographical 'race', solely based on morphological, phenotype-observable features, the seemingly logical addition has been made to allow molecular evidence as well. These two types of evidence can become conflicting, of which Vipera aspis atra (morphology challenged by genetics) and Salamandra salamandra alfredschmidti (morphological delimitation not even that consistent) are possible examples. However, by embracing molecular evidence as a (if not, the) criterion, I feel you need to draw a bottom threshold of some sort.
So, what does define a subspecies? Is clear-cut congruence between nuclear, mtDNA, morphology and whatnot sufficient, even in divergence is very low? Or do we adopt a certain basal level of divergence (albeit absolute or relative)? To me, defining subspecies becomes pointless with low levels of divergence, even if supported by all relevant sources of information. What's the point of distinguishing something you can not distinguish morphologically and which has no reproductive isolation? How far do we go with this (and also e.g. conservation implications, cf. ESUs)? On the other hand, I understand subspecies as being manageable concepts with added information when distinguished. But what if there's no morphological ground? I can understand the value of cryptic species, but cryptic subspecies ... ?
I don't think you can ever have a fixed threshold of distinction for any taxonomic category, because every situation is different. In some African lake cichlids, species that are clearly distinct and reproductively isolated have virtually no genetic differences due to the recency and rapidity of speciation. On the other hand, in other cases, there is virtually no morphological divergence despite long periods of differentiation and a lack of gene flow. Whether we can tell the difference between the animals or not is immaterial if the genetic data suggest that the animals can!
However, that applies to species. I tend to agree that this does not leave much room for subspecies. The one situation where subspecies
might be of some use is in situations where lineages (evidenced from genetic data, e.g., mtDNA, as well as morphology) have diverged in the past, but where there is evidence of recent genetic exchange across the boundary -
Vipera aspis francisciredii and
V. a. hugyi might be good examples of that. However, very few of the many subspecies described in herpetology over the years have actually been studied throughly and with that kind of philosophy in mind, so whereas the concept of subspecies may have some use in fairly specific situations, the vast majority of subspecies recognised in the literature are based on much earlier descriptions using morophological data only, and are therefore of limited use. In my experience, once a polytypic is analysed properly, around 60-70% of recognised subspecies will turn out to be just local variants of a single taxon, 25-30% will be speparate species, and 5-10% might usefully be recognised as subspecies
Personally, I would not bother with subspecies except in situations where they have been tested using modern methods, and where they are either suspected species, or at least clearly defined ssp. Just to give an example, in the case of
V. aspis, I would record the ssp.
hugyi,
francisciredii,
zinnikeri and
aspis (but not
atra), whereas in the case of
V. ammodytes, where there is even more disagreement between existing genetic data and morphology, I would not bother with ssp. at all.
Just my $ .02