Good Point Imberbe
Toko's link from National geographic sort of refers to the fact that these can be classified as different species even if they are able to produce fertile young and that is true as far as I understand it. A species is a taxonomic unit, a biologically relevant one but still a TU and often a subjective one at that.
Species are defined by what are known as species concepts, which work in defining some or other discontinuity in the traits of organisms, so basically how different two populations of organisms must be in order to be classed as two separate species. I know of 8 different species concepts and although it is difficult to see where each is applied taxonomists use these to classify organisms and this study is a perfect example of taxonomists trying to determine which species concept (SC) is most appropriate.
Imberbe explained how separate species can be classified by the ability to produce fertile offspring. That is the biological species concept
and is used quite often. Basically here a species is a group of organisms potentially or actually reproductively isolated from another group of organisms by pre or post mating mechanisms . This includes hybrid viability (post) and sexual isolation(pre, sperm characteristics etc). Until recently it was the most widely accepted.
There are 7 other species concepts such as the morphological and phenetic SC's
. These two are similar in that they both distinguish species based on morphological characteristics, so each species is morphologically distinct from its closest relative and this is based on a type specimen which is the first to be found of that species and described (its best to collect one from the edge of its known distribution). The phentic SC is different from the morphological SC only in that it confers some exact degree of morphological difference using a phenetic distance statistic .
Then you also get the evolutionary, Ecological, phylogenetic cohesion and recognition species concepts. in the evolutionary SC
a species is seen as an independent evolutionary lineage with actual reproductive isolation. So in that SC any population that is sufficiently isolated genetically or biogeographically could be considered a separate species and is the one I think has been used in these studies.
The ecological SC
defines a species as a group of organisms exploiting the same environmental niche. Here if two very different organisms interbreed the hybrid in theory would be maladapted because it has genes that are not co-adapted to exploit a particular resource.
The phylogenetic SC
defines species as a group of organisms in which all individuals share unique derived characteristics which are not present in their relatives. This does use morphological characteristics of the organisms but the derived characters (apomorphs) can be identified genetically so this is quite a powerful one.
ok some of these are not as easy to understand as the rest and the cohesion species concept
, which is the newest one as far as I know is very, very difficult. Im pretty sure I do not quite understand it. It refers to a species as the most inclusive group of organisms having the potential for phenotypic cohesion (physical appearance) through demographic or genetic exchange abilities. It involves tons of other theory but I dont think its necessary to explain here.
The last SC and one of my favourite is the recognition species concept
. Here a species is a population which shares the same specific mate recognition system (SMRS). an SMRS included calls, behaviours, appearance and a suite of other components used to choose a mate. This concept is conceptually much better than the biological SC because it does not rely on mating to define a species and every species can be defined based on a unique SMRS.
So taxonomy is a very interesting but frustrating field of science as how to define a species is not cast in stone and often a suite of SC's are used to classify organisms. The simplest, which is the morphological species concept is used often by all of us in field guides and we all know about the biological (or at least the idea of it) SC. So really it is subjective and is most effected by the scale at which the species is examined.So the savanna and forest elephant can easily be classified as separate species using a number of the concepts and as the same species by others.