Todays post is on a rather theoretical topic, namely the definition of a species. It is very basic, but a necessary requirement for other issues such as hybridization or the subspecies concept, which are relevant for several units of “breeding-back”. At first it might seem easy to define a species, but everybody who has a basic deeper knowledge of biology will know that it is actually a very tricky issue. There are several concepts and definitions of a species, and none of them qualifies as the ideal and only one. This is why there is the saying “in a room of n biologists there are n+1 species definitions”.
The reason for this problem is that the taxonomical system that classifies organism is an artificial one made by us humans in order to work efficiently with the diversity of living beings. Classical Linnean ranks, thought to us in school and common to most people having a basic biological knowledge, are painfully artificial and subjective which is why they are usually avoided in modern phylogenetic systematics and cladistics. The basic biological entity that systematics are based on are individuals (even here we have blurred lines, see animal colonies; however, this blog deals only with vertebrates so let us ignore those for now). The next level are populations, and above that, biological ranks begin and so does subjectivity. Strictly spoken, a species is more of a hypothesis than an entity. In this post, I want to outline the different approaches to how define a species, their problems and also give some examples.
We want to focus on vertebrates here. For other organismic groups, such as the asexually reproducing bacteria, there are other approaches on how to define a species. The most common species definition was coined by Ernst Mayr and is widely known as the biological species concept. As “biologic” is rather generic, I refer to as the Mayr’s definition in this post. It defines a group of individuals or populations that actually or potentially reproduce and produce fully fertile offspring. This concept works well in many cases, which is why it is widely used. But there are also examples where it becomes problematic. One problem are hybridization in the wild; many species have hybrid zones with closely related, neighbouring species. Prominent examples are fire-bellied toads (Bombina), or Canis, where wolves hybridize with golden jackals in Eurasia1 and coyotes in North America, and so do the so-called pariah dogs2. A special case of hybridization in the wild that makes the Mayer’s definition problematic are the so-called ring species. Another example that is relevant for us are American and European bison (Bison bison and Bison bonasus). Both are geographically isolated because they dwell different continents, but when brought to the same area they reproduce readily and produce fully fertile offspring. This is why some authors have suggested listing them as one species, giving them subspecies status (although both the European and American bison are further divided into two subspecies themselves). However, most zoologists still consider both different species. One reason is another level that is relevant for species recognition, morphology. There is the so-called morphological concept or morphospecies that differentiates species based on phenotypical traits. Palaeontologists mostly have to work with morphospecies only due to a lack of genetic, ethologic and ecologic data. The problem of morphospecies are intraspecific variation, which can also lead to a morphologic overlap between closely related species, as much as phenotypic plasticity. The ambiguity of the morphospecies concept shows for example in the case of lion (Panthera leo) and tiger (Panthera tigris). The skeleton of a female tiger and a male lion are almost indistinguishable3. However, apart from other morphological differences such as coat colour and hair growth (mane), lions and tigers have different social behaviour, hunting behaviour and despite an overlap in range do not reproduce in nature, which is why they are considered separate species. This, as much as the bison example, shows that in practice not just one species definition but often a mix of several levels (genetic, morphologic, ethologic and ethologic) is used to determine and differentiate species.
The third relevant definition of a species is called the phylogenetic concept, which deals with species that evolved through cladogenesis (that is the split of evolutional lines; the direct evolution from one species to another without cladogenesis is called anagenesis). In this case a species is a monophyletic clade of one or more population that ends either through speciation (the evolution of a new species) or extinction. This concept is only useful under the frame of phylogenetic systematics or cladistics.
Having had a look at what species actually are, we can dive deeper into the issues of hybridization and subspecies, which is directly relevant for “breeding-back” related topics concerning the wisent, quagga and others that have been covered here on this blog.
1 Moura et al. 2013: Unregulated hunting and genetic recovery from a severe population decline: the cautionary case of Bulgarian wolves.
2 Gonzalez 2012: The pariah case: some comments on the origin and evolution of primitive dogs and on the taxonomy of related species.
3 Weishampel, Dodson, Osmolska, 2004: The dinosauria