Monday, 19 February 2018

Biological basics pt. III: Subspecies

In Biological basics III: The species concept – a complicated issue, I gave a little overview over how a species is usually defined and what a species actually is. Today’s post is going to look at subspecies. The understanding of the concept of a subspecies is necessary for some aspects of “breeding-back”, such as the issue of using zebus in aurochs projects and, more essentially, the Quagga project. Subspecies are sometimes called a cop-out, nit-picking or something for bored taxonomists, and I am going to outline why this critique is not fact-based.

Subspecies – something for bored taxonomists?

The short and definite answer to this is No. As Charles Darwin already noticed absolutely correctly and perfectly subsumed, subspecies are incipient species and the line between varieties, subspecies and species is gradual. As he noted in his opus magnum, On the origin of species, species are basically “strongly marked varieties”, and subspecies are a state in between. And as the previous post on hybridization has shown, even the line between species is blurred because many closely related species are able to hybridize with limited or unlimited fertility. In many cases there is uncertainty on whether two or more species should be regarded as subspecies of one species, or whether two or more subspecies should be elevated to species level. However, for this section on the concept of a subspecies and what justifies it from a biological point of view, I will try to focus on examples for which there is a consensus.

Subspecies are in a beginning state of cladogenesis, a speciation mode that I already explained in part I. It is the case when populations diverge and become more or less isolated, and the allelic frequencies start to change due to genetic drift, mutation and selection. These changes not only concern neutral genetic variations but all biological aspects: morphology, behaviour, ecology. This is especially enforced by the fact that the different populations often experience different selective pressure due to a different environment. This is particularly true in species with a large geographical range, especially when it is disjunct (isolation). Thus, the different populations become regionally adapted additionally to coincidental differences caused by genetic drift. In some cases, the differences between populations or group of populations assigned to different subspecies may concern only single traits like colour variants, but in some cases the differences are well-marked and concern a number of biological aspects. I am going to go over some examples.

The classification of subspecies experiences the typical taxonomical problems of subjectivity and tradition, and therefore is not always consistent. In some cases two subspecies might be as distinct as a pair of species, and vice versa. There is no clear line as taxonomy is an artificial system that aims to classify the complex reality of organismic relationships.

One example clear example of a species that is divided into two subspecies is the American Bison Bison bison. This species is divided into a northern subspecies, the wood bison B. b. athabascae, and a southern one, the plains bison B. b. bison. Both differ ecologically and morphologically; wood bison dwell boreal forest regions and are adapted to lower temperatures, while plains bison live in more open regions and are less (but still very) cold-adapted. Wood bison are larger, have a darker pelage and less hair on the forelegs and beard. Also, the hump is shaped differently.

The African buffalo, Syncerus caffer, is divided into five subspecies. The nominate form, Syncerus caffer caffer, has the southernmost range, is the largest subspecies and very dark in colour. S. c. brachyceros, is smaller, weights only half as much and is lighter in colour. The smallest subspecies, the forest buffalo S. c. nanus, differs considerably ecologically and morphologically. It inhabits the swampy forest areas of Central and West Africa instead of dry grasslands, it is very small (withers height less than 120cm), a bright orangish-red colour instead of greyish to black-brown, small horns and brushy hair on the ears. Some authors have suggesting listing it as a separate species, and since the Caucasian wisent which differs from the nominate form less drastically is considered a separate species now (Bison caucasicus), I definitely list the forest buffalo as a separate species, Syncerus nanus. This is a good example that shows that taxonomy is often subjective and not consistent.

A prime example for a species with a large geographical range and many different subspecies that are morphologically and ecologically distinct is the wolf, Canis lupus. Its large, Holarctic distribution includes more than a dozen subspecies that all differ in colour and morphology, body size, environmental adaptions, and to a certain degree behaviour. For example, large subspecies such as the nominate form C. l. lupus weight twice as much as smaller subspecies such as the Arabian wolf  C. l. arabis or pale-footed wolf and have a stronger and more robust build; smaller wolves live in smaller packs and are less macropredatory. Wolf subspecies can be adapted to totally different climates; the Arabian wolf for example inhabits the hot arid climate of the Arabian peninsular, while the polar wolf C. l. artcos obviously is adapted to arctic conditions. Wolf subspecies also differ in colour nuances.
Another predatory species that has a large range that includes many different subspecies with different morphological characteristics is the lion, Panthera leo. Lion subspecies can be adapted to different ecologic environments and may differ in body size and mane for example, and the diversity in the species becomes even greater if you include Pleistocene forms.

There are many other examples in all possible vertebrate groups that show that subspecies are clear evolutionary lines with distinct morphological, ecological or ethological traits. In some cases the situation is more clear and distinct as in others, but subspecies are definitely not something for “bored taxonomists”. It makes sense to differentiate subspecies not only from a taxonomical standpoint, it also appreciates the evolutionary, ecological, morphological and ethological situation within a species. Therefore, subspecies are a very useful and justified concept, which is followed by all zoologists (quite frankly, I have never heard of any zoologist deeming the subspecies concept as not useful and not using it).

A special case of evolutionary variation within a species is the so-called cline. In this case there is not a clear differentiation into several lineages, but variation along a geographic or ecologic gradient. This is comparable to the concept of a ring species that has been explained in the previous post. The quagga, Equus quagga quagga, has been suggested by some to represent the end of a cline rather than a distinct subspecies, although I think this should be backed up by empirical evidence (see this post on the quagga).

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