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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