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What’s in a Name? Searching for a Species Concept

Any child can tell you that a human is different to a chimpanzee or a buttercup is different to a dandelion. Humans, chimpanzees, buttercups and dandelions are all groups of individuals with certain characters in common, not shared by the other groups. Furthermore, there are no ‘butterlions’ or ‘dandecups’ in existence, not to mention half-human, half-apes, despite the claims of a well-known science fiction film. Therefore, it seems simple to define what a species is: just look for groups of similar individuals with no intermediates. However, looking more closely at nature reveals that matters are not always that straightforward.

Consider, for example, the Larus gulls. There are seven gull populations, which form a ring around the North Pole; each can interbreed with neighbouring populations but not with populations further away. For example, the British herring gull can mate with the American herring gull, which in turn can mate with the lesser black-backed gull of Eastern Siberia. The British and the Siberian gulls, on the other hand, are too different to be able to breed. This raises a confusing conundrum: do you define the whole ring of populations as a single species, or do you consider each population of gulls to be a different species? The former means that you end up with a species where not all individuals can interbreed and the latter with different species that can.

Similar arguments on how to define a species have been debated for years, and searching for a concept of what defines a species that satisfies everyone has so far proven difficult.

A historical sketch

The quest to accurately describe the living world has occupied naturalists and philosophers alike for quite some time.  Aristotle, for example, did not believe in the uniqueness of species, but thought that seeds from one plant could germinate into another species. Subsequently, with the spread of monotheistic religions, the idea that species were fixed and constant gained popularity. Each species was considered the product of a single creation event and any variation was the imperfect expression of an underlying ‘essence’. The Swedish naturalist Carl Linnaeus used this idea in his creation of the binomial nomenclature system that we now use to name all things living. However, like all essentialists, he struggled in finding traits that occurred in all individuals of a species.  The reason for this is evolution. Charles Darwin turned biology and world upside down by publishing the Origin of Species. Although not the first to put forward the idea of evolution, natural selection was the first satisfactory mechanism to explain the diversity of life.

To Darwin, species were groups of individuals that share physical characters not shared by any other group. In this view, species have no intermediates, as they had been lost throughout evolutionary time, although even Darwin admitted to ‘problem cases’, like the Larus gulls above, where biologists had to decide whether the differences between groups were great enough to merit the title of species, or whether they were just different varieties on a single species.

The Biological Species Concept

The species debate, however, did not end with Darwin. The Harvard biologist Ernst Mayr developed the Biological Species Concept (BSC). The BSC states that a species is a group of organisms that can breed with each other to produce fertile offspring, and are reproductively isolated from other such groups. The feasibility of using such a concept in defining a species has dominated the species debate for the last 70 years.

The BSC is an easily testable concept. By its definition, if two organisms make fertile offspring they are the same species, otherwise they are not. This cleared up any perceived subjectivity of Darwin’s definition. Species were now defined by reproductive isolation from other organisms, regardless of physical resemblance. However, the BSC is not without its problems. Perhaps most importantly, reproductive isolation is not simply all or nothing.

Many species show partial fertility with each other. For example, many plants show total reproductive isolation in the wild because of their different pollinators, but can freely interbreed if artificially crossed. Asexual organisms pose another problem for the BSC. Here, each individual is reproductively isolated and by the BSC they should be considered a separate species. This is obviously not practical, nor does it reflect biological reality.

Despite the dominance of the BSC, there are still over 20 suggested species concepts thrown about. Are they all necessary? To some extent they serve their purpose in different fields of biology, but many biologists and philosophers are attracted by the idea that there is an underlying ‘essence’ of what a species is. Is it possible to define this elusive ‘essence’ without confusing the matter by falsely merging multiple views to create a single view that no one is happy with?

Moving towards a single definition?

Using DNA techniques, hybrids in the wild are now more easily detectable and recent estimations are that 6% of mammal, 16% of butterfly and 25% of plant species form fertile hybrids. This frequency of interbreeding is much higher than what was originally thought and is directly contrary to what the BSC says about species. Even blue whales, the largest animal ever to live, have recently been shown to interbreed with their closest relative the fin whale! However, the species are nevertheless considered to be distinct because the intermediates are few and the species are readily distinguishable, regardless of the fact that individuals may sometimes interbreed.

These advances in molecular biology have led some biologists to suggest that species are simply part of a continuum ranging from ecotypes via varieties to species. London-based geneticist James Mallet champions this argument. His views are built upon the observation that physical differences can remain between groups of organisms, in spite of a lack of reproductive isolation. The argument is therefore that species are simply distinguishable clusters of organisms with similar appearances (phenotypes) and gene combinations (genotypes) with few or no intermediates in nature. Thus, when we distinguish buttercups from dandelions, we do so by recognizing that they cluster according to their phenotype and that the independent characteristics that create these clusters are correlated. Adding genetics to these clusters means that we observe two species rather than one if there are two distinct genetic clusters without any intermediates. Reproductive isolation will play a part in this, but will not be the only determining factor.

What’s in a name?

Not all biologists are convinced that Mallet and his colleagues are correct, or indeed that a single definition can be found at all. Perhaps calling a rose by any other name that would still encapsulate its sweet smell may not create havoc, but as seen here, a seemingly minor issue, like whether to name something a species or not, can actually impact important issues.

Aside from purely theoretical matters, it has great implications on where we draw the line between two potential species. To start, a key problem in evolutionary biology is understanding how one species is transformed into another, a process known as speciation. If we do not know how to formally distinguish one species from another, this task becomes practically impossible.

An accurate definition of species is also vital in the conservation of endangered organisms. The US Fish and Wildlife Service, for example, do not offer protection to hybrids (the offspring of the mating of individuals of two different species). The red wolf is an endangered top predator in the southeastern US, but its status is threatened since recent molecular analysis showed it originated as a hybrid between the coyote and the grey wolf. Deciding whether it is a species or not, therefore, drastically affects the red wolf’s hopes of survival. Recently, hybrids between polar and grizzly bears (so-called ‘pizzly bears’) have also been reported. Mating between these two previously geographically isolated species is due to changes in their patterns of movement as a consequence of global warming. It is still early days for hybrids such as the pizzly bears and it is yet unclear whether they will develop into a new species or if mating will remain an occasional occurrence. What is clear, though, is that current conservation regulations are not up to date with biology.

The prospect of one single universal definition of species is exciting. Moving beyond the alluring simplicity of the BSC will be difficult, but the living world seems to be more complex than the dichotomy it offers. Although the human brain likes to frame concepts in black and white, we all know that while white and black are truly different, the gap can be bridged by a continuous gradient of grey in between. Species are like this; the clusters of phenotypes and genotypes that comprise them may not be totally distinct. However, this fuzziness around the edges does not prevent their existence. A species concept truer to this reality is surely a step in the right direction.

 

Arvid Ågren and Chris White both graduat- ed from the School of Biological Sciences in 2010 and are now pursuing graduate studies at Universities of Toronto and Cambridge respectively

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