What is a species?

Having discussed how difficult it is to define ‘species’ in my last post, I feel I should stress that biologists still absolutely must use the concept all the time. There is no good in studying an animal, or group of animals, if you don’t name the group to which they belong.  Members of a species should always be more closely related to one another that they are to any member of any other species: as an example, doves all look more like each other than they look like a pigeon. This much is objectively true, but the subjective problem lies in where exactly you draw the line.

So doves all look more like each other than they look like pigeons  (and vice-versa) but the dove-pigeon grouping all look more like each other than they look like crows (and vice-versa). Are doves and pigeons and crows three distinct species or are dovepigeons and crows two distinct species and doves and pigeons are subspecies of dovepigeons? The more species you add, and the deeper back in time their genetic split goes, the more complex this gets.

This is mainly the domain of 19th century biologists, of course, as we no longer just look at animals and say “I think…” Modern biologists have a whole host of morphological, behavioural and molecular tools at their disposal, and in the past decade, researchers have been working towards developing a tool to give an honest, universal and discrete individual identifier to all living things on Earth. The emerging technology is known as ‘DNA barcoding’.

Firstly, geneticists must carefully chose a particular gene or DNA sequence so that it (a) has sufficiently low variation (<2%) within a single species (b) has experienced enough mutation to have a large (>2%) sequence difference between closely related species, and (c) has remained conserved enough so as to remain recognisable across very distantly related species. The gene chosen varies depending on which taxa one is analysing (e.g. when working with animals, the Cytocrome Oxidase 1 mitochondrial gene is used), so as to satisfy these requirements.

Once the target gene is selected, species identification can begin. Excitingly, biologists no longer need a whole live animal to know what species it is. We can work with clumps of hair, meat, teeth, and horns, etc. All that is needed is a source of undamaged DNA. The technology has found uses outside of academia; in the trading of exotic animals and animal parts, it can be used to spot those breaking the law. Back in the laboratory, geneticists can use this technology to spot errors in deeply ingrained species. The African elephant was recently split into two separate species – the African savannah elephant and the African forest elephant.  Early biologists were puzzled by two species of parrot, one of which they only found males, the other they only found females. DNA barcoding showed that they were in fact one species, only with massive sexual dimorphism.

Biodiversity research requires honest and clear definitions of species in order to properly measure species numbers, or study the ecological interactions between species. If we are erroneously over or underestimate genetic divergence, this at best, introduces statistical noise into our data, or worse, leads us to draw conclusions that are actively false.

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Filed under Biology

5 responses to “What is a species?

  1. Interesting stuff, especially the bit about the dimorphic parrots. For all the intellectual stock we put in Darwin and his peers, it’s always good to be reminded that the sciences are in perpetual flux.

  2. This deserves a wider audience. Are you promoting it using hash tags and keywords via Twitter? What about Facebook? It’s really good stuff.

    • Thank you, you’re very kind! Yeah, every time I post something new I plug it on Twitter and Facebook. Tried one or two hash tags but I’m a bit of a twitter novice…

  3. paul b

    Wow! Love it and well up my street. So well written too. Have you written anything about the notion that we could well be a long-forgotten Simms style computer game that is still chugging away? The maths that underpins much of what we class as nature could suggest a logic-based framework and there is the issue of the moon appearing to be the same size as the sun in a total eclipse. Gaming has moved on so rapidly in the last 20 or so years that it is not inconceivable that in a 1000 years time gamers are creating and manipulating (and then discarding) Earth-like worlds for their pleasure. Just a thought.

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