Most students of biology will be familiar with the Biological Species Concept - that individuals belong to a species if they can interbreed to produce fertile offspring. This is a reasonable starting point, but is more or less impossible to use or test. For paleontologists looking at two fossils, or forestry workers wanting to know if a particular insect is a dangerous pest or harmless, it is not going to be possible to try and engineer a mating! Similarly, we cannot test a new discovery using the biological species concept. A further problem is it is not always (or indeed often in the case of plants) in agreement with what we think of as species. Species hybrids (the offspring of a mating between parents of different species) exist, and can be fertile: with similar hybrids, with either parental or even sufficiently closely related species.
An example of a hybrid 'species' Papilio nandina, a cross between P. dardanus and P. phorcas |
Where does this leave us? Certainly if we are interested in purely pragmatic identification, variants of Ecological Species Concepts are attractive. These all start from the assumption that all members of a species occupy the same ecological niche: they live in the same kind of habitat, have the same lifestyle and eat the same things. This is great if we want to tell edible from poisonous or pest from harmless, but these ideas are not without their problems. For starters, in many organisms (such as butterflies) different life stages have different lifestyles - they occupy different niches:
P. dardanus caterpillars |
P. dardaus adult feeing on nectar |
If we wish to learn about the biology of such organisms, it is going to be more than a little bit inconvenient to have different life stages in different species! This (perhaps unfair) criticism of ecological species concepts is in addition to other practical difficulties - primarily that it is very difficult to determine an organism's niche (or to see if two similar organism really are in the same niche) without extensive study. For a bucket full of beetles in alcohol or a drawer of pinned butterflies, we simply don't have the data to make a purely ecological idea of species workable.
Some scientists have taken advantage of technological advances to suggest new concepts, such as using particular pieces of DNA as a molecular barcode: if a specimen has the barcode belonging to a species, then it is from that species! There is now an initiative to generate a barcode from as many species as possbile - some advocates of this approach promise Star-Trek like machines which can use DNA barcodes to identify any biological material which is fed into it.
Whilst we obviously all want to own a tricorder, I'm afraid I don't think this is an end to the problem of species. We might want what we define as a species of bird today to be comparable with a species of dinosaur, for instance.Personally, I think that if we want our species to be biologically meaningful, we need to think more broadly, to be (dare I say it) a bit Post-Modern. What we call species (and I'm tying my flag firmly to the mast of the 'species are real' camp here) are best described and circumscribed by using a range of metrics, reflecting the vast range of reasons people might want to identify a species and the range of data available for different taxa.
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