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A wheatear controversy and the origin of the species

Rare bird alert made me aware of a bird that certainly piqued my interest yesterday afternoon.  It was reported as either a (very rare) female eastern black-eared wheatear or a (rare) pied wheatear. Later in the afternoon RBA “confirmed” that it was the former and I immediately set about planning a trip for today. The eastern black-eared wheatear spends its summer in the eastern mediterranean and over winters in the Sudan so this individual was somewhat lost!

The bird was located on the Lancashire coast and as such involved a drive on my beloved (sic) M42 and M6. I was faced with the choice of either going early before the presence of the bird had been reconfirmed, hence avoiding the Monday morning rush hour, or waiting for it to be reconfirmed around 7 am. I gambled, luckily as it turned out, on the former and left home at 05:30.

The weather on arrival was cold, wet and windy and, very wisely, the bird was hiding almost out of sight under a rock. After an hour or so it jumped out and was actively flying and feeding along the sea defences composed of large granite rocks.





The question of whether it is of the pied or eastern black-eared species has subsequently been re-opened but a pending DNA analysis of collected droppings should eventually be definitive.

A juvenile cuckoo was also hunting insects in the spinach field behind us.

Although I have always had a very keen interest in wildlife and photography, fostered by growing up in a very rural environment, I came to “serious” birding comparatively late in life.  One of the most surprising things I have subsequently come across is the ambiguity around the definition of a species.  This may have something to do with the fact that I am a physicist who is more use to seeing science as exact  and precise definitions and formula.

Just how ambiguous the definition of a species is becomes clear as soon as you pick up a dictionary or a biology book. Typically it will read something like

 “A group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions.”

Now this strikes me as a very broad and grey definition very much open to interpretation. For example, how about self-sustaining groups of hybrids – these seem to be a species based on the above definition?

In ornithology there is an almost continuous debate about what is a species or a subspecies in a particular set of birds. Although sometimes what were previously defined as separate species are reclassified as subspecies, there seems to be a general trend to split birds into more and more species. For example Western and Eastern black-eared wheatears are currently regarded as subspecies but there are some papers which now argue that they should now be treated as separate species.

I wonder if this is at least partially driven by our desire to attached definitive labels to things and have tick lists. ? It seems to me that nature often produces a continuum of forms where we desire to impose discreteness – some gull groups seem to be a particularly good example of this.

With modem rapid and cheap DNA analysis surely it should be possible to create a much more concrete definition of a species based on DNA differentiation and our ability to trace DNA difference in similar creatures back to common past parentage and their family trees or am I missing something fundamental here?

Comments

  1. Hi Jim - it is a knotty question, and while one might think DNA sequencing offers a solution, or at least new information (which it does in some cases), it also reveals more complexity. For example, different parts of the genome of two species may show different degrees of differentiation owing to both chance sorting as lineages diverge, and (in principle) because of variation in the extent to divergent selection. To use a linguistic analogy, French and English are not 'equally' divergent throughout the dictionary - there are still some words that we have in common, or nearly in common (e.g. blue and bleu) through shared ancestry, whereas others are quite divergent (black and noir). Add to that the possibility that there is flow of genetic material in both directions between species, particularly in the early stages of divergence (e.g. 'the weekend' and 'le weekend'), that lineages may separate and then recombine, and the divergence in the genome can be net-like rather than tree-like. Defining species is a bit like defining languages... we don't have any problem seeing that French and English are different, but that they have a common ancestor, but are different dialects in the UK different enough to be called languages, and if not what's the criterion for deciding they have become different enough to be languages? As you say, this is partly a problem of forcing discrete categories onto continuous variation, but there is a 'lumpiness' to that continuous variation which we also need to confront.

    Nice blog by the way!
    Cheers

    Ben

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