Sunday, 23 March 2014

Evolution Has More to Crow About

Carrion crow, Corvus corone corone
A few days ago I mentioned at the end of a blog about the evolution of crows in the presence of the great spotted cuckoo, the fact that carrion crows and hooded crows are now regarded as different species. This illustrates neatly the problem of defining a species, especially where two closely related species are at a stage in their evolution where they have not fully diverged and retain the capability of interbreeding. I'll discuss this more later, but first, the case of the Eurasian crows.

As you drive north from England across southern Scotland and up towards the Highlands, you might, if you're interested in the birds, notice that the ubiquitous glossy black crows you will have seen almost everywhere from town parks to country fields and woodlands have quite suddenly been replaced by an equally common crow with a black head, tail and wings and a grey back and underparts. If you drive eastwards from Western Europe through Germany towards Poland or down into Austria or the Balkans you'll see a similar change. In both cases the plain black carrion crow, Corvus corone, has been replaced by the black and grey hooded crow, C. cornix. A similar change occurs as you drive from France into Italy south of the Alps.

Hooded crow, Berlin, Germany, Corvus cornix cornix
What you may not have noticed as you drove north in Britain however was a narrow band where the crows were both black and grey and black, and a whole range of intermediates between the two. In this narrow band, which has apparently moved over time, the two species of crow behave like a single species and interbreed freely, producing all the different intermediates.

There are similar zones of interbreeding between France and Italy. Everywhere else in Europe, they behave like two perfectly respectable species, just like, say, the song thrush, Turdus philomelos, and the blackbird, T. merula, or the house sparrow, Passer domesticus, and the tree sparrow, P. montanus.

Hybrid between carrion and hooded crow, Corvus corone x cornix
If you continue driving eastwards from Europe across the Yenisei river into central Asia, you'll find another sudden change - the hooded crows will disappear, to be replaced by what looks like slightly larger carrion crows. In fact, most authorities think that's exactly what the eastern crow is, and call it C. c. orientalis, the carrion crow being called C. c. corone to show that they are merely subspecies and not distinct species in their own right. Others disagree and think that, because the two subspecies are geographically separated, and have been for a long time, they do not interbreed and so form a single species with a single gene-pool, they should be regarded as two distinct species, C. corone and C. orientalis. However, there is a zone of interbreeding between C. cornix and C. C. orientalis just as there are zones of interbreeding between C. cornix and C. c. corone.

And it gets worse! Go from the north down through Iraq towards the Arabian Gulf and, as you come to ancient Mesopotamia between the Tigris and Euphrates rivers, the grey parts of the hooded crows become much paler so they look almost black and white. There they are known as the Iraqi pied crow. Taxonomists regard these as a subspecies, C. c. capellanus. All in all, there are four subspecies of hooded crow; the other two being one which lives in a band running from western Siberia down between the Black and the Caspian seas through the Caucasus Mountains and into Iran (C. c. sharpii), and one found in Turkey and Egypt (C. c. pallescens).

Eastern carrion crow, Corvus corone orientalis
The current hypothesis is that several populations of the stem species for all these crows got separated during the Ice Age and for tens of thousands of years evolved in isolation as effectively different gene-pools. As the ice retreated one of the populations which had evolved into the hooded form expanded northwards forming a block between the western and eastern forms which had retained their all-black plumage.

These eastern and western forms have since continued on their different evolutionary trajectories, the eastern form becoming a little larger and their tail feathers becoming more tapered. Meanwhile, the hooded form, which happens to occupy a more diverse habitat has formed local subspecies. All the crows are sedentary species, never straying far from the area they were born in. This helps to minimise any gene flow from adjacent subspecies.

So why is this? Why does the science of taxonomy find it difficult to tell if closely related species are the same species, subspecies or full species in their own right? Why is there now agreement that hooded crows and carrion crows, which do interbreed in a few small areas, are distinct species, yet disagreement about the status of the eastern crow, even though they never normally meet carrion crows in the wild and so do not normally interbreed? And, if the hooded and carrion crows were merely subspecies, what would be the status of the four subspecies of hooded crow?

The reason for this apparent confusion and imprecision, is that 'species' is a human concept; a tool used by science to classify all the different living things. Nature doesn't read our rulebook and doesn't have any obligation to produce neat divisions between living things. Nature is quite happy about the distinction being blurred so any device we come up with to try to divide living things up into our neat compartments is bound to lack precision because nature itself lacks precision. The reason that C. corone and C. corvix are now regarded as species rather than subspecies, incidentally, is because studies have shown that the hybrids lack breeding vigour, indicating that divergence of the species had progress towards the point where interbreeding would either be impossible or the offspring would be sterile. This is in fact a departure from the commonly accepted definition because, although they may lack vigour, they may well be fertile, and one definition of 'species' is a distinct population capable of interbreeding and producing fertile offspring.

Even though it is not disputed that species is a taxonomic rank, this does not prevent disagreements when particular species are discussed. Consider the case of the Baltimore Oriole (Icterus galbula) and Bullock's Oriole (I. bullockii), two similar species of birds that have sometimes in the past been considered to be one single species, the Northern Oriole (I. galbula). Currently, biologists agree that these are actually two separate species, but in the past this was not the case.

Disagreements and confusion happen over just what the best criteria are for identifying new species. In 1942, Ernst Mayr wrote that, because biologists have different ways of identifying species, they actually have different species concepts. Mayr listed five different species concepts, and since then many more have been added. The question of which species concept is best has occupied many printed pages and many hours of discussion.

The debates are philosophical in nature. One common disagreement is over whether a species should be defined by the characteristics that biologists use to identify the species, or whether a species is an evolving entity in nature. Every named species has been formally described as a type of organism with particular defining characteristics. These defining traits are used to identify which species an organism belongs to. For many species, all of the individuals that fit the defining criteria also make up a single evolving unit, but it might not be known whether that is the case. These two different ways of thinking about species, as a category or as an evolving population, may be quite different from each other.


... I was much struck how entirely vague and arbitrary is the distinction between species and varieties

Charles Darwin, On the Origin of Species. (p. 48)
It was Charles Darwin himself who pointed out that if it hadn't been for evolution, which has caused living things to diverge and differentiate and which has emphasised and worked on differences, converting minor differences over time into major ones, that we have all the different taxa from kingdoms to subspecies and varieties in the first place. Evolution has carved out the current taxa from what would otherwise be a confusing mass of undifferentiated life forms. In fact, it would not have been possible for living things to evolve any of their solutions to problems like where to get basic resources from even to make copies of themselves. Life could not have progressed beyond simple autocatalytic replicators without natural selection to work on the slight differences produced by imperfect replication.

What we have in the crow example above is an example of evolution in progress, just like a ring species. These different populations of crows are seen at the current stage in their evolution and diversification. They are obviously not the same species but their progress to separate species status has not yet progressed to completion. In some ways they act like one species; in other ways they act like subspecies and in still other ways they behave like different species altogether. There is no requirement on nature or the process of evolution to complete this process suddenly or even within a given time frame, or in front of an eye-witness. If it takes 10 million years, or ten thousand years then that's what it takes. If you expect to see it happen, you have a loooooong wait.

In the case of our own divergence from the common ancestor we share with the chimpanzees, recent evidence has suggested that we may have interbred with chimpanzees for over a million years before finally diverging. The process of speciation started some 6.3 million years ago and took 1.2 million years to complete! Later on in the evolution of Homo sapiens, we were still able to interbreed with H. neanderthalensis and maybe with H. antecessor, H. habilis, and H. erectus and other species yet to be discovered (or should they be subspecies?).

This exposes the lie behind the idiotic mantra which creationists chant when you show them some examples of recent evolution or indisputable evidence that evolution occurred in the very recent past, "That's microevolution. Macroevolution is impossible! No one has ever seen a new species arise!" No-one who knows anything about evolution would expect to see a species arise (except in the rare examples of new species arising by hybridization - which have been seen) because it takes tens or hundreds of thousand or even millions of years. Evolution is a process, not an event.

By what sane logic have creationist pseud-scientists concluded that evolution can proceed within a species and can happily account for the evolution of subspecies and yet conclude that progressing a little further to the status of species is impossible? The terms are purely human constructs and do not mandate or constrain the process of evolution in any way. Are we really expected to believe that when humans classify two crows as C. c. corone and C. c. cornix evolution is perfectly adequate to explain how they differentiated from one another, yet when the same humans decide to re-classify them as C. corone and C. cornix, all that magically changes and the differentiation we previously accepted somehow becomes retrospectively impossible? Only someone who sincerely wants to be fooled could fall for that 'reasoning'.

The creationist frauds who have sold that lie to the gullible fools who eagerly buy their cures for cognitive dissonance really should be taken to task for releasing their unfortunate victims onto the Internet to make such fools of themselves with that simplistic nonsense.

And the crow example can only be understood as an evolutionary process. There can be no intelligent reason for creating different overlapping populations with different forms and behaviours and then allowing them to interbreed, simply to produce offspring with reduced vigour which soon die out when they interbreed with one another, in a zone which needs to be constantly replenished with new hybrids from the two parent species. Any creator which intended that outcome would be an idiot. Any creator which did it accidentally would be incompetent. The same frauds are selling this 'Intelligent Design' nonsense to the same gullible fools too.


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1 comment :

  1. In the Fundamenta Botanica (1736) Carolus Linnaeus asserted that "there are as many species as the Infinite Being created in the beginning." But soon Linnaeus during his classification work was exposed to a variety of hybrids. These experiences eventually led him to change his views and question/challenge the immutability dogma. Why didn't God rest on His own day of rest?

    Read more about how Carolus Linnaeus, Peter Simon Pallas, Jean-Baptiste Lamarck and others tried to solve the problem with hybrids here: http://www.macroevolution.net/carolus-linnaeus.html#.UzBojvl5OaI .

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