Readers’ wildlife photo, and an evolutionary lesson: speciation in action!

January 8, 2015 • 9:13 am

Reader pyers from the UK sent a photo of a bird at his feeder that brings up a cool evolutionary story. Birders and biologists have known for more than 40 years that this species, the Eurasian blackcap, may be splitting into two species before our eyes, as birds going to their original overwintering ground have now split into two different groups overwintering in different places.

But I’m getting ahead of myself. First I’ll show the photo and then let pyers tell a lot of the story, supplementing his comments with what I know about this case. It may involve a form of evolution that creationists say simply doesn’t happen: new species forming before our eyes—in real time!

Pyers’s comments are indented, mine are flush left.

This not brilliant photo  (it is dark and gloomy outside today); it’s of a Eurasian Blackcap (Sylvia atricapilla)  having a nom at my bird feeder.

Here’s a bit of evolution in action, which I hope will interest you.

Now normally this wouldn’t be considered for any of your wildlife photos – except for one thing: it shouldn’t be here [in the UK] – at least not in winter!
Blackcaps are normally summer visitors to the UK but is a new(ish) change to their behaviour.The normal migratory route for these birds is to Spain & tropical Africa, but over the last 50 odd years (say from the 1960’s) a small population from Germany, where they breed,  has come to overwinter in the UK because of human-provided food (like my feeder) . What is fascinating is that the change in their behaviour is genetic and has evolved over a very few generations. My only slight complaint is that they are rather thuggish birds, even if they have a beautiful song, and are perfectly capable of bullying a robin (which are normally the birds which deserve an ASBO).

Here’s a map of the different migration routes taken from the Berkeley “Understanding Evolution” website (the African overwintering sites aren’t shown):


What pyers doesn’t add is that the birds from Spain and Africa both return to the summer breeding grounds a bit later than the birds from the UK, because the latter have less distance to cover on the return.  Since birds form mating pairs soon after they get to the breeding grounds, the change in migration, perhaps based originally on small genetic differences, has led to two subpopulations that mate largely among themselves, so that gene flow is reduced between the two groups. Early-arriving birds mate with other early-arriving birds, and the same assortative mating goes for the late arrivals. That means that the interbreeding that normally prevents genetic differentiation is minimized, and the two differentially-migrating populations can diverge from each other by either natural selection or random genetic drift.

One might indeed suspect that, subject to different selection pressures—including adaptation to very different food and temperatures—the two groups of birds might indeed diverge by selection. And that seems to be what’s happening. There are already genetic differences in body form. The south-migrating birds have pointer wings (favoring flight for long distances) as oppose to the rounder wings of the UK-migrating birds. And the southern migrators also have shorter and stouter beaks, which may reflect a difference in diet (fruit, insects, and pollen for the southerners, suet, seeds, and other human-provided foods for the UK migrators).

These body-form differences are supplemented by other genetic differences that have accumulated: DNA analysis of birds on the breeding grounds can tell where they have overwintered with about 85% accuracy.

Over time (and this has happened in only a few decades), the blackcap species may eventually fragment into two groups that, while breeding in the same location, mate only with members of their own group and not with those from the other. Since this would have a genetic basis (the genetic propensity to migrate one way versus the other is probably increasing over time, since stragglers wouldn’t find good overwintering grounds), what we would have is speciation: the formation of two reproductively-isolated groups descended from an original common ancestor. Certainly what we see now is what evolutionists call speciation in statu nascendi—speciation in the “nascent state”.

Creationists say that we have no cases of this, but, as I show in WEIT, we surely do, especially in certain plants—something that Greg will write about shortly.  And we have lots of cases of species that are likely in the process of forming, as in the blackcaps.  But pyers has more to say:

I have 2 issues. Firstly: as I  said, they are thuggish birds so what effect will they have on the native birds?, and, secondly, this is a human driven change (not deliberate, I know) so what are the long-term chances for the bird? (I am not suggesting that we in the UK stop putting feeders out—we DO love our garden birds—but over thousands of years).

You can find more here on the change to the Blackcap.

And there are papers here (the first one is good)

and here ($$$ required!)

Also check out the link from Understanding Evolution given above.

We don’t often get readers’ photos that tell stories like this, so h/t to pyers and thanks for the references.


47 thoughts on “Readers’ wildlife photo, and an evolutionary lesson: speciation in action!

  1. Interesting post, I’m curious as to how long it would take till the two populations would be considered separate species.
    Even if we can show the creationists documented speciation,they’ll just move the goal posts and say that evolution is the change from one type(however they chose to define that) to another. I’ve had this conversation and it’s no use offering evidence and rational arguments to those who have already made up their minds.

    1. Its a judgement call about how much gene flow must be eliminated before we call things different species. Even well-recognized species may have a tiny bit of gene flow between them. I discuss this issue in my first book, Speciation (with Allen Orr), and show that it’s a continuum. If the two forms can maintain their genetic differences and body forms despite a bit of hybridization, they’re good enough for me to call species, although gene flow must be reduced to zero before we can call them FULL species.

      The “microevolution” caveat for creationists doesn’t usually involve speciation; their line is that change within a lineage is simply microevolution, and we don’t see members of a lineage changing into something very different. They don’t extend this, I think, to speciation by saying that the formation of a new species is simply microevolution.

      1. And I would like to point out that under that definition, there are pretty much no full species within the family Anatidae, including some species that have been separate-ish for 20 or 30 million years. That’s why we consider your first criterion good enough.

        1. this reminds me of another human action driven change in an animal species. In this case racks on deer that the best racks are killed before mating by hunters so the less spectacular are the ones who are winning by this human predatory elimination from the local gene pool are left to be chosen from. The same with rattle snakes. So many are rounded up that those with little or no rattle are having a better chance of surviving such massive hunts. Which could eventually spell danger for people when there is no rattle sound just a sting of a bite.

      2. …and even in the case of a total absence of extant gene flow, it’s not possible to exclude that a future change, e.g. in the ecological conditions, could reverse the situation. To be acknowledged as “full”, species should be separated by strong postzygotic barriers (non viability or total sterility of hybrids). But the most insteresting species are those “on the way”!

        1. I disagree; if reproductive isolation is complete, they are full species. We never know if speciation can be reversed,and, indeed, there are some very rare mutations that can reverse complete postzygotic barriers like complete sterility.

          1. One major problem for taxonomists, and an automatic “win” for Creationists is the difficulty in deciding about sub-species versus complete speciation. I suppose full genomic maps will eventually answer that question.

            I recall way back in the 1980’s, about North and South leopard frogs who seem to be in the process of speciation. They were starting to show difficulties in mating together. Though other factors could be in play. I do not know if more was found out one way or the other with them.

            1. How is that a win for creationists? The continuum of species-vs-subspecies was one of Darwin’s arguments for (gradual) evolution, and is absolutely not predicted on the hypothesis of separate creation (or punc-eq).
              It’s not necessary to be gracious in victory to creationism, it’s just crap.

    2. Good question – until they could no longer breed together might be the easy answer, but if humans intervene we can get populations breeding together that would not otherwise, eg the Ruddy Duck & White Headed Ducks.
      These are separate species but when Ruddy Ducks Oxyura jamaicensis & White Headed Oxyura leucocephala bred together it threatened the integrity of the White Headed ducks as a species. This is Jerry’s baby, speciation, & I am not competent to comment in detail, but one could argue that these are still nascent species & should be grouped together. Other nascent species, presently subspecies, are the various wagtails – Motacilla. Look them up – they are very variable.

      1. The problem with ruddy and white-headed ducks being mere incipient species is that they are not each other’s closest relatives. In order to unite them you would have to add in pretty much all the rest of the genus Oxyura. There’s a clear middle ground between “incipient” species and “full” species, and that middle ground is where the average bird species lives. These are not nascent species.

        McCracken, K. G., J. Harshman, M. D. Sorenson, and K. P. Johnson. 2000. Are ruddy ducks and white-headed ducks the same species? British Birds 93:394-398.

          1. Humanity is a force of Nature. We do all kinds of things without planning for them or thinking or caring about the outcomes. Why coyotes and wolves have been found to be mating lately. And the hybrids seem to have the best of both species. Now as to fertility, that is another matter.

        1. Interfertility is symplesiomorphy!
          (Never synapomorphy. Loss of a recently-acquired reproductive barrier by mutation – mentioned by Jerry above – would be autapomorphy in one lineage only.)

    3. Shorter version:

      Yeah, but they’re still blackcaps. Wake me up when you see a kitten hatching from a blackcap egg.

      1. Typical and expected Creationist answer. Does’t fit into anything just pulled out of the same place their deity is hiding.

  2. My question to ornithologists is what is lacking in the British Isles that means they migrate in the spring to the continent? What species would be out-competing them if they stayed? Is it just an ingrained need to migrate?

    1. I don’t know to be honest. I would guess that both Germany and the UK would have similar summer residents ( swifts, swallows etc etc) so I doubt that there are huge differences in ecological niches.

      Thinking about it though, there is, however, one species that would compete with it in the UK and that is the Blackcap itself.We have large population of “normal” migratory birds that return from Mediterranean climes for the summer so the payoff for our winter visitors might be greater if they return to Germany.

      1. Revisiting this page & just seeing your comment …
        Then are UK Blackcaps separating as well, & why don’t they stay??? It is like swapping houses with other people in a chain & then swapping back!

  3. I always find a certain awe in knowing that we are watching evolution at work. It is something that takes place on grand time scales far greater than a human life and usually longer than even written human history. So when there is an example of it happening at we can say “look look, species are about to split. New branches are being added to the tree of life!” that just kind of blows my mind.

    I’m glad you kept your website, Jerry. I’ll definitely buy your book. This is kind of an aside that doesn’t have to do with this topic, but…I grew up a Jehovah’s Witness, and in my early 20s I realized that I had been lied to my entire life. When I got out of that crazy cult, your book was the first I bought. I wanted to know about evolution, and your book didn’t disappoint. I’m sure the albatross won’t either. Thanks for all the writing both scientific, and philosophical…and also cats.

  4. I’m curious how much the mating behaviour of these birds affect the rate of accumalated genetic differences. I’m assuming that this species is fairly monogamous if they are seperating so quickly.

    1. Good point! A lot of ‘officially’ monogamous birds have high frequency of extra-pair copulation, and this woulod tend to swamp the influence of assortative pair-formation driving genetic divergence.

  5. I’m asking this as a rank amateur, so hope someone can help me understand. If this is not microevolution, is it macroevolution? (I know, I hate the terms.) Is this simply existing genes being selected, or are new genes evolving?

    1. The boundary between microevolution and macroevolution is arbitrary and is usually set at speciation. Given that speciation isn’t instantaneous this boundary is quite fuzzy. So this is macroevolution if you think there are now two species of blackcap but microevolution if you think there’s still only one.*

      The boundary between microevolution and macroevolution has absolutely nothing to do with whether or not new genes have evolved. (And I should also point out that the difference between new genes and old genes is another fuzzy, arbitrary boundary, for similar reasons.)

      *I am not, however, one of those folks who thinks that macroevolution is solely the accumulation of microevolution. I think there are some processes that deserve to be called macroevolutionary. How important they are is a matter for discussion.

  6. It seems to me that since the two populations breed in the same area in Europe, but at different times of the year, that this can be regarded as a kind of sympatric speciation. I regularly hear that that kind of speciation generally gets poo-pooed, but I keep coming across good cases for it.

    1. Well, no it isn’t really sympatric speciation because the genetic divergence causing reproductive isolation occurred in allopatry, by differential selection in different places, and would not have occurred had they not migrated to different places. It’s allopatric speciation.

  7. Fascinating. This also seems to me to be a good example of how artificial the distinctions between “artificial selection” and “natural selection” can be.

    In this case human activity has driven the evolution of another species. It was not intentional, or purposeful. Humans did not set out with the goal of changing the species in some way, they just like having birds around their gardens.

    This is no different than other non-human species’ behavior driving evolutionary change in another species, which is of course the unavoidable norm.

    The only useful, in certain limited contexts, distinction between “artificial” and “natural” selection is that artificial selection denotes awareness, intention and a specific goal on the part of the species driving the evolution of another.

  8. Very interesting stuff.

    Do you also find that the northern-migrators are better at queueing and the southern-migrators are better at football?

  9. I’m wondering if the ongoing slaughter of migratory birds around the Mediterranean has any correlation here; I would suspect that it is “favoring” those birds who winter in less murderous places.

  10. Thanks for the fascinating post and related articles. I’m interested to see how divergent their phenotype will eventually become.

  11. The Junco project has a series of videos that cover similar changes in separate Junco populations. One of the more interesting video segments is about a Junco population that has taken to living year-round at UC San Diego (if I remember right) and how quickly genetic and behavioral changes have accumulated in this population.

    You can see the videos at

  12. ” this is a human driven change (not deliberate, I know) so what are the long-term chances for the bird?”

    Isn’t it still natural selection if humans are participants? Even if humans stopped feeding winter birds, wouldn’t that change be gradual enough for them to learn to find other food sources?

    1. I’m not sure how rigidly anyone defines it, but presume that to be artificial selection, humans would have to be directly selecting for the novel phenotype (or against the ancestral one) rather than participating less directly. It’s arguable that this is the case here (but AS is best regarded as a special case of NS anyway).

      1. Alternatively, winter provisioning by humans is purely incidental, and the real reason for more Blackcaps going to the British Isles in recent decades is climate change. That would probably be indirect enough causation that nobody would call it artificial selection.

  13. Cool indeed.

    Now I’m no biologist (I wish I was!), but – and I’m sure Jerry or someone will have something to say here – if they’re mating in differing climes at differing times (that rhymes!), presumably sexual selection within the 2 populations will be manifest in differing ways. Presumably, over time, certain sexually preferred traits will dominate in the UK population and others in the Spanish population?

    I would be interested to know if anyone has studied any(?) differences or has plans to. Song complexity/length might be an obvious area of study with the Blackcap.

    The Blackcap has a complex and beautiful song (it’s sometimes called the ‘nightingale of the North’). I work near Regent’s Park in London and frequently hear/sometimes see them when I’m out running in the park at lunch.

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