We saw these two wild geese on on lake near Oxford the other day when we went for a long country walk with our grandson and his parents. My grandson would much rather talk about Minecraft and wasn't even interested when I showed him the hole where the little gall wasp came out of an oak-apple gall - how can that not be interesting? He'll soon be old enough to have that copy of Richard Dawkins' "The Magic Of Reality" I bought him when he was about 4.
Anyway, what we noticed about these geese was that, while they are obviously a pair and the one on the right is a perfectly normal-looking graylag goose, the other was a slightly odd-looking Canada goose. Canada geese are an alien species in Britain but have spread very rapidly throughout the Thames Valley and beyond.
graylag goose, Anser anser
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Normal Canada goose, Branta canadensis
The obvious conclusion is that the odd goose is a hybrid between a Canada goose and a graylag - hybridization if fairly common in the Anatidae group (swans, geese and ducks) especially where they occur together in quite large numbers, but why would a normal graylag pair up with a hybrid which looks far more like a Canada goose than a graylag? And why is hybridization so frequent in the Anatidae. This is where it starts to get really interesting - well, interesting to me, anyway.
There are believed to be two main causes of hybridization in this group, in which the males are unusual in birds in that they have retained a penis which nearly all other birds have lost in evolution (see, How Birds Lost Their Penises). The first is by, not to put too fine a point on it, rape. One reason why males of this group of birds may have retained a penis is because male genes benefit from their carrier's ability to force a female to copulate. In species where sexual dimorphism is pronounced, as in most ducks, suggesting a high degree of female sex selection, there is an obvious survival advantage in the males having a strategy for overriding it. There is no morality in evolution.
The second is a sneaky strategy some females have, so they can produce more descendants in a year than they could raise themselves. They are partial parasites, like the cuckoo, and will lay eggs in other nest. Normally, this will be those of the same species, but not always, so their offspring can end up being reared by a different species. Given that the young of this group are active from hatching and are not fed by their parents, it actually makes very little difference to them who is keeping them warm at night and protecting them from predators.
But, geese, especially, 'know' what species they are by imprinting on the first moving thing they see when they hatch, so these 'cuckoo' goslings will think they are a different species and will seek out members of that species for a mate.
So, that's why we have frequent hybridizations in this group, and probably why our hybrid Canada/graylag goose was paired with a normal graylag, having been raised as a graylag, probably conceived by rape.
But now we have another question: why is stable, fertile hybridization so common in this group in the first place and why is it biologically possible when it's normally rare in mammals, and usually results in either non-viable or sterile offspring? For all sorts of reasons, once speciation occurs there are distinct advantages to both sister species to evolve barriers to hybridization.
Biologists classify these as prezygotic and postzygotic barrier. Prezygotic barriers are ones which prevent egg and sperm ever coming into contact, basically, by preventing mating. In birds, the barriers often involve mating rituals and displays, involving colour. Birds have the advantage there over most mammals in having colour vision. Most mammals don't have colour vision, apart from the simian branch of which we and the other great apes are members.
Postzygotic barriers are those which prevent the fertilised egg from developing. Normally, this will be because of genetic incompatibility in that the genes of one species are organised differently to those of possible mates, so, even if mating occurs and the egg is fertilised, a viable offspring won't develop. In mammals there can be an additional postzygotic barrier in that the female's immune system might detect and destroy a 'foreign' embryo developing with a placenta.
This isn't available to birds, however because the female's reproductive tract quickly seals the fertilized egg with a shell, and expels it. For this reason, birds are much more dependent on prezygotic barriers and mammals on postzygotic barriers. Hybridization is much more common in birds other than just the Anatidae than it is in mammals, and this partly explains the brilliant colours of many bird species.
So, that just leaved genetic incompatibility to explain away.
Typically, mammals will have a widely differing number of chromosomes between different species because the mammalian chromosomes seem to be more prone to fusing into one or breaking into two. Most mammal chromosome numbers fall within the range of 18-30 pairs (humans have 32 pairs). Most birds, by contrast, fall within a much narrower range of 38-40 pairs and all the geese have 40 pairs. This evolutionary stability means there are few genetic barriers to hybridization either, even for species that have been geographically separated for tens of millions of years.
The result is, that if a male can get through the prezygotic barriers he has a good chance of a successful, viable fertilisation and that his offspring will be fertile.
So, the combination of a retained penis, a rape strategy as a mode of mating, and the female's parasitic strategy for egg-laying, all conspire to help the male get past the prezygotic barriers and the lack of postzygotic barriers makes this strategy viable.
So, that's probably why we saw slightly odd-looking Canada goose paired with a perfectly normal graylag goose, and in finding out I learned about prezygotic and postzygotic barriers to hybridization, cuckoo geese, rapist ducks and chromosome evolutionary stability in the Anatidae. Somehow, I find this hugely more satisfying than the simplistic and arrogantly dismissive "God did it! And He did it all for me!" answer you will get from any proudly ignorant creationist.
Now, if I can just make my grandson find that more interesting than Minecraft, I could make a scientist of him, like his mother is.
Further reading:
- AskANaturalist.com - What is this strange goose?
- Randler, C. Do forced extrapair copulations and interspecific brood amalgamation facilitate natural hybridisation in wildfowl? Behaviour 142(2005):477-488.
- Randler, C. (2006). Behavioural and ecological correlates of natural hybridization in birds. Ibis, 148(3), 459-467.
- Randler, C. (2006). Extrapair paternity and hybridization in birds. Journal of avian biology, 37(1), 1-5.
- Randler, C. (2008). Hybrid wildfowl in central europe – An overview. Waterbirds, 31(1), 143-146.
- Kraus, van Hooft, P, Megens, H, et al. (2012). Widespread horizontal genomic exchange does not erode species barriers among sympatric ducks. BMC evolutionary biology, 12;45.
- Ellegren, H. (2010). Evolutionary stasis: The stable chromosomes of birds. Trends in ecology & evolution, 25(5), 283-291.
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