Sunday, 23 June 2013

Evolution in Progress - A Tale Of Three Sparrows

Italian sparrow, Passer italiae
While sitting enjoying the shade in a very hot Villa Borghese Park in Rome the other day [sniff!] I noticed all the male sparrows were different to our 'normal' house sparrow that we see in Britain and most of the rest of Europe. They all have chestnut coloured heads whereas the usual house sparrow males have grey heads. Their backs looked a little brighter too. The females are indistinguishable (to me) from the usual female house sparrows. I have seen these 'Italian sparrows' before, in Switzerland on the south side of the Alps just a few miles from the Italian border. It's the kind of small difference that makes you think, "there's something different about that bird!" Most people who have no interest in wildlife probably wouldn't even notice it.

House sparrow, Passer domesticus
The status of this sparrow has always been something of an enigma to taxonomists. It used to be listed as a geographical colour variant of the house sparrow, Passer domesticus. My 1987 edition of Collins New Generation Guide to Birds of Britain and Europe lists it as a variety but discusses the difficulty with nomenclature since it interbreeds with P. domesticus in a narrow band in the Alps and with the Spanish sparrow, P. hispaniolensis in Sicily. At first sight this looks like a classical ring species such as we commonly see as a species begins to speciate in different parts of its range.

However, a Norwegian team, using DNA from blood samples of these three sparrows have come up with a slightly different conclusion. They believe DNA analysis shows that the Italian sparrow is a recent new species derived from a stable hybrid between P. domesticus and P. hispaniolensis which, over most of its range has set up barriers to hybridisation and hence should be regarded as a distinct species, P. italiae.

Homoploid hybrid speciation is thought to require unusual circumstances to yield reproductive isolation from the parental species, and few examples are known from nature. Here, we present genetic evidence for this mode of speciation in birds. Using Bayesian assignment analyses of 751 individuals genotyped for 14 unlinked, nuclear microsatellite loci, we show that the phenotypically intermediate Italian sparrow (Passer italiae) does not form a cluster of its own, but instead exhibits clear admixture (over its entire breeding range) between its putative parental species, the house sparrow (P. domesticus) and the Spanish sparrow (P. hispaniolensis). Further, the Italian sparrow possesses mitochondrial (mt) DNA haplotypes identical to both putative parental species (although mostly of house sparrow type), indicating a recent hybrid origin. Today, the Italian sparrow has a largely allopatric distribution on the Italian peninsula and some Mediterranean islands separated from its suggested parental species by the Alps and the Mediterranean Sea, but co-occurs with the Spanish sparrow on the Gargano peninsula in southeast Italy. No evidence of interbreeding was found in this sympatric population. However, the Italian sparrow hybridizes with the house sparrow in a sparsely populated contact zone in the Alps. Yet, the contact zone is characterized by steep clines in species-specific male plumage traits, suggesting that partial reproductive isolation may also have developed between these two taxa. Thus, geographic and reproductive barriers restrict gene flow into the nascent hybrid species. We propose that an origin of hybrid species where the hybrid lineage gets geographically isolated from its parental species, as seems to have happened in this system, might be more common in nature than previously assumed.

Spanish sparrow, Passer hispaniolensis
Of course, the existence of the Spanish sparrow itself is interesting since it is obviously closely related to the house sparrow, until we see the propensity of the house sparrow to diversify into geographical colour varieties. It was introduced into the New World in about 1850, some 160 sparrow generations ago yet we can already see if diversifying. It is noticeably paler in colour in desert areas and darker in hot, humid areas. Egg clutch size has also changed, being only one or two in tropical conditions. In Costa Rica the clutch size is typically two; in Britain three to six is the norm with four or five being typical.

So what we could see in the Spanish sparrow is a regional form which became isolated, maybe during the last Ice Age as I described in Creationist's Macroevolution Lie with regard to finch evolution, and there evolved into a distinct species.

Now, in mainland Italy (presumably, though this could of course have happened on an island like Sicily), the Spanish sparrow and house sparrow hybridized and produced the Italian sparrow which for reasons which remain unclear, has established barriers to further inter-breeding and so has established itself as a distinct species. This barrier could have been something as simple as female sex selection favouring brown heads.

The significance of the Italian sparrow having mtDNA derived from both parent species, though mostly of the house sparrow type, is also interesting. mtDNA is inherited from the female only (sperm, unlike ova, don't have any mitochondria) so this indicates that hybrid mating was most frequently, though not exclusively, between male Spanish sparrows and female house sparrows.

Here again, when we look at the detail instead of giving nature a mere superficial glance, we find things which can't rationally be explained by anything other than descent with modification and diversification from a common ancestor. We also see that nature doesn't read our rule book and is not constrained by our imperfect taxonomic system. Species can occasionally arise spontaneously by interbreeding between species which normally behave like perfectly normal isolated gene-pools, as well as by diversification over time in isolated gene-pools. Whatever the mechanism involved in producing the Italian sparrow, and it now looks like hybridization, we have another example of geologically very recent evolution to give us a new species of sparrow, something the Creationist loons and liars keep telling us can't happen.

Note: I fully expect a creationist to assure us now that, because it's still a sparrow, 'macro-evolution' is now possible at the level of species, which is now reclassified as 'micro-evolution, but not at the new 'macro-evolution' level of genuses, which is still impossible.

Or more probably, they'll just ignore this evidence against their daft Bronze-Age notion of it all being due to magic, in the belief that that's the best way to make unwanted facts go away.

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