Sunday, 8 November 2015

Evolution In Progress In Europe's Smallest Bird

Female Western goldcrest (R. r. regulus)
Photo credit: Missy Osborn Source: Wikipedia


We had to go to Kidlington to check on some work being done to a cottage for an absent relative, so stuck for something to do we decided to explore this village to the north of Oxford which claims to be the biggest village in the world. It's a strange form of inverted snobbery that insists Kidlington is a village and not a town because it's the largest. I used to live near Charlbury in Oxfordshire which, despite being a village, insists it's a town because it has a charter for a market that hasn't been held now for generations and has never had a town council or any of the civic trappings of townhood. But that's by the by.

Kidlington sits astride the main Oxford to Banbury road and grew by ribbon development, mainly as Oxford overspill as the car factory in Cowley caused the demand for housing to explode in post-war times. Most of this development is thankfully away from 'Old' Kidlington so it was to the older parts we went to see what we always manage to miss when we drive through.

Having driven down a cul-de-sac we found ourselves at the entrance the the village church car park, off which lead a couple of footpaths across what are essentially the water meadows in the floodplain of the River Cherwell. Within a couple of hundred yards we had seen two flowers that have no business flowering at this time of the year, a great-spotted woodpecker, a peregrine falcon and, quite unexpectedly and within a few feet of us, a bush full of goldcrests. Goldcrests literally a few feet away, almost at arm's length and completely unconcerned by our presence! I was too afraid to move enough to get out my iphone and take a picture! Annoyingly, I had only put it in an inside pocket a few minutes earlier.

Goldcrests are rather special little birds and ones I only saw rarely when I was young. I probably saw twice as many goldcrests this morning as in the entire rest of my life.

This spurred me to do a bit of research when we got home, so much of what follows is from Wikipedia and other online sources. When creationists come blundering onto the social media showing off their ignorance and scientific illiteracy whilst pretending to know better than scientists, it's hard not to tell them to spend a few minutes online doing what I did, and actually educating themselves. With so much information now freely available, their ignorance can only be wilful or feigned, yet they wonder why evolutionary biologists and other scientists give them such short shrift and treat their ignorant arrogance with such contempt.

Anyway, what makes goldcrests special is that they, along with the related firecrests, are our smallest European birds. They are also interesting in another way, and a way which makes them the subject of this blog. They are a great example of evolution in progress. They are members of a group allied to the Old World warblers, the kinglets, so called because of their crown of yellow, orange and/or red are all in the Regulus genus.

The Eurasian kinglets are believed to have diversified into the goldcrests (Regulus regulus) and the firecrests (Regulus ignicapillus) in the Middle Pleistocene. The goldcrests have since begun to differentiate into a multitude of regional sub-species in various degrees of genetic isolation, the most recent of which appears to be the divergence of a Central Asian form into the Himalayan goldcrest (R. r. himalayensis) and the Sikkim goldcrest (R. r. sikkimensis) only recently, having initially diverged from the Western goldcrest about 2.8 million years ago.

The picture is slightly complicated by the status of a number of island species such as the Taiwan firecrest (Regulus goodfellowi) which was believed to have been close to the firecrests but has now been shown to be genetically closer to the Himalayan goldcrest, as is its territorial song. The Canary and Azores island groups are also an interesting microcosm mirroring in some ways the diversification of Darwin's finches on the Galapagos Islands.

The Canary Islands appear to have been colonised in two waves; the first colonising Tenerife and La Gomera 1.9–2.3 million years ago, followed by a separate colonisation of El Hierro and La Palma 1.3–1.8 million years ago. The descendants of these two colonisations are now recognised as the Tenerife and the Western Canary Island goldcrest. Significantly because of the way barriers to hybridization work in birds, they are distinguishable by song.

The Azores group of islands were probably colonised as little as 100,000 years ago and have since diversified into three subspecies, the São Miguel Goldcrest (R. r. azoricus), the Western Azores goldcrest (R. r. inermis) and the Santa Maria goldcrest (R. r. sanctaemariae).

All together, the number of subspecies currently recognised throughout Eurasia is 14:
  • R. r. teneriffae. Found only on Tenerife, Canary Islands; it is a distinctive, small subspecies with a black forehead and pink-buff underparts.
  • R. r. ellenthalerae. Resident on La Palma and El Hierro, Canary Islands.
  • R. r. azoricus. Found only on São Miguel, Azores; it is like R. r. inermis, except the underparts are more olive-buff.
  • R. r. inermis. Resident on Flores, Faial, Terceira, São Jorge and Pico, Azores; its upper-parts are a darker olive-green than those of the nominate form, and the underparts are also darker.
  • R. r. sanctaemariae. Found only on Santa Maria Island (Azores); it is paler than other Azores subspecies and whitish below.

So, the questions evolutionary biologists need to answer here are:
  1. Why are there so many subspecies?
  2. Why have they remained subspecies and nor progressed to full species

Distribution map of the Eurasian Godcrests
Yellow: Breeding summer visitor
Green: Resident year-round
Blue: Winter visitor
Source: Wikipedia
The first question can be answered by looking at the geographical range and in particular the different types of habitat, couples with the food species these birds normally live on. And their relatively sedentary existence over most of their range, with a partial migration south of the Northeastern Eurasia subspecies, especially in harsh winters.

The second question demands a rather more complex answer.

A sedentary existence across an ecologically diverse range (montane, northern pine forest, broadleaf deciduous woodlands, agricultural, etc) will encourage diversification of foraging strategy and food species, but, a regular interbreeding or injection of genes from neighbouring subspecies, provided the barriers to hybridization are not what are termed, post coital barriers (in other words, once mating occurs there is nothing to prevent fertile hybrids being produced) then the only barriers to overcome are matters such as mating rituals and sex-selection.

Imposed on this picture of course is the longer term climate and tectonic changes which have shaped Eurasia over the last few million years, leading to various periods of isolation as interglacial periods allowed the species to move north, to be driven back as the ice sheets advanced again and spread from mountain ranges such as the Alps, the Caucasus, the Urals and the Himalayas forming very effective barrier to intermixing for hundreds of thousands or millions of years.

In common with many related birds, the commonest barriers to hybridization are pre-coital barriers as reflected in plumage, territorial songs and mating rituals. These act to prevent mating taking place at all, so questions of cross-fertility don't normally arise. It's not surprising therefore that the main differences between these different subspecies of goldcrest, and between the closely related firecrests, are plumage and song. Other factors driving plumage colour are such factors as predation and camouflage and, not insignificantly, random genetic drift.

So, what normally drives evolution by natural selection to set up these barriers to hybridization in the first place? Here it gets a little harder to explain but is almost certainly related to the fact that the food species doesn't change very much between the different sub-species nor does where they forage for it - mostly on tree branches but sometimes on the ground. It is comprised of small arthropods such as springtails, so there would be no pressure to diversify features such as bill size or shape or internal features such as digestive enzymes etc. So, when two subspecies hybridize, the offspring are not at any special disadvantage such as in being unable to find and eat their food species. Where they may be at a disadvantage however, is in having the wrong plumage or song for the local population, so making it difficult to find a mate.

In short, the evolutionary pressures to set up barriers to hybridization are not strong so can be expected to produce change only very slowly, and what the goldcrest has not had is enough time to progress to full species status. They have only been around for a few million years since they diversified from the firecrests and earlier, from the Old World warblers. What they have had however is enough geographical genetic isolation, at least in parts of their range, and in particular on islands, to diversify by genetic drift. In fact, the latter could well be the most significant factor in the diversification of this species, not necessarily natural selection.

In other words, what we have in the Eurasian goldcrests is an absolutely classical case of evolution in progress. All this diversity, all this variation, all explainable in simple, easy to understand terms and by an inevitable, elegantly simple process which fits a species into its local environment almost as snugly as a hand in a glove.

And it's equally possible to show just this with any number of species.

I suppose a hard-pressed creationist, looking for a way to fit these known facts into their childish view of a special creation just a few thousand years ago could always fall back on their get-out-of-jail-free chant, "God did it!", and persuade themselves that their magic creator must have had a reason for making all these geographical variations but it's all a mystery. They might even fool themselves into thinking this is some sort of answer.

Their problem is that this isn't anything of the sort. It can't be verified or falsified and actually tells us nothing. It just adds a massive layer of complexity because the answer now needs to include an explanation of how this magic creator arose and how they detected it a priori to designating it as their default answer. In other words, their answer is infinitely complex with unnecessary, unproven entities; it is evidence free, unscientific and doesn't answer the questions satisfactorily.

Creationists find this far more convincing than the logically consistent, verifiable scientific one based on real evidence.

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