- Part 1: ‘Well I heard that…’
- Part 2: ‘They look the same to me…’
- Part 3: What is a species anyway?
- References and useful links
After reading two papers providing compelling evidence for a three-way dunnock split, satisfied with the methods and conclusions reached, I sat down, downloaded the PDFs of each of the papers and got ready to annotate them, ready to draft a new post on the blog. However, a quick google search later, and I quickly became rather disheartened at the general attitude towards the findings.
As for all posts or drafts I do on this blog, I like to check online to see if the topic has already been covered. It often already has been to some extent, but I like to see if I can add another angle to the discussion, rather than regurgitate information already sufficiently explained elsewhere. So, I clicked on the birdguides article covering this split and checked the comments to see what people had to say.
All 8 comments voiced dismissal at the proposition of the split. The words ‘silly’ and ‘boring’ were used and the majority of comments voiced concern at the ‘worrying’ increase in splits. I believe that this comment section perfectly illustrates the rift between hobbyist birdwatchers (often determined listers) and taxonomists. Growing up as a passionate birdwatcher, I have clear memories of other birdwatchers sat in hides at some NNR or another voicing annoyance at useless splits and ‘bad-science’. Often, I was inclined to agree.
As I gained a passion for taxonomy at university, my opinion on the matter changed, and now I find myself ever-increasingly on the splitter side of the fence. In this post I aim to defend the ‘split-crazy’ direction that taxonomy is headed in, and voice my opinions as to why I think so much frustration is generated when splits are proposed.
Part 1: ‘Well I heard that…’
One conversation that I still remember from when I couldn’t have been older than 16 was sat in a hide, overlooking a disappointing flood with very little to look at. One veteran birdwatcher sat in the corner started talking about how crazy splits were starting to become.
“They’ve recently split Eastern and Western Bonelli’s Warbler”, he said. “Can you believe it?”.
Having seen neither, nor read anything about them, I hardly had an opinion.
“I’ve seen both many-a-time, and they look the ****ing same to me”, he continued. “You see, they’re splitting all these birds nowadays using genetics. I read the paper, and do you know what percentage divergence they had?”
I shook my head.
“0.3 percent”, he said. “And they want to split those two birds off of a difference that small, what a joke. They just want a new species named after themselves, that’s all”.
I couldn’t help but agree. It did all seem rather ludicrous.
Two years later, I decided to write a dissertation on Phylloscopus warblers. The memory from that conversation still firmly embedded in my mind, I opted to start with the fateful paper which lead to the Bonelli’s Warbler split. Mockingly, I began reading, expecting to see the authors attempt to justify tiny differences. But when I got to the results section, a cytochrome b divergence of 8.5%, not 0.3%, was the last thing I expected to read. For context, Common Chiffchaff and Mountain Chiffchaff have cyt. b divergences of only 3.6% yet breed next to one another, supposedly without hybridisation. Bonelli’s Warblers more than doubled this divergence, and diverged at least 4.5 million years ago. 4.5 million years ago, we looked like this. Does this look like the same species to you?
Now I don’t mean to strawman ‘anti-splitters’, but this is a conversation that, I’m sure, almost all birders have had at some point regarding some split or another. Had I not actually read the source material, I would still have believed that Western and Eastern Bonelli’s Warbler were fraudulent taxa. The problem is twofold:
- Humans are too trusting, and more than once I have believed something that someone else has said in the birding community, later to find out it was a load of bollocks. This problem is magnified by the fact that experienced birdwatchers when it comes to identification, as this man was, are often misrepresented as knowledgeable taxonomic authorities.
- People form opinions without actually reading the source material. I understand that most people don’t have the time or desire to read any of the literature behind these controversial splits. In any case, is it not fair to simply concede that you don’t know enough about the split in question to have a strong opinion on the matter?
Part 2: ‘They look the same to me…’
I believe that a large source of frustration regarding avian splits are cryptic species complexes, or splits that result in two species that look all-but identical. Birding is a hobby that requires extensive knowledge, but sometimes not being able to clinch ID due to not having any spread-tail photos can be exceedingly annoying.
One comment on the BirdGuides article sums this sentiment up very well:
“Splits which recognise visible differences are encouraging, but not this sort of split.”
Evolution is a phenomena whereby traits are accumulated in populations due to natural selection, sexual selection or sometimes due to chance. For many taxa, especially birds, of which the majority are prey items of other animals, camouflage is an essential trait for survival.
Small birds that spend most of their time in foliage often match the colour of the plant in question, see Acrocephalus, Phylloscopus and Hippolais warblers. Birds that spend most of their time on the ground often match the colour of the substrate, see pipits and larks. The truth is that sometimes bright colours are selected for sexually, and lead to large, obvious differences that birdwatchers can readily discern. However, often colour differences represent miniscule differences in genetic makeup. Think about how much hair, eye or skin colour can vary in humans. It can hardly be considered sufficient evidence for species elevation. Changes from a green hue to a yellow hue can often be due to 1 single base pair change in a single gene. A fantastic example of colour changes can be seen in Yellow Wagtails, which I’m sure all European readers will be familiar with. A bird with huge colour variation across its range, but comparatively little variation in genetics.
Contrastingly, some colour schemes and morphologies just make sense for the environment and niche which they inhabit. Oak woodlands in the Eastern Mediterranean and those in the Western Mediterranean aren’t that different. Similar environments often lead to similar selective pressures. It’s therefore no surprise that two small Bonelli’s Warblers that both live in dense green foliage, eat the same food and live in the same climate ended up looking very similar as there was no selective pressure for either population to look very different. However, the two populations diverged longer ago than the Maned Wolf and Bush Dog (see below). The common ancestor of these two canid species would have been a North American canid that had just crossed the newly formed land bridge in Panama and stumbled across South America. This new continent would have had vastly different environments, food sources and organisms with which to compete. Hence, extremely strong selective pressures led to extremely rapid evolution. Now, the two species look absolutely nothing like each other. Yet they have more in common genetically with one another than the Eastern and Western Bonelli’s Warbler.
Difference in morphology (and hence diagnosability) often correlates with genetic difference, but natural selection often seems to follow the mantra: ‘If it ain’t broke, don’t fix it.’
Dunnocks are a ground-foraging bird that are coloured in a way that maximise their camouflage matching leaf litter in woodland and scrubby habitats. Are there large differences in the colour or make-up of leaf litter in Spanish, British or Armenian woodlands? Do the insects that they feed upon differ much? Do the predators which they intend to evade differ much? The answer is, of course, no. As such, there is a strong selective pressure to maintain the plumage characteristics of all Dunnocks, to continue living life on the forest floor, undetected. This does not mean that large genetic rifts haven’t occurred, and that, most importantly, were the Dunnock populations to meet one another once again, they would interbreed.
Suppose, hypothetically, a mutation in one gene lead to a distinctive blue colouration in an Irish Dunnock and this mutation was considered a sexually attractive trait to other Dunnocks. Perhaps this gene could spread rapidly to all Irish Dunnocks in, say, 10,000 years. Irish Dunnocks would thus be distinctive and readily diagnosable. This would be an ‘encouraging’, ‘visible difference’ in the words of the commenter. Perhaps listers would flock to tick a vagrant ‘blue’ Dunnock that made its way over to Wales or Scotland. Yet the difference between this population of blue Dunnocks would have diverged no less than 300,000 years more recently than Iberian or Caucasian populations.
The study in question employed powerful statistical analyses that showed complete genetic isolation and a lack of gene flow, and found a greater difference between the three Dunnock populations than between Radde’s and Black-throated Accentor, two accepted species in the same genus as Dunnocks. Yet this well supported proposed split was met with complete dismissal. Granted, the BirdGuides article provided none of the relevant evidence, and I understand that few people would opt to read a paper on Dunnock taxonomy in their spare time. However, I hope this hyperbolic ‘blue’ Dunnock analogy expresses my frustration at the dismissive comments.
Diagnosability is obviously hugely important from a human perspective of what a species is, but it is sad to think that where science is supposed to be free from subjectivity or opinion, our notion of species status is based on whether we can tell the difference visually between two populations or not, as opposed to whether they can. Us humans seem to think that our eyes are the ultimate arbiters of objective species delimitation, and if it looks the same to us, it can’t possibly be taken seriously.
Part 3: What is a species anyway?
The final comment on the article sums up the controversy of taxonomy very well:
“Is there actually an agreed definition of what constitutes a species? Surely that is needed first.”
The crux of the problem behind ever changing taxonomy is simple: a species is an arbitrary man-made concept. Biologists have come up with over 20 definitions (species concepts), all of which have obvious upsides and downsides. Take the most popular for example, the Biological Species Concept:
“Species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups“
Mayr, 1942
Besides the obvious downfalls of the concept, such as a failure to mention asexually reproducing organisms (over 2000 animal species, all bacterial species and many plant and fungal species) or fossils, the concept also has more nuanced problems, particularly in relation to birds.
Firstly, would anyone dispute Lesser Black Backed Gulls and Herring Gulls being different species? Barn Swallows and House Martins? Both of these are examples that 1) are clearly different species and 2) hybridise in natural environments. Furthermore, what of populations that don’t overlap in range (allopatric populations)? How is a taxonomist to know if they would freely interbreed should their ranges overlap? The answer is that no one really knows.
One solution is to see if they would interbreed in captivity. Dubious ethics aside, imagine Lesser Black Backed Gulls and Herring Gulls didn’t overlap in range. If scientists had taken this approach, the two would have interbred in captivity (as they occasionally do in the wild) and the conclusion would have been that they were the same species… which they clearly aren’t.
Before the age of genetics, there was no solution to this problem. As such, birds that looked ‘sufficiently’ different (whatever that means) were considered different species.
Nowadays, more clever methods are used. Genetics can be used to test how much divergence is present between two closely related birds that overlap in range and we can thus say with certainty are different species. For example, Caucasian and Common Chiffchaff or Short-toed and Eurasian Treecreeper. This difference can be compared to two populations that don’t overlap (allopatric populations), and if the percentage difference in the sympatric species is smaller, it can reasonably be concluded that the two allopatric populations probably wouldn’t interbreed often and would hence be different species.
This is the approach taken in most splits nowadays. In the case of the Dunnock paper, the difference between Dunnock populations was compared to the difference between Radde’s and Black-throated Accentor which aren’t sympatric. This is a large caveat to the paper which would have been mentioned in my write-up about the Dunnock split. However, the point the authors intend to make is that if Radde’s and Black-throated are considered separate species, why aren’t Iberian, Caucasian and European Dunnock, given larger genetic divergence.
Many characteristics, such as vocalisations and morphology are often very important for mate choice, and thus chance of interbreeding. As such, morphology and vocalisation are great predictors of species status. However, as seen in the previous chapter, sometimes similar morphologies are misleading, and with the rise in genetic sequencing many cryptic species are being discovered using the rough approach outlined above. That, to me, is exciting, not boring.
References and useful links
The two Dunnock split papers:
The Bonelli’s Warbler split paper:
The Maned Wolf and Bush Dog species radiation (a particularly interesting read) :
The BirdGuides article: link
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