Okay, so what’s the alternative?
Such problems and counter-arguments have led to a large number of alternative species definitions, with the best known arguably being the Phylogenetic Species Concept (PSC), introduced in 1983. Here the critical factor is not reproductive isolation, but diagnosability. This is to say that if a population possesses unique characters that can be consistently discriminated, that population represents a ‘monophyletic terminal taxon’ with its own evolutionary lineage, i.e. a leaf in the evolutionary tree, and is a species. As a result, consistently identifiable subspecies are all afforded species status under the PSC. Conversely, populations (‘subspecies’) that do not have consistent discernable differences are not considered valid taxonomic entities. An example of the latter would be a sample randomly taken from a cline – a gradient of subtle change in a character across the geographic range of a species. The PSC therefore renders subspecies obsolete, and if strictly applied, would result in a world bird list of around 25,000 species (as opposed to the current 10,000).
But how close do you want to look? One problem with the PSC is that our ability to differentiate diagnosable characters is not fixed. With increasingly sophisticated technology we can define finer and finer differences in characters. Also, we as humans see birds very differently to how they see themselves. For example, birds are able to see ultraviolet light, and many species have additional markings that are revealed in UV, but are not visible to the human eye. Also, and here I can already hear a collective groan from the birding community, some birds may be outwardly identical, but have small (but consistent, and therefore diagnosable) differences in their genetic code. So essentially, the closer you look, the more you’ll see. And the more likely you are to find some character that will define a population as a species under the PSC.
Especially since the development of technologies to incorporate genetic information in taxonomic arguments (since the 1980s, but especially in the last decade or so) there has been an increasing trend for ‘splitting’ birds into new species. Such splits often rely on increasingly tiny differences between ‘species’. And who doesn’t love a new split? Chances are that you’ll get a nice new ‘armchair tick’ to add to your lifelist. Despite scientists’ commitment to impartiality, academics that perform such studies, especially when they require huge investments in time and money, also feel subconscious pressure to interpret their finding in such a way as to reveal new species. Thirdly, elevation of a subspecies to species-level results in more resources being allocated to conservation and study of that taxon.
All of this, together with the speedy domino effect by which newly proposed splits pass unchallenged into the popular literature can be alarming. It takes only one hasty decision by a publisher to include a ‘new species’ in a field guide, and that species will be accepted without question and almost irrevocably engrained in the minds of birders. And who can blame them? Every amateur birder cannot be expected to conduct his or her own critical analysis of newly proposed taxa – we accept what the book says, and assume that the decision was based on sound research.
A dangerous assumption, I’m afraid. Just from a southern African perspective, some splits that turned out to have been overly optimistic and premature, include the now obsolete ‘Kimberley Pipit Anthus pseudosimilis’, ‘Long-tailed Pipit A. longicaudatus’, ‘Agulhas Clapper Lark Mirafra marjoriae’ and Levaillant’s mysterious ‘Namaqualand Clapper Lark M. rufopilea’, as well as Cinnamon Reed Warbler Acrocephalus cinnamomeus’ and ‘Purple Widowbird/Whydah Microchera haagneri’. Although not widely known to the birding community, we recently also came very close to getting a new species of scrub robin! Not all of these were rejected because of erroneous application of species definitions of course; the ‘Purple Widowbird’ turned out to be a hybrid between an indigobird and whydah.
And who doesn’t love a new split? Chances are that you’ll get a nice new armchair tick for your lifelist.
This is probably a topic for a whole new blog post, and I still have some errands to run later this afternoon, so I’ll just make some brief comments. We must first understand that life is continually evolving, and that division of life forms into taxonomic units such as species and subspecies is merely an artificial way for humans to make sense of life’s diversity. Despite the PSC’s rejection of the subspecies concept, the subspecies rank is a useful unit to understand within-species diversity in the continuum of evolution. In a way, subspecies give us an early warning that those populations are embarking on their own evolutionary path that will one day, in a few millions years or so, lead to a completely new species. This is an important motivation for the conservation of subspecies, as well as species! The Checklist agrees, and makes a compelling argument for the retention of the subspecies rank.
In fact, one of the book’s greatest attractions of The Checklist is its depiction of all major subspecies in the plates. The authors are quick to point out that their already monumental work is focused primarily at the species-level, and that time and resources did not allow a critical, in-depth re-evaluation of the validity of all proposed subspecies. However, a succinct commentary is still provided on the various subspecies proposed under each species.
One interesting hypothetical case argues against the merits of the PSC (illustrated below). What if the middle section of a cline of variation went extinct, leaving only two disparate populations from the two ends the variation spectrum? In the absence of an intermediate form between two such extremes, they would be classified as species under the PSC. In reality there was loss of actual biodiversity, but measured biodiversity increased! Tsk, tsk.
I’m confused – what is a species then!?
So if the BSC and its limitations in regards to interbreeding is not the answer, and the PSC with its over-splitting based on diagnosable characters is not the answer, how do we define a species? Do not feel alone – this is a question that many biologists are struggling with, and why I can’t give my son a straight answer. There have been some commendable efforts to reach a middle ground, such as the Comprehensive Biological Species Concept. Also, nowadays there is greater emphasis on splits supported by multiple characters. The question now becomes ‘how do we interpret the significance of differences?’ The problem is that different taxonomists allocate different weights to variations.
Professor Minerva McGonagall might consider a slight colour hue difference between two cisticola populations as a diagnostic difference indicative of being different species, while Professor Albus Percival Wulfric Brian Dumbledore might think they should merely be classified as subspecies. Party pooper Severus Snape might counter that this is just clinal variation, and that the species is monotypic (one species, without any subspecies). The fundamental problem is that there are no fixed criteria to define these different classification levels. Each taxonomist has his or her own view on how different two things have to be before they are different enough. And this problem is particularly relevant in regards to genetics!
Genetics: don’t trust what you see on TV
Surely genetics/molecular data is an objective and reliable way to define a species? Yes, and no. Firstly, a large phenotypic (physical) difference does not necessarily correspond to a large genetic difference, and vice versa. Remember that molecular genetic studies typically look at only a tiny fragment of the entire genetic code of a bird, and studies have shown that a detectable signature of an outwardly difference might not be obvious in the sampled gene.
In nearly all earlier and many modern genetic analyses, the favourite gene-of-choice was mtDNA. This Draco Malfoyian data packet was initially the golden boy of genetics, but has now grown into a rebellious teenager. MtDNA is derived from mitochondria, tiny organelles located in cells that help convert food into energy, and is inherited only from the mother. Mitochondrial DNA has a very limited amount of base pairs (the information units in genetic studies, denoted by the letters G, A, T and C) compared to nuclear DNA though, but a rapid mutation rate that makes it useful for phylogenetic studies. There are many limitations to using MtDNA however (most of which I don’t understand), and nuclear DNA for that matter. Some researchers are now strongly opposed to using mtDNA, and are today questioning even their own confidently published findings of a decade ago.
This Draco Malfoyian data packet was initially the golden boy of genetics, but has now grown into a rebellious teenager.
The authors of The Checklist provide some fascinating and shocking examples. Two populations of Common Redstarts Phoenicurus phoenicurus in Germany, despite lacking any apparent difference in appearance, differ by as much as 5% in their mtDNA signatures. Conversely, a whole series of monarch bird species (Monarchus spp.) with strikingly different plumage and calls differ by only 2%. Likewise, Common Swift Apus apus and Pallid Swift A. pallidus, which are universally considered different species, are genetically indistinguishable (at least based on the sampled genes). So where do we draw the line?
The bottom line is that a worldwide bird checklist, defining species based only on genetics, is not even remotely possible at this stage. Molecular data have, at the higher taxonomic levels of order and family, revealed a relatively stable framework (but only from as recently as about 2008). But at the species-level, not even to mention the subspecies-level, genetic data need to be interpreted with caution. So, still no joy. We’re still stuck with the same question, namely how do we objectively define what constitutes a species?
Tobias et al. to the rescue!
In October 2010 a team of evolutionary biologists from the universities of Oxford and Cambridge, together with BirdLife International, published a 23-page paper in the Journal Ibis (available here). The article is entitled ‘Quantitative criteria for species delimitation” and is cited in the literature as as Tobias et al. (2010) – the team of authors was led by Joseph Tobias, but also includes our own Cape Town-born Claire Spottiswoode. The team proposed an innovative, objective method for assessing whether taxa are species or not, based on allocating a numerical score to various characters: if the score is 7 or greater, you’ve got a species. If not, then no cigar. Simple, robust, transparent, objective. The HBW/BirdLife International Checklist courageously, but in my opinion very wisely, adopted this single taxonomic adjudication system as the basis of all the species definitions in their book.