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Microbiology and The Biological Species Concept: Response to Jerry Coyne
Henry Taylor
Introduction
Recently, I wrote a popular article for The Conversation about the species problem in biology, where I discussed (amongst other things) the biological species concept (hereafter the ‘BSC’), cladism, and species eliminativism (the view that there are no species). Almost immediately, the biologist and popular blogger Jerry Coyne wrote an aggressive response to the article on his blog.
My article was a popular piece designed to introduce a wider audience to the species problem in biology, and I was restricted by a small word limit. As such, I couldn’t delve into any of the issues in any depth. The present piece thus represents an opportunity to expand on the original piece that appeared in The Conversation. As well as considering and responding to Coyne’s points, this piece will also touch on some neglected issues in the area, such as microbiology. When read alongside the piece in The Conversation, this article also represents a slightly more in depth introduction to the species problem.
In his response to my article, which is titled ‘A misguided philosopher claims that species don’t exist’, Coyne says that ‘I won’t say that philosophers in general have nothing to contribute to debates about the nature of biological species, but this philosopher certainly does: Henry Taylor’.
He goes on to say that I am ‘Ignoring nature completely (has he been outdoors?)’ and describes my article as a ‘lame effort to topple the BSC’. He then describes one of my claims as ‘risible’. He says that: ‘The danger here is that those who don’t know much about biology and evolution will read Taylor’s piece and think he’s onto something. He isn’t.’ He summarises by describing my article as ‘Taylor’s nonsense’.
My hope is that the present article will demonstrate that there is significantly more value in reflecting on these issues than Coyne claims.
My original piece is here:
Coyne’s response is here:
Section 1: The species problem
The species problem, put simply, is the issue that there are a wide variety of alternative proposed definitions of ‘species’ in biology. One recent book on the topic lists 32 potential definitions of ‘species’ (Zachos 2018). Another lists 28, plus four more that are intended as replacements for the notion of a species (Wilkins 2018). The various alternative definitions of ‘species’ are known as ‘species concepts’ (in fact, Wilkins prefers the term ‘conception’, but that won’t matter for what I have to say here).
Section 2: The biological species concept
Coyne focusses most of his discussion around the BSC, which is one popular way of understanding species. This concept (developed extensively by Ernst Mayr, Theodosius Dobzhansky and others) states that species are:
‘interbreeding natural populations reproductively isolated from other groups; all individuals that produce fertile offspring (actually or potentially-this varies among definitions)’ (Zachos 2018, p.80).
There is a lot of variety in the way that different thinkers have specified the basic idea of the BSC. For example, some claim that we should also include the criterion that individual members of a species must have mate recognition systems (see Patterson 1985 and Mayr 1970 for more on this). However, the fundamental idea is that species are relatively isolated gene pools: they are groups of organisms around which genetic material can be passed in a relatively free manner, where gene exchange between members of the group and non-members is relatively minimal, if it occurs at all.
There are four issues about the BSC that require discussion:
1) Hybridisation
2) Species Pluralism
3) Microbiology
4) The popularity of the BSC in biology
Hybridisation and the BSC
In his response to me, Coyne says:
‘Now, why does Taylor reject the BSC, and along with it all species concepts? He gives two reasons.
1.) Polar bears and grizzly bears, once living in different places (“allopatric”) are now meeting each other in nature due to the global-warming-induced disappearance of the cold habitat to which polar bears were once restricted. There is some hybridization between the two groups that now meet, and some of the hybrids are fertile.
Taylor says this shows that the two bears weren’t reproductively isolated, and thus weren’t species. But this is bogus: the two groups were biological species, isolated by what we call “ecological isolating barriers”: genetically based preferences for different habitats that kept two species from encountering each other.’ [emphasis in original]
This is a misrepresentation of my argument. I never claimed that hybridisation between these two groups of bears is problematic for the BSC. My exact words were this:
‘Mayr’s way of thinking about species has some amazing consequences. Recently, due to rising temperatures in the Arctic, polar bears and grizzly bears have been coming into increased contact, and have been producing fertile offspring. The offspring are (adorably) called grolar or pizzly bears. What this suggests is that polars and grizzlies may actually be the same species after all, despite radical differences in size, appearance, hibernation behaviours, diet and so on.’
As can be seen in this quotation, the hybridisation between these two populations of bear was presented as an ‘amazing consequence’ for the BSC, not as a problem for it.
Indeed, I never claimed that this hybridisation has the result that the bears are somehow not a species. Rather, I suggested that (since the advent of increased interbreeding behaviours) the BSC leads us to the conclusion that they all belong to the same species now.
In the above quotation, Coyne mentions ecological isolating barriers, and says that these barriers kept the two groups of bears from encountering each other. He takes this point to support the claim that they were separate species. Of course, it may well be that, because of these ecological isolating mechanisms, the two groups were indeed kept from mating, and thus (by the BSC) were separate species in the past (though we must not forget that some mating between these groups has actually been going on a long time (Cahil 2013)). My point was that changing climate is now breaking down these ecological barriers, as it is forcing polars and brown bears to come into increased contact, and to produce fertile offspring. Given the breakdown in the ecological isolation barriers, the BSC is leading us toward the conclusion that they are now the same species.
Species Pluralism
Here it is worth delving a little more into the subtleties of the case. My official view is not strictly speaking that polar bears and brown bears belong to the same species. Rather, it is that they are members of the same species for some theoretical purposes, and not members of the same species for others. As I mentioned in my article, polar bears and brown bears are very different from each other in lots of ways (behaviour, appearance, diet, etc.), and for some purposes, it will be a good idea for us to group together organisms based on similarities in these traits. Specifically, it will be useful to group together the bears that have developed similar adaptations to similar environments. When we do this, we will group the polar bears and the brown bears into separate species. Indeed, there are species concepts which classify organisms into species in just this way, based on organisms’ ecological niche (the environmental pressures that they have been exposed to, and the adaptations they’ve evolved to deal with them).
However, for other purposes, it will make sense for us to group together the animals that can exchange genetic information with each other (this is the core idea at the heart of the BSC). For this theoretical purpose, we should be grouping together the polar bears and the grizzlies as members of the same species because they exchange genetic information (at least, this will be appropriate in locations where polar bears and brown bears frequently encounter each other). I argued for this official view extensively in a paper that Coyne himself mentions, though he does not engage with it (Taylor 2019a).
These ideas reflect a view called ‘species pluralism’. The view is that there are a variety of species concepts available, that some of them are useful for some purposes, and that some are useful for others. Which concept we use should be determined by the theoretical purpose we have in mind (what we want to do with the concept in question). This view does not imply that any species concept is just as good as any other of course, the view is not ‘anything goes’. On this view, we should keep a wide range of species concepts in our toolkit, and select the right one for each particular job. Biology is messy. We should not expect just one single concept to be able to do all of the work that we need doing.
Microbiology and the BSC
The real reason I gave in my Conversation article to reject the BSC was to do with microbiology. I said that:
‘The [BSC] makes use of the notion of interbreeding. This is all very well with horses and polar bears, but smaller organisms like bacteria do not interbreed at all. They reproduce entirely asexually, by simply splitting in two. So this definition of species can’t really apply to bacteria.’
Coyne’s response to this issue is somewhat puzzling. He says ‘Taylor then trots out the old canard (if ducks can trot) that organisms that don’t interbreed can’t be subject to the BSC. DUH!’ Coyne then goes on to say that: ‘it’s hard (but not entirely impossible) to imply a reproductively-based species concept to bacteria’.
There is a tension in Coyne’s claims here. In the first quotation, he takes it to be obvious that the biological species concept cannot apply to organisms that don’t interbreed (he includes the term ‘DUH!’ to indicate the obviousness of this idea). However, almost immediately after, he says that it is hard but not impossible to apply the BSC to bacteria. So does he think that the BSC can apply to bacteria or not? It is very unclear what his view is on this issue.
This tension arises at several points in Coyne’s response. Sometimes he says that the BSC was ‘not even meant to apply to asexual organisms’, so it looks like he is saying that it doesn’t apply. He also says that ‘the BSC cannot be applied to species that lack sexual reproduction — like many species of bacteria’. At other points he is more circumspect, saying that:
‘there have been several attempts to discern bacterial species using reproductive criteria. The question hinges on whether there’s a problem to explain in bacteria: are they “lumpy,” like sexually-reproducing species, or do they form more of a continuum, and thus there’s not a biological observation that needs explaining? This question isn’t yet settled.’
In this latter quotation, it looks as though here he is saying that we may be able to apply the BSC to microbiology after all, but we just don’t know whether this will be possible yet. The main point of my article was to point out the widespread uncertainty on the nature of species. By admitting that it is unclear whether the BSC applies to microbiology, Coyne seems to agree with me. We just don’t know whether it can apply or not.
We should not underestimate how great this uncertainty about species in microbiology is. Asexuality is not an obscure or fringe case in the biological world. It is the norm. Asexual organisms make up most of life. As the philosopher of biology David Hull says:
‘[sexuality] turns out to be rare on every measure suggested by evolutionary biologists — number of organisms, biomass, amount of energy transduced, and so on’ (1988, p.429, quoted in Ereshefsky 2010, p.414).
By admitting that it isn’t clear whether the BSC applies to microbiology, Coyne admits that we don’t know whether it can account for most of life. This is a very high level of uncertainty about the nature of species, which is precisely what my original piece was pointing out. Obviously, Coyne may just bite the bullet at this point and say that the BSC doesn’t apply to microbiology, but this option leaves us completely without an answer to the species problem for most of life.
A brief aside on microbiology.
Let’s take a step back from Coyne and look at the wider issue of the species problem in microbiology, because it is interesting and important in its own right (here I follow work by Ereshefsky (2010)).
Some concepts of species in microbiology are indeed similar to the BSC (Fraser et. al. 2007; for an interesting discussion of the BSC in microbiology, see Franklin 2006). Others define species in terms of similarity in the ecological adaptations of their members (Cohan 2002). Another (especially popular) one takes its inspiration from cladistics, by claiming that microbiological species must be monophyletic. That is, they must include one ancestor and all of its descendants. This approach then uses data about RNA and DNA to ascertain this (Rosselló and Amann 2001)). In microbiology too, we find a variety of alternative approaches to species.
Here a further complication can be thrown into the mix (thanks to Richard Hayward, a senior lecturer in microbiology at the University of Cambridge, for pointing this out to me). There are huge differences between different microbiological groups, meaning that some of them might be amenable to some species concepts, whilst others may be amenable to others (or none at all). For example, some of the order Chlamydiales constitute a reasonably stable lineage. What this means is that there is relatively little movement of genetic material outside of the lineage of organisms. The relative stability of this lineage, and its relative lack of gene exchange with organisms outside of the group, means that it may be possible to apply the BSC (or some similar concept) to it. The reason is straightforward: recall that the BSC was designed to capture relatively isolated gene pools, where gene exchange within the group occurs in a reasonably free manner, whilst it does not occur as freely outside of the group. By contrast, in other groups of microbiological organisms (such as members of E.coli) gene exchange with organisms outside of the group is rife. As a result, E. coli does not represent a stable lineage in the way that members of the Chlamydiales order do, and so it may be the case that E.coli is not suitable to classification using the BSC. We may need a piecemeal approach even within microbiology.
Now is not the time to explore these issues further, but one thing is definitely clear: that there is uncertainty about which approach to species is best when it comes to microbiology. Of course, I did not have time in my original article to go into these issues, because I was restricted by the word limit. However, when we scrutinise microbiology, we find that it supports my view.
Notice that my argument has nothing to do with fuzziness in the concept ‘species’. There is nothing at all wrong with a species concept being fuzzy. I have argued at length that natural kinds often have vague and fuzzy boundaries, and that this is no bad thing. So, we should expect concepts that refer to these natural kinds to also be fuzzy in their extensions (Taylor 2019b, 2018). The problem is not that the BSC does a good job of picking out groups of organisms, but that it has a few fuzzy edges. The problem is that we don’t really know how to apply it to most of life in the first place.
The popularity of the species concept in biology.
From reading Coyne’s response, one may gain the impression that the BSC has been agreed upon within biology, and that the matter is now settled. He says:
‘these criticisms of the BSC have been made many times before, and dispelled equally many times — I do it in my book, which is 15 years old’
But the matter is not settled. To see this, we can return to the recent surveys on the species problem that I mentioned above (Zachos 2018, Wilkins 2018). Some of the many concepts listed in these surveys are similar to the BSC (Wu 2001). Some of them contain elements of the BSC and merge its insights with those of other species concepts (Stamos 2003). Others depart from the BSC very significantly (Ridley 1989, Mishler and Theriot 2000). The issue is anything but settled.
In the quotation above, Coyne says that my objections to the BSC have been made many times before. This is true (I myself referenced several works where they have been made). My piece was designed to introduce a wide audience to the species problem, not to introduce novel and original objections to the BSC.
Section 3: Species Eliminativism
At the end of my article, I mentioned species eliminativism: the view that there are no species.
Coyne interprets me as arguing in favour of species eliminativism. Strictly speaking, I did not argue for species eliminativism. I introduced the idea (which comes from Charles Darwin) that we should do away with the species concept, then suggested that it would be a natural next step for biology, and tried to convey some of the importance of the step. Again, the article was a popular piece intended for a wide audience, and the aim was simply to present an extreme position and explain its merits. My official view is not species eliminativism. As I argued in the piece that I earlier mentioned (Taylor 2019a) I am a species pluralist: I think that there are several alternative definitions of ‘species’, and that some are useful for some purposes and others are useful for others.
Nonetheless, it certainly is the case that I presented species eliminativism in a positive light, as a reasonable and sensible view. And I certainly do believe that it is a reasonable and sensible view. One version of the position is similar in spirit to species pluralism in that it argues that there are a variety of different concepts of ‘species’ and that each of them has a place in biological theorising, but there is no overarching privileged category of ‘species’. What makes this view eliminativist is that it takes the further step of arguing that biology should abandon the concept of ‘species’ altogether and replace it with other concepts (Ereshefsky 1992). Put another way, the idea is that we can certainly classify together groups of organisms that are similar with respect to interbreeding (as the BSC has it) or that form natural and complete lineages (as cladistics has it). Though these alternative classifications may be useful for biology, there is no single notion of a ‘species’ that can correctly capture all the various taxa of interest to biology, and biology would be better off not worrying about the species question itself. As I pointed out in my original piece, this is reasonably similar to Darwin’s own view.
Species eliminativism comes in a few varieties. Several biologists and philosophers have argued that the notion of a species should be rejected and replaced with some other taxonomic concept (Pleijel 1999, Mishler and Wilkins 2018). There is nothing unreasonable in any of these views.
Coyne points to the ‘lumpiness’ of nature as evidence in favour of the existence of species. Presumably he is referring to the fact that certain groups of organisms tend to share a wide variety of traits to each other, and that these traits tend to stay within one group of organisms (you don’t tend to find a whale that suddenly develops bear fur), hence the ‘lumpiness’.
Now, it is true that some sets of organisms are lumpy in this regard (though maybe not all: remember how much horizontal gene transfer there is in E.coli, resulting in a huge amount of variation within this group). However, it doesn’t follow that we should be classifying organisms into species based on these lumps of similarity, or that doing so will provide us with a correct answer to the question of what a species is. Much recent work has argued against this emphasis on similarity on the grounds that many taxa that are important for biology don’t contain members that are very similar to one another at all (Magnus 2011). Lumpiness does not a species make.
Section 4. Conclusion
My hope is that a reader will at least be convinced that the species problem is a real problem, and one that must be confronted. I further hope to have shown that the positions (such as pluralism and eliminativism) are much more sensible and robust than Coyne claims in his response.
Acknowledgements
Thanks to Natasha Kriznik and Peter Vickers for comments on a previous draft of this article.
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