By Tom Wells
I was once told not to think too much about the definition of the word “species” if I valued my sanity and career. In the depths of lockdown perhaps both of these things seemed fairly precarious anyway, because the Scotland group decided to turn our collective attention to that knottiest of biological conundrums: the “species problem”.
This decades-old argument about how best to define species continues to nag at the subconscious of many biologists, with most following my former-lecturer’s advice and trying not to think too much about it. No single definition has ever satisfied everyone, and there are now well over 30 competing definitions – termed “species concepts” – in existence. The most famous, and perhaps the most popular, is Ernst Mayr’s (1963) Biological Species Concept (BSC), which emphasises reproductive isolation as the ultimate test of different species, but even proponents admit this definition is far from perfect (Gao and Rieseberg 2020; Coyne and Orr 2004). Most notably, it is extremely difficult to test empirically and cannot be applied to asexual organisms. Even in sexually reproducing animals and plants however, the BSC is increasingly undermined by evidence of widespread historic and contemporary hybridization between what are otherwise highly distinct species; the “Pizzly” Bear for instance – lonely and occasional offspring of Polar Bears and Grizzlies – or the possession by modern humans of significant quantities of Neanderthal DNA.
In contrast to elements in Chemistry and particles in Physics then, Biology’s best-known unit – the species – remains ill-defined and disputed, and yet it continues to be used by researchers. This problem is un-nerving because an indefinable unit is a potentially incomparable one. Biologists use species in their research all the time, whether that’s in ecology, systematics, evolutionary biology or conservation. Knowing which species a given organism belongs to is therefore integral to studying and understanding both that organism and the wider world. And if different researchers are using different definitions of the term species, can their results be compared?
Most of us feel intuitively like we know what a species is, even if we might struggle to give an airtight definition when put on the spot. This instinctive acceptance of the notion of species is backed up by the largely concordant delimitation of species by modern science and what is rather dismissively known as the “Folk Taxonomy” of indigenous communities around the world (Mayr 1963; Diamond 1966; Bulmer and Tyler 1968; Berlin 1973, 1992; Berlin et al. 1974; Bulmer et al. 1975; Majnep and Bulmer 1977; Atran 1998; Coyne and Orr 2004; Ludwig 2015; Slater 2015). These different knowledge systems might have very different classification systems thanks to their basis in differing cosmologies that emphasise criteria other than evolutionary relatedness, but they all appear to possess a category more or less identical to species. This suggests, at least to an extent, that the term species refers to real, observable phenomena.
Nevertheless, many have argued that acceptance of the theory of evolution by natural selection sounded the death knell for an essentialist view of species’ reality (Mayr 1959, 1963, 1976, 1982; Simpson 1961; Hull 1965, 1976, 1988; de Queiroz 1988; Sober 1994; Dupré 1999; Okasha 2002; Rieppel 2009) with comments by Darwin himself in the Origin of Species that appear to equate species with mere varieties often being cited in this regard (Darwin 1859, p. 52).
While it is far from clear that this was Darwin’s definitive view on the subject (Wilkins 2009; Richards 2011a; Stamos 2013), or that naturalists who used the term species before him had a simplistic and static notion of what they might be (Boyd 1999; Winsor 2003, 2006; Wilson et al. 2007; Wilkins 2009; Rieppel 2010; Richards 2011b), it does seem clear that evolution poses serious challenges to defining what exactly a species is. This is because evolving species are necessarily variable entities across space and over time: they are “born” through the process of speciation, “die” through extinction, and are continually changing in terms of their constituent parts between these two points. Variability is an inherent and natural consequence of evolutionary forces such as mutation, gene flow, genetic drift, developmental happenstance, and adaptation to selective pressure. As Darwin realised, this variability is also what allows one species to give rise to another. And this continual change raises a fundamental question about the nature of species; an issue that Rieppel (2009) described in terms of a Heraclitus Paradox: if all the elements of which something is composed are exchanged or replaced, can it still be considered the same entity? Fans of nineties British comedy Only Fools and Horses may recognise this in terms of Trigger’s broom, which Trigger claimed to have been using for 20 years, but has had 17 replacement heads and 14 new handles.
Despite the pervasive disagreement within the academic community about the reality and definition of the term species however, species continue to retain an important place in biological research. What we know about species in practice – such as species richness and distribution patterns – has led to important scientific insights including the latitudinal biodiversity gradient (Hillebrand 2003), biodiversity hotspots (Myers et al. 2000), biogeography (von Humboldt 1805; de Candolle 1855; Wallace 1876; Brundin 1972) and, not least of all, the theory of evolution by natural selection itself (Darwin 1859). Not only did a commitment to the idea of species facilitate this work, but denying species their reality might be seen to threaten or undermine the theories themselves (Richards 2010). What is more, if species are figments of our imagination, why should we be concerned if large numbers of them are threatened with extinction (Hull 1997)?
In this sense theory and practice seem to be out of alignment in biological research. In theory species are, if not arbitrarily, then at least inconsistently defined and identified. And yet in practice they are the bedrock of much research, where they are counted, compared and contrasted all the time. We need a comparable unit, but instead we have a highly variable one. And trying to come up with ever more specific definitions has not solved the species problem. As David Hull (1997) pointed out, the more theoretically applicable a species concept is, the less practically useful it is. So, perhaps a different approach is required.
Species delimitation and identification are essentially decision problems: “Is a group of organisms best described as one species or two?” and “does a given organism belong to one species or another?”. Heuristics are a means of making decisions in the face of irreconcilable uncertainty. We need an answer based on the evidence we have in order to proceed, and we need that answer to be as good as possible, though it can never be perfect. The conclusion we arrived at is that if the term species is treated as a heuristic one, then perhaps this can alleviate some of the difficulty arising from the species problem.
Treating species as heuristic also fits nicely with both the theoretical ideas of what species are and the practical ways we identify and use them. If species are constantly changing networks of variable individuals, then we are forced to use proxy characters as a heuristic way of recognising them. And this makes sense practically, because as humans we can only perceive a tiny sample of all the potential members of a species through time and space, and we do so with a limited set of senses. We never have access to all the “data” and even if we did, it wouldn’t help us. These are exactly the kinds of problems heuristics were designed for.
Perhaps most importantly, recognising species as a heuristic brings us back to what we need species to be – a more or less comparable unit of biodiversity. Something that can be used to understand the biological world around us. And if this is the case then it also provides implicit guidelines for how we should treat species. That is as “clusters of closely related individuals responding in a similar manner to comparable sets of evolutionary and ecological forces”. This in turn provides further guidelines for how to delimit them: through the “congruence of contingent properties indicative of those forces”. In this sense treating species as a heuristic allows us to reconcile theoretical ideas of what species are with the practical ways they can be identified and used in biological research.
Problem solved? See for yourself here: https://academic.oup.com/sysbio/advance-article/doi/10.1093/sysbio/syab087/6407163 and make your own decision.
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Tom Wells is a D.Phil. student at the University of Oxford, Department of Plant Sciences. He and colleagues published “Species as a heuristic: reconciling theory and practice” in Systematic Biology, October 2021.