What are ‘good species’?

March 6, 2014 by · Leave a Comment 

Recently in TREE, Mallet [1,2], argued for an operational, concept-free definition of species as ‘genotypic clusters’, asserting ’that species are man-made groupings’ [2]. However, Mallet resorts to the traditional notion of ‘good’ species for final arbitration regarding what degree of variation is appropriate for the species-level taxon.

This is a poor species definition for two reasons. As an operational definition it leaves us with no means for dealing with the great complexities of biological systems of descent. Moreover, the decision as to what constitutes species-level variation is based on an essentialistic perspective offered by a ‘good taxonomist’s or naturalist’s definition’ [1].

If species are, in Mallet’s operational terms, ‘groups that remain recognizable in sympatry because of the morphological gaps between them’ [2], it is important to realize that they are nothing more than the sum of the operations that serve to identify them [3,4]. We are left with the arbitrary decision of how large these gaps must be and what frequency of intermediates would lead us to accept two species rather than one. These difficulties are problematic for the diagnosis of a ‘species’ under any definition or concept, due to the fuzzy nature of groups resulting from, or participating in, the evolutionary process (i.e. natural groups).

However, they are more severe for a concept-free definition because we have no theoretical guideline with which to sort variation into hypotheses about natural groups. A conceptfree definition of species as ‘genotypic clusters’ must also deal with the discrete morphological variation manifest, for example, between genders of many plants and animals. These are ‘genotypic clusters’ of sorts. However, neither today, nor in Darwin’s time, do biologists wittingly hypothesize different species for different genders. Without an ontological context with which to sort variation in biological systems, we find ourselves perplexed by situations as straightforward as sexual dimorphism.

In stark contrast to his purely operational definition, Mallet alludes to such an ontological framework by reference to ‘good’ species [2]. But what are ‘good’ species? Mallet endorses the traditional position that, in the most difficult cases, the ultimate authority of the existence of ‘good’ species is the taxonomist or naturalist.

The implication of this deference to the taxonomist is that ‘good’ species exist, but that their essential nature is hidden; a taxonomist’s contribution is to reveal ‘good’ species through description, case by case. Thus, the notion that ‘good’ species can be revealed to us by taxonomic authorities is  steeped in the essentialistic outlook that Mallet [1,2] (and others [5]) seek to condemn. Furthermore, the definition of a species becomes ‘a group of organisms that is recognized as a ‘good’ species by the taxonomist or naturalist.’

This is obviously undesirable. Although taxonomists may point to groups that they believe exist, species will only have objective value if the general properties of ‘good’ species (the species taxon) are revealed to the rest of us. Whereas a purely operational definition causes us to forego the question, ‘what is the nature of the group that we might call species?’, asserting the existence of ‘good’ species (even if we knew their properties) demands that all groups of organisms, that we might call species, exist in the same ways. A definition that results in one or both of these outcomes should be avoided, particularly in studies of speciation where we are interested in all the natural groups produced by a pluralistic process of evolution. It is a step forward for students of speciation to acknowledge that different sorts of natural groups have valid claims to the term ‘species’ [6,7].

Similarly, it is regressive to undermine the notion that the species taxon (whatever natural group we choose for it to designate) has underlying properties that make it worth studying. One possible solution is provided by a nominalistic approach [6-9], which formulates a species definition explicitly while retaining the ontological meaning that a purely operational definition leaves behind. Such a definition would embody a statement of the necessary and sufficient properties for the diagnosis of species in any particular case. The important distinction between a nominalistic definition and Mallet’s is that our avenue of inquiry would lead us to explore the nature and evolution of natural groups (as opposed to some notion of a ‘good’ species), with or without a coextensive relationship between such groups and ‘good’ species (whatever they are!).

Kerry L. Shaw
Dept of Organismic and Evolutionary Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138, USA

[1] Mallet, J. (1995) Trends Ecol. Evol. 10,294-299
[2] Mallet, J. (1995) Trends Ecol. Evol. 10, 490-491
[3] Hull, D.L. (1968) Syst. Zoo/. 17,438-457
[4] Baum, D.A. and Donoghue, M.J. (1995) Syst. Bot. 20, 560-573
[5] Mayr, E. (1982) The Growth of Biological Thought, Belknap
[6] de Queiroz, K. and Donoghue, M.J. (1990) Cladistics 4, 317-338
[7] de Queiroz, K. (1994) Syst. Biol. 43, 497-510
[8] Popper, K.R. (1966) The Open Society and its Enemies (5th edn), Princeton University Press
[9] Baum, D.A. and Shaw, K.L. (1995) in Experimental and Molecular Approaches to Plant  Biosystematics
(Hoch, PC. and Stephenson, A.G., eds), pp. 289-303, Missouri Botanical Garden

How many species are there?

March 24, 2012 by · Leave a Comment 

An interesting research note just came out in the American Naturalist by Hamilton and colleagues entitled quantifying uncertainty in estimation of tropical arthropod species richness. I retweeted a Science Daily twitter feed on this that had a terribly misleading opening line: “New calculations reveal that the number of species on Earth is likely to be in the order of several million rather than tens of millions“. This is, of course, absolute rubbish because the authors only looked at estimating tropical arthropod richness, not all species on Earth. The number of protists alone is probably > 4 million species, and there are an estimated > 1.5 fungi.

That whinge about crap reporting aside, this is what Hamilton and colleagues concluded:

  • using stochastic models, they predict medians of 3.7 million and 2.5 million tropical arthropod species globally
  • estimates of 30 million species or greater are predicted to have < 0.00001 probability
  • uncertainty in the proportion of canopy arthropod species that are beetles is the most influential parameter
  • in spite of 250 years of taxonomy and around 855000 species of arthropods already described, approximately 70 % await description

Interesting, but I didn’t give it much notice until New Scientist contacted me to get an assessment (their article will appear shortly). This is what I had to say:In general, I commend the authors for attempting to shed some mathematical light on the problem of species richness estimation. I believe that many species richness estimates are inflated for a number of taxa given the paucity of reasonable data with which to make extrapolations. I therefore support the notion that some estimates (e.g., > 30 million tropical arthropod species) are unrealistic.

That said, I believe that the approach potentially underestimates the influence of beta diversity on simple alpha diversity algorithms. Although they acknowledge that changing specialisation across a species’ range is possible (but could not correct for this), their algorithm completely ignores three MAJOR driver of biodiversity patterns: (1) the community of local competitors, (2) the community of local predators and (3) the biogeographical history of a particular ecosystem. These will shift enormously across a species’ range and impose a plethora of constraints that tend to promote speciation (i.e., greater number of niches).

Additionally, but related to the above, taking a single dataset from one island nation and extrapolating it to the entire tropical region is fraught with potential error. It makes for highly uncertain scientific predictions because it cannot capture all the nuances of species distributions elsewhere. Every biological community is different.

My overall conclusion is that while the algorithm provides some direction about the upward bias in existing estimates of arthropod species richness, their prediction is also likely to be far too conservative to be realistic. I would predict the ‘true’ species richness lies somewhere between their estimate of 2.5-3.7 million and existing estimates of > 30 million.

My other concerns include:

  1. It seems to me that the major assumption is the degree of specialisation – this is perhaps the most imprecise parameter and possibly prone to underestimation, especially in light of the high specialisation values observed for most tropical invertebrates.
  2. The sensitivity analysis is basic and does not take into account partial correlations. A multivariate ‘global’ sensitivity analysis using logistic regression is more robust (McCarthy et al. 1995. Biol Conserv 73:93-100); thus, I suspect that their rankings of parameter sensitivity are incorrect.
  3. I very much doubt the parameters in equation 1 (except number of herbivorous canopy beetles) followed uniform distributions. At the very least, I suspect these to be Poisson, log-Normal, Normal or beta (depending on type). The authors discuss this, but I disagree that the Pert is a good alternative distribution. For example, the proportional parameters (i.e., proportion of species that are beetles, the proportion of arthropods in the canopy, etc.) might in fact have a ‘central’ tendency much closer to an extreme between 0 and 1 under say, a beta distribution. Therefore, I believe that the authors have severely underestimated the variance (especially of high richness values), indicating that the upper confidence bounds are too conservative.

Why is any of this important for conservation? Without good estimates of species number and distribution, we have no idea how much we stand to lose/are losing as habitats are destroyed. This is essential information for predictive conservation biology, so we need to get it right. Good on Hamilton and colleagues for stepping in and moving the discipline forward. CJA Bradshaw


Hamilton, A., Basset, Y., Benke, K., Grimbacher, P., Miller, S., Novotný, V., Samuelson, G., Stork, N., Weiblen, G., & Yen, J. (2010). Quantifying uncertainty in estimation of tropical arthropod species richness The American Naturalist, 176 (1), 90-95 DOI: 10.1086/652998