Saturday, June 22, 2024

Classification in Biology, Medicine, and Psychiatry


Plecoptera sketch

Classification in Biology, Medicine, and Psychiatry

I was a double major in college – biology and chemistry. Anyone with similar experience who has done fieldwork in zoology, botany, or associated systems disciplines like ecology is aware of various forms of biological classification. I spent months doing population surveys of plants, aquatic invertebrates, and plankton in undergrad courses and working on water quality surveys for the southern tributaries of Lake Superior.  In the winter, a friend and I would go out on the ice in various locations, chop a hole in the ice, and pump about 80 gallons water through a plankton net – and then carefully take that concentrated sample back to the lab and count plankton species under a microscope. It was painstaking work – but the payoff was a much greater knowledge of the life forms around you.  The lakes, streams, and rivers, and even the ground you were standing on is teeming with life.  Hundreds of species that were unknown until you took those courses and learned how to collect and classify them.

All of that classification was done using a Linnean binary classification system – genus followed by species. As an example, Loblolly pine is Pinus taeda.  Douglas fir is Pseudotsuga menziesii. The Douglas fir example is given because it is not a true fir tree (genus Abies) despite the common name.  The difference is based on morphological characteristics although the entire genome was characterized in 2017. Both the common and scientific names illustrate how species are named.  The common name was after David Douglas a Scottish botanist who described it and the species name after Archibald Menzies – a Scottish physician and naturalist.

Other physicians have been involved in the taxonomy of both human diseases and biological species most notably Carl Linnaeus (1707-1778) – a Swedish physician who is credited with founding the binomial nomenclature used in all modern taxonomy.  He also recognized the necessity of organizing illnesses so that physicians could know they were discussing similar problems. By 1746 he had organized them into nine classes – critici, phlogistici, doloritici, mentales, privatii, spastici, deformans, evacuatorii, and chrirurgia.  Linnaeus was trained as a physician and got his doctorate degree in 1735, but worked as a physician intermittently and for a short period of time until 1741 when he became a professor of botany and theoretical medicine.  

Linnaeus also focused on a species diagnosis rather than a description, meaning features that could clearly distinguish one taxon from another. He went as far as saying the diagnosis should not exceed 12 words and should include previous references and diagrams (6). According to the same reference, the concept of species diagnosis is included in modern nomenclature for species as: “a statement of that which in the opinion of its author distinguishes the taxon from others.” (p. 1090).

Linnaeus taught medical students a course in the diagnosis of disease and in 1759 published a book - Genera Morborum (Varieties of Disease).  He described 11 classes of disorders, 37 orders and 327 genera.  Mental disorders (Mentales) were divided into 3 orders and 25 genera (7). Inspecting the list from that reference suggests that delirium, mania, melancholia, hypochondriasis, anxiety, sleep, and appetitive disorders were all included.  Vertigo was also included and today is considered a neurological or otolaryngological diagnosis.  The overall tenor of reference 7 is that the various systems devised by 17th and 18th century physicians would never attain the level of certainty as the biological taxonomy used in natural history that we now know is also controversial.

That brings me back to the problem of biological classification – specifically speciation. That has been a predominate area of study in biology since Darwin and it is not without controversy despite the broad use of these methods in all of biology and science. What is equally interesting is that as far as I know there have been no direct comparisons to medicine and psychiatry.

Let me start by touching on the controversies of speciation from a biological standpoint.  I qualify these remarks by saying my training was at the undergraduate level and all of what I say here is based on recent readings.  I am very interested in hearing from professional biologists on this topic and so far have not been able to get any email responses. My overriding hypotheses are that the same mechanisms leading to speciation controversy in biology may be responsible for classification controversy in medicine and psychiatry.

Coyne and Orr (1) break down speciation into three phases.  The first phase began with Darwin and they point out that he had more to say about changes within species than the origin of new species.  Natural selection was seen as the most important force. The second phase began in about 1935 and was also known as the Modern Synthesis.   It was also marked by a critique of the species concept by Dobzhansky a noted evolutionary biologist.  Reproductive isolating mechanisms were stressed as a necessary cause of speciation. Ernst Mayr another noted evolutionary biologist came up with the biological species concept (BSC).  He defined species as interbreeding populations that are reproductively and geographically isolated from one another.  The third phase started in the 1980s. Coyne and Orr suggest that “more work on speciation has been performed in the past two decades than the entire period from 1859 to1980.”   They speculate that this explosion of work was due to several factors including the new tools of molecular genetics, more extensive use of mathematical models, a new emphasis on ecology, and a use of comparative studies.  The old hypotheses about speciation were re-examined and many new concepts were introduced (in addition to the BSC).  They list nine possibilities grouped as based on interbreeding, genetic or phenotype cohesion, evolutionary cohesion, and evolutionary history (p. 27).

Coyne and Orr described how the species concept was validated over the years.  First, arguments from common sense.  That is that anyone can see clusters of species and recognize they are real. This same phenomenon occurs with physical and psychiatric illnesses.  A second stronger argument is that there is concordance between folk and scientific species.  In this case researchers will look at the species in a given area and see if it is separated out and recognized by local people as being a unique cluster of traits.  That has also been referred to as folk taxonomy.  The concordance is typically high, and I suggest that the same type of concordance experiment is not only possible in medicine – it may have been done using the disease concept (2,3). And finally statistical identification of clusters. This has been done with both medical and psychiatric diagnoses to examine diagnostic features and also to determine if they separate different clusters adequately enough from one another.

The authors carefully explain the exceptions to the various species concept and settle on their own version of the BSC as the more viable one. But even the modern-day species concept is a problem because it does not clearly define all cases.  Several authors refer to it as the “species problem” and there is ongoing controversy in the literature. At the extreme there have been suggestions that the idea of species stems from a limitation of human cognition (we must lump things into categories) or there is a biological continuum that we are marking up arbitrarily.  Any reader of this blog realizes that these are frequent arguments made against psychiatric diagnoses and by extension other complex polygenic medical conditions.  Keep in mind that as far as I know all biology majors working on taxonomy in the past 50 years use the same binary system of classification that I used in college.

To their credit the authors propose how the species concept is useful and they come up with the following observation most biologists want an operational classifier that allows for systematic classification, describes what is seen in nature, helps develop an understanding about how things occur in nature, is consistent with evolutionary history, and applies to most organisms. In other words, it is not very different from what physicians expect diagnostic classifiers to do.  They realize that these are imperfect complex, multidimensional classifiers that are nonetheless useful for daily work and can be fine-tuned for improvements in the individual case.

That is what I am working on so far. I think I have demonstrated how biological classification even using all available methods and directly observable data is difficult if not impossible in many cases. The same can be said about medical diagnoses. That is because both the medical diagnoses and binary species designations are complex, multidimensional variables rather than basic physical structures. It does not mean that groupings in biology do not exist.  The key questions for my additional focus will be on the underlying mechanisms.  I have already described stochastics as a basic biological mechanism introducing some degree of uncertainty into biological systems – but I am sure there are many more.  In this era of proposed alternate diagnostic systems for mental disorders (Research Domain Criteria (RDoC), The Hierarchical Taxonomy Of Psychopathology (HiTOP), etc) – my reading so far suggests that there has been very little input from biological sciences. In most cases it seems like just a reshuffling of existing theory and measurement. My hypothesis going forward is that biological theory has a lot more to offer.


George Dawson, MD, DFAPA




1:  Coyne JA, Orr HA.  Speciation.  Sunderland, MA Sinauer and Associates, 2004: 1-82.

2:   Tikkinen KA, Leinonen JS, Guyatt GH, Ebrahim S, Järvinen TL. What is a disease? Perspectives of the public, health professionals and legislators. BMJ open. 2012 Jan 1;2(6):e001632.

3:  Tikkinen KAO, Rutanen J, Frances A, Perry BL, Dennis BB, Agarwal A, Maqbool A, Ebrahim S, Leinonen JS, Järvinen TLN, Guyatt GH. Public, health professional and legislator perspectives on the concept of psychiatric disease: a population-based survey. BMJ Open. 2019 Jun 4;9(6):e024265. doi: 10.1136/bmjopen-2018-024265. PMID: 31167856; PMCID: PMC6561450.

4:  Hey J.  Genes, categories, and species. NY, NY. Oxford University Press, 2001.

5:  Broberg G.  The man who organized nature.  Princeton, NJ. Princeton University Press, 2023; p. 221.

6:  Renner SS. A Return to Linnaeus's Focus on Diagnosis, Not Description: The Use of DNA Characters in the Formal Naming of Species. Syst Biol. 2016 Nov;65(6):1085-1095. doi: 10.1093/sysbio/syw032. Epub 2016 May 4. PMID: 27146045.

7: Munsche H, Whitaker HA. Eighteenth century classification of mental illness: Linnaeus, de Sauvages, Vogel, and Cullen. Cogn Behav Neurol. 2012 Dec;25(4):224-39. doi: 10.1097/WNN.0b013e31827de594. PMID: 23277141.


Graphics Credit:

Stonefly drawing by Carpenter, George H. (George Herbert), 1865-1939.  Public domain per Wikimedia Commons.  Genus is Plecoptera and it was one of the many species I studied in freshwater streams of Wisconsin and Michigan as an undergrad.  Click on the graphic for details. 

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