The Classification Problem - Fungal Considerations

 THE CLASSIFICATION PROBLEM - FUNGAL CONSIDERATIONS

by Wilfred Franklin

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Taxonomists and systematists are interested in understanding evolutionary relationships between living organisms.  This in turn is motivated by an interest to understand the forces and factors that influence evolution within taxa and in so doing, evolution in general. 

Mycology, the study of fungi, is an interesting test case for systematist because to infer evolutionary process the researcher must first come to some consensus on the pattern from which one is inferring that process.  Species is the unit of measure that biologist use to categorize the patterns of living diversity, but fungi display a unique set characteristics that make it very difficult to delineate species.  There are many versions of the species concept and fungi are problematic for all of them.  In other words, how does a researcher infer process (evolution) from pattern (species), when the organisms defy classification - when fungi do not fall into a clear pattern. This presents a philosophical consideration I call the Classification Problem that has implications for epistemology in general, the nature of reality and constructs of it. What follows is a brief overview of relevant fungal characteristics to set up the Classification Problem. 

With the exception of the single celled fungi, yeast, all fungi are made up of long  narrow cells called hyphae.  Theses hyphae, branch and multiply to form a web-like mass (the fungal body) called mycellium.

Thus, if this is fungi and this is also fungi, how does one classify these into species? There are no morphological traits by which to circumscribe a species. For a large number of fungi these characteristics rule out the morphological species concept for the vast majority of fungi.

Seasonal fruiting bodies have some morphological traits by which to differentiate organisms. Imagine all tree species were the same, like all mycellium. However, tree flowers and or fruits are unique.  One could use apples and oranges to distinguish the two different organisms.  For the most part, that is what occurs with fungi.  Researchers look at fruiting bodies, of which a sub-group are known as mushrooms.

The comparative analysis of fruiting bodies works well enough for the macroscopic and commonly seen fungi, but the large majority of fungi do not reproduce sexually (Compare the Life Cycle of a Mushroom  to the Life Cycle of an Asexual Fungi). This again makes the morphological species concept in adequate for most fungi and in addition, rules out the biological species concept in which species are delineated by populations of individuals that can sexually reproduce.  

That leaves researchers with several lesser known species concepts, such as the ecological species concept and the phylogenetic/evolutionary species concept. Each concept has some use for separating some taxonomic groups, but both are still very problematic at the level of species.  Theses concepts are both a bit more technical, but boil down to a few key characteristics about fungi.

The ecological species concept uses niche and environmental resource partitions to aid in the delineation of species.  Two organisms can be very closely related, perhaps considered to be part of the same species but two groups use different parts and resources within the environment.  With fungi, most are saprophitic or decomposers of organic debris and this species concept boils down to separating species by the type of organic material it decomposes.  Some fungi are generalist, but others are specialist on one type of tree, plant or insect.  In these cases an ecological species concept can help, but when considering generalist, this concepts adds no discriminating power. 

That leaves a researcher with the phylogenetic species concept.  In essence it is a concept that ignores sex, environment and life history and instead analyzes organisms at a genomic level and looks for discrete, non-overlapping gaps in genomic variation across individuals and populations. Once enough information of this type is gathered a researchers can make an argument that two groups should be considered separate species given that they represent a unique, genomically bounded clade - a genetically cohesive group on its own evolutionary trajectory.  This idea does not place emphasis on any one biological consideration, instead only looks at the results of any and all evolutionary forces at play on individuals and populations.  

The phylogenetic species concept has much appeal, but is problematic for some of the same reasons the other species concepts have problems, namely, when looking at a mycellium, where does one individual end and another begin?  Where does one population end and another begin?  Before a mycologist can identify discrete, non-overlapping groups in genomic structure that researcher must have collected samples and assigned them to a distinct individuals and or populations.  Add this is no trivial task when at least one fungal individual has been recorded at over 9 square kilometers. Not to mention the laboratory effort and energy needed just to begin to assign samples as pertaining to one individual or another.  What amount of genetic difference signifies a different individual, different population, etc?  And all of this has to be worked out before any large scale genomic analysis a variation can be started.

To summarize, fungi have a set of characteristics that make them very hard to assign to species, let alone deciding any level of variation that might be meaningful especially since most do not reproduce sexually.

Fungi then, presents one particular case of a much larger issue that has been recognized for some time, namely:

“Classification is not the recognition of something that exists in nature, for nature is not classified. It is the invention by man of ways in which events and things can be grouped, for the sole purpose of increasing his ability to deal with the events and things.”

From Systematic Zoology. (R. E. Blackwelder, 1959).

And perhaps Einstein said it best with respect to physical constructs:

"Physical concepts are free creations of the human mind, and are not, however it may seem, uniquely determined by the external world”

(Einstein & Infeld, 1938).

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