Challenges Faced by Biologists When Defining a ‘Species’
Of the Age-Old Species Problem
It is common knowledge that planet Earth boasts a diverse array of living organisms; from single-celled bacteria to complex human beings. This diversity lends itself to the classification of organisms(mammals, angiosperms, primates, etc.) — with the species being the lowest level of biological classification. It may seem ironic that a science that prides itself in discussing life forms does not have a unanimously agreed upon definition of what it considers its lowest grouping. But life is much more complex than meets the eye.
A layperson would perhaps wonder why biologists cannot agree on this aspect when everyone else intuitively knows that an elephant and a housefly are characteristically different, and thus cannot possibly be the same species. This view, known as the Typological Species Concept (TSC), stands as the oldest way of recognizing species. Its proponents viewed a species as a group of organisms whose members share certain characteristics that distinguish them from other species. It stems from the Aristotelian philosophy of Essentialism, summarized below:
Granted, this way of thinking does make a lot of sense. However, the TSC breaks down when faced with intraspecific differences based on the age and/or stage in the life cycle of the organism. Take a caterpillar and the butterfly or moth that it turns into, or the fierce antlion and the lacewing fly it morphs into. These drastic differences in morphology between the larval stages and their adult forms would lead a typologist to misclassify them as distinct species.
It is a well-known phenomenon that there tends to be marked variation of forms within a population of a species — such as the four variant blood groups in human beings. This phenomenon is known as polymorphism. I performed a little test on my younger brother, Neil, who happened to have been taught the typological species concept. I showed him a picture of a black panther and asked him if he would classify the big cat like a leopard (jaguars also have the melanistic variant). His response was succinct — ‘I do not think this is a leopard because it doesn’t have the essential characteristics of a leopard: yellow skin with dark spots grouped in rosettes’.
Resolving such problems demanded restructuring the species concept entirely. This overhaul led to the formulation of the Biological Species Concept (BSC), proposed in 1942 by Ernst Mayr, one of the greatest twentieth-century Darwinians — although Mayr himself recognized a number of biologists before him who had proposed similar criterion. There are many variant wordings for the BSC, but it generally describes species as groups of actually or potentially interbreeding populations, which are reproductively isolated from other such groups.
Reproductive isolation is a rather multifaceted term that refers to the various evolutionary mechanisms — pre-zygotic (before fertilization) and post-zygotic (after fertilization) — that prevent members of different species from producing offspring. Pre-zygotic isolation includes mechanisms such as different mating rituals between animals of different species while post-zygotic isolation includes hybrid sterility, as is the case of the liger or mule. Mayr belabored the applicability of these isolating mechanisms in plants — because it was not favored by numerous botanists — by alluding to the specialization of species of flowering plants to specific pollinators.
As with the TSC, there are a number of criticisms drawn against the BSC. Among others, the BSC is inapplicable to asexually reproducing organisms. In his treatise, Mayr himself rightly pointed out that even the most enthusiastic supporters of BSC have to admit that this concept does not fit asexually or uniparentally reproducing lineages.
At the root of this discourse, I found most interesting an article published in The Economist on October third, 2020 titled ‘How hybrids have upturned evolutionary theory’. In it, the author genuinely reports of a phenomenon where two different bird species — the large cactus finch (Geospiza conirostris) and the medium ground finch (Geospiza fortis)— mated and produced hybrid offspring. Unlike the common trend of infertility with hybrids, these hybrid finches have gone on reproducing successfully among themselves. They are reproductively isolated from either parent species and, apparently, have earned the status of species.
Given the complexity of life, it is not implausible that nature would present us with something drastically different from our conventional knowledge. However, what I find most ironic is that these hybrid finches have been granted the species status using the Biological Species Concept yet, under the same concept, the two supposed parent species would logically have to be recognized as the same species (due to their successful interbreeding). Perhaps it is a stretch on my part, but we consider domestic dog breeds as the same species — Canis familiaris or Canis lupus familiaris — because of their interbreeding potential, despite the huge variations among them.
The above are but two of the twenty-six recognized concepts of the species, each packed with its own limitations. It is from such quagmire that others have held the position that — though the organisms are real — species are nothing more than a human construct employed to aid in cataloging and studying organismal diversity. Such a position, while quite attractive a philosophical stand, would perhaps require us to reconsider some biological concepts. If indeed species are not real, can we objectively discuss mechanisms of speciation — the process by which populations evolve to become distinct ‘species’? At what stage during ‘speciation’ will we consider the budding population as fully distinct from the parent population to earn the status of ‘species’? Can conservation biology objectively ascertain endangered ‘species’? The reader at this point should be careful not to regard the species problem as evidence against biological evolution. In fact, it is because of evolution that we land in this problem in the first place. By analogy allow me to quote the novelist Herman Melville:
“Who in the rainbow can draw the line where the violet tint ends and the orange tint begins? Distinctly we see the difference of the colors, but where exactly does the one first blendingly enter into the other?”
It is by reason of the evolution of organisms that it becomes hard for biologists to agree on where to draw the line for what should define a species. Lest I digress, allow me to close my discourse by quoting the man whose theory unified all of biology.
“Hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgement and wide experience seems the only guide to follow.” — Charles Darwin.
Cathcart, T., Klein, D. (2007). PLATO AND A PLATYPUS WALK INTO A BAR: Understanding Philosophy Through Jokes.
Kottler, M.J. (1978). Charles Darwin’s Biological Species Concept and Theory of Geographic Speciation: the Transmutation Notebooks.
Love, A. (2013). THE BIOLOGICAL SPECIES CONCEPT AND ITS EVOLUTIONARY STRUCTURE.
Mayr, E. (1992). A LOCAL FLORA AND THE BIOLOGICAL SPECIES CONCEPT.
