What makes a species… a species?

What makes a species… a species?  This, in some ways, is an easy question to answer and in other ways can get quite complicated.  In its simplest form, a species is a living organism that is capable of producing viable offspring.   Typically, this is by interbreeding between two sexes, but can occur in asexual ways as well (I’m looking at you, sponges!).  In 2011, it was estimated that over 8.7 million species existed on Earth and around 2 million lived in the ocean (Mora, Tittensor, Adl, Simpson, & Worm, 2011); and of these, approximately 86% remain undescribed.  Now, that number seems extreme, and there is a chance it might be… we’ll get into that later.  But if we consider that, of these millions of organisms, many are very, very small, or live in an extreme environment that humans do not frequently visit (the top of a mountain or bottom of the ocean), then it’s not hard to imagine that there could be lots of unknown species that exist on our planet.   But how do we determine if something is a new species or not?

The science of classifying things is called Taxonomy (I know, I know… it’s surprising it has nothing to do with accounting).  All organisms on earth are sorted into different groups and arranged into a ‘taxonomic hierarchy’.  A species is one of the most refined levels of an organism.  These levels of classification have fluctuated throughout history and there is still debate on whether some of them should exist or if we should be creating a hierarchy of life at all.  Humans, by nature, tend to try and find order in a system… but unfortunately, our natural world doesn’t always follow this same principle.  Originally, a man named Carl Linnaeus split all life into plants and animals, but then someone noticed that fungi don’t fit into either of those categories and later still bacteria were separated into their own kingdom of life… and don’t even get me started on protists.  So even at the highest level (the one that should encompass all life in some way) scientists historically struggled to agree if there are 2, 3, or up to 9 different groupings (Cavalier-Smith, 1981)… but I digress.

Now, don’t get me wrong, I actually love taxonomic categorization (with all its flaws).  Understanding what makes a species unique is a complex mystery to solve and we use different tools to achieve this goal.  Unfortunately, these tools also further complicate our “well-organized” hierarchy.  Back in the 1700’s, when taxonomic classification began, scientists had to use visual identifiers to distinguish between species.  We call these morphological traits.  Features like the number of legs, colour, patterns, even internal anatomy can all be used to distinguish between species.  The problem is that, in many cases, species are so similar in appearance that finding those physical differences becomes an adventure in and of itself.  For instance, we could have one clam with ten ridges along with its shell, while this other nearly identical clam has eleven.  Some species have characters that exist on a spectrum, human eye colour is an example of this.  Even at the species level, dramatic morphological differences exist (by this logic, a dachshund and a husky should be completely different species… but they are not).  This zoological game of Where’s Waldo can be frustrating, but extremely satisfying when you actually find a common trait amongst a group of specimens.  Now I admit, this method of identification is extremely subjective, which brings us to the second tool we can use to identify a new species… genetics!

Technological advances in the 1970’s allowed us to sequence the genetic code of an organism (Sanger et al., 1977).  This means that scientists are able to identify a species based on a short genetic marker on their mitochondrial DNA, and scientists propose that a difference of only 2% can constitute a new species (Panko, 2017).  This makes the resulting identification less subjective.  Although DNA sequencing is a pretty precise science, there is still room for error.  This could be due to how recently two species lines separated from each other, as recently diverged species may not yet be distinguishable based solely on their genetic code (Narendran, 2008), or samples could become contaminated during testing.  On top of that, genetics can be expensive, so it is not always an option for all researchers.  It does simplify the process of determining a new species though, as you can ID something based on a small piece of tissue even if you don’t have access to the entire organism. 

So you’re probably asking: why isn’t this method used all the time?  Great question!  A drawback of this method of species identification is that it is based on someone else’s prior ID.  In order to ensure that the DNA sequence matches the correct name, scientists need to go back to the previous method, physical characters, and double-check that the organism they are researching matches the original one described.   This can often be a huge struggle, as original specimens (called “types”) can sometimes be several hundred years old and often are poorly preserved.  Imagine trying to identify the type of sandwich that has been hiding at the back of your fridge for the past 6 months; this would be a similar endeavor.  Accurate identification of a new species will combine both methods to ensure a comprehensive description of the organism, both from a molecular and morphological perspective. 

            Now, remember those 8.7 million species living on the Earth, many of which have yet to be discovered? This is of course an approximation, and we will never know all the species that exist.  That number can also greatly fluctuate based on which taxonomist you talk to.  Some scientists will split one species into two, others determine that several older species were actually a single one with dramatic physical variation (remember our dog example earlier?)  This process of splitting and lumping species ID’s is particularly controversial and has important impacts on how we view the natural world.  Garnett and Christidis (2017) even went so far as to call our current taxonomic practices a “free-for-all” and “anarchic”.  I can’t disagree with the sentiment (although the wording is a little dramatic) and can see how all these complexities would be frustrating; especially when we consider the impacts it has on animal conservation.  This critique of taxonomy was refuted by many taxonomists and conservation biologists alike who found their viewpoint to be narrow and unsupported by evidence (Thomson et al., 2018).  I have no doubt that this debate will continue in the future. 

            If you’ve made it this far (my congratulations), you might be wondering why all this is important?  Understanding species biodiversity is imperative from many different scientific lenses.  In its most accessible use, taxonomy allows the public to enjoy recreational activities in nature.  Have you ever gone bird watching or picked wild mushrooms in the forest?  These activities require a field guide with species ID’s that would have been determined by a taxonomist.  This would be particularly important when considering mushroom edibility... that’s an ID you’d definitely want to ensure is correct!  But taxonomy is also a vital part of species conservation.  The better we understand species diversity, the more knowledge we can gather about ecological interactions, and create effective legislature relating to conservation efforts. 

Although complex, and arguably subjective, taxonomy is a vital process.  Unfortunately, funding for taxonomic projects is diminishing, as priorities are shifting in favor of more modern research methods.  But I advocate the importance of this science as humans continue to expand our urban areas, pollute natural environments, and harvest natural resources.  Because of these actions, large numbers of species, many undescribed, will conceivably go extinct.  Extinction is a natural process, but rates are greatly increased due to recent human activity (Smithsonian, n.d.). In conclusion, I’d like to quote Narendran (2008) “At no time has there been a greater need for taxonomists than now when the crisis facing biodiversity is escalating”. 


Aaron Evans


Reference list:

Cavalier-Smith, T. (1981). Eukaryote kingdoms: seven or nine? Biosystems, 14(3-4), 461–81. doi: 10.1016/0303-2647(81)90050-2 

Garnett, S., & Christidis, L. (2017). Taxonomy anarchy hampers conservation. Nature, 546, 25–27 https://doi.org/10.1038/546025a

Mora, C., Tittensor, D. P., Adl, S., Simpson, A. G., & Worm, B. (2011). How many species are there on Earth and in the ocean?. PLoS biology, 9(8), e1001127. https://doi.org/10.1371/journal.pbio.1001127

Narendran, T. C. (2008). Taxonomy and its Relevance. Research Journal, 1(2), 9–14.

Panko, B. (2017, May 19). What does it mean to be a species? Genetics is changing the answer. Smithsonian Magazine. https://www.smithsonianmag.com/science-nature/what-does-it-mean-be-species-genetics-changing-answer-180963380/

Sanger, F., Air, G., Barrell, B. et al. (1977). Nucleotide sequence of bacteriophage φX174 DNA. Nature, 265, 687–695. https://doi.org/10.1038/265687a0

Smithsonian (n.d.). Extinction over time.  Smithsonian Museum of Natural History. Accessed November 13, 2021. https://naturalhistory.si.edu/education/teaching-resources/paleontology/extinction-over-time#:~:text=Regardless%2C%20scientists%20agree%20that%20today%E2%80%99s%20extinction%20rate%20is,species%20per%20every%20one%20million%20species%20per%20year

Thomson S.A., Pyle R.L., Ahyong S.T., Alonso-Zarazaga M., Ammirati J., Araya J.F., et al. (2018). Taxonomy based on science is necessary for global conservation. PLoS Biol, 16(3), e2005075. https://doi.org/10.1371/journal.pbio.2005075

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