What is a species and why should we care?

When you read about an animal on Wikipedia or in a textbook, one of the first things mentioned is its scientific name — genus and species. But, how do we define a species? 

First, why are we concerned with classification? Humans have an innate need to categorize. It’s ingrained in our minds from our days as hunter-gatherers, when we needed to remember what each plant was and whether it was safe to eat or poisonous.

Taxonomy is a hierarchical system that classifies the biological continuum. It comes from Greek philosophers (including Plato and Aristotle), who ranked animals as closer or farther from God. Darwin then focused on common ancestors, and his ideas on how to draw phylogenies are still used today. In 1758, Carl Linnaeus finalized the classification system into what we use today: Kingdom, Phylum, Class, Order, Family, Genus, and Species. His system is ubiquitous, and the only modification has been to define a level above Kingdom (Domain: bacteria, archaea, and eukarya).

At this point, you would think that the definition of a species is clear and concrete. But, that is not the case; there are still multiple definitions of a species.

The first hurdle is that variation is continuous, but classification is discrete. This is a holdover of Linnaeus’s belief that species were sharply delineated, but it obviously creates some friction. Continuous variation is the case for much of nature. For example, light is a spectrum, but colors are discrete bins. We get around this by classifying colors as “blue-green” or “red-orange.” Species are no different, except we don’t readily create new descriptors for animals that defy categorization. In other words, there’s no “bear-cat” in scientific language.

This system has led some to adopt the view that species don’t exist at all and are simply a made-up concept. This view was popular in France in the 1800s, but is still held today by some scientists today, mainly botanists.

Although most people believe in the biological reality of species, the definition has changed over the years. John Ray introduced the first biological species definition in his book History of Plants in 1686. He included an interbreeding criterion: that mating cannot occur between different species. However, he did not know about evolution, so he said that existing species don’t give rise to new species and that variation is caused by the environment.

George Buffon added that all members of a species must be able to reproduce, and he was so confident in his identifications that he wrote thirty-six volumes about the physical attributes of animals.

Overlying all of these historical definitions is the importance of viability and fertility of offspring. In terms of fitness, non-fertile or non-viable offspring are a waste of the mother’s limited resources. Non-viable offspring do not live, and so their parents are of different species. Similarly, non-fertile offspring cannot reproduce, and so their parents are of different species. Mules are an example of non-fertile offspring; because they are unable to reproduce, they do not contribute to the propagation of either species. Offspring with lower fitness are sometimes produced as well. These offspring may be fertile, but they are less fit than both of their parent species and  cannot compete reproductively. An example of offspring with lower fitness are dog breeds that are susceptible to serious illness or have difficulty breathing or walking. The breeding of some species enables offspring that are less fit to survive.

In cases where offspring are less fit than their parents, reproductive isolation mechanisms may result. Complex reproductive organs have evolved in many creatures (including cats, birds, and flies) to combat hybridization of species. These organs make it very clear what can and cannot mate in nature: a “mechanical coupling” is required, wherein “the male ‘key’ must fit the female ‘lock.’” The downside to this mechanism is that it is unlikely to be preserved in the fossil record, because it mainly affects soft tissue that does not easily fossilize. These mechanisms typically result from species interbreeding in close proximity.

Today, the main biological species definition is from Ernst Mayr in 1942: “species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.”

Mayr’s definition still has some issues. What about asexually reproducing organisms? This definition also hasn’t been tested for most species, and it can’t be tested in fossils either. Also, reproductive isolation mechanisms evolve in response to reduced fitness of hybrids. Thus, distantly-related species may not have evolved these mechanisms if hybridization is unlikely to occur in the wild. 

Another issue with this definition is ring species, in which several species are in close proximity around something like a mountain or a valley, and each species can interbreed with the species on either side, but the first and last species in the barely unconnected ring cannot interbreed. Where would the species boundary be drawn with ring species under Mayr’s definition?

The biggest strength of Mayr’s Biological Species Concept is that it provides a falsification criterion. Hypothesis: Two populations are distinct species. Test: Can they interbreed?

Other species definitions focus on species as a unit. It may be a genetic unit, an intercommunicating gene pool through reproduction in which each individual is a holding vessel containing a portion of the gene pool. Or maybe it’s an ecological unit, where species interact with other species as one in the same environment. There can also be a focus on the reproductive community, with a wide range of mechanisms that allow for intraspecific reproduction between populations.

Why should we bother with species at all? In the U.S., funding for conservation, the legal system, and the Endangered Species Act are all built on species-level identification. This issue is particularly evident with hard-to-define species that require conservation efforts. The International Union for Conservation of Nature is also based on species. 

Different definitions of a species could mean everything for an endangered species. Making sure that we all use the same definition is the first step. The second step is making sure that our definition works for all creatures, which will be much more difficult. 

Acknowledgements:

EAS-G 690 Invertebrate Paleontology allowed me to gather information about speciation, which is related to my thesis project. This post is derived from a portion of a lecture that I developed and presented as part of that course.

 

Edited by Ben Greulich and Clara Boothby