“… The term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for mere convenience sake.” — Charles Darwin

Darwin wrestled with describing what a species is, but ultimately gave up. Defining this cornerstone of natural history, the basis of measuring biodiversity, the legal foundation for conservation-oriented policy, eluded the person who first identified the process of how species come to be. It still eludes scientists today. Whereas chemistry has clearly defined categories for the chemical elements, carbon or calcium, silica or sulfur, and the others, each defined by specific combinations of neutrons, protons, and electrons, biology is messy with no single definition that allows each species to fit into neat and exclusive boxes. Scientists needing things to be binary, black or white, become chemists or physicists. Scientists comfortable with shades of gray become naturalists and ecologists. 

At first blush, distinguishing between species seems straightforward enough. Most of us can look at any two organisms and come up with a pretty good guess as to whether they might be the same or different species. Different flower structures, different shapes, sizes, or colors are all good hints, but those clues may not be enough. Take the desert dune cockroach. The males look very cockroach-like except they are a pale blond color. The females are very different; they are a dark red brown, lack wings, spend their lives burrowed into the sand searching for bits of dead plants to eat, and appear to be a closer match to a tidepool chiton in overall appearance. For decades, insect taxonomists considered them separate species, but on closer examination they realized that they were males and females of the same species. The flip side of this paradox is that just because individuals look alike, does not necessarily mean that they are the same species. My 1983 first edition of the National Geographic Society’s “Birds of North America” lists just two species of western titmice, small gray relatives of chickadees, the plain titmouse and bridled titmouse. In my more recent, seventh edition of the same book, there are four species, the oak, juniper, black-crested, and bridled titmouse. The “new species” look nearly identical to each other, but their genetics tell a different story. Due primarily to different habitat preferences, the four species do not interbreed. Appearances alone can be deceiving.

Early on, the most widely used species definition was the Biological Species Concept, the idea that if any two individuals can breed and produce viable young and if those offspring can also breed and produce viable, fertile young, they are the same species. If the answer is no to any of the above, then they are different species. A horse and a burro can breed and produce healthy offspring (mules), but except for very rare instances mules are sterile. Ipso facto, they are different species. However, it is not always so easy. In Arizona and New Mexico there are several recognized species of whiptail lizards (genus Aspidoscelis) that occasionally mate with other recognized species of whiptails and when they do the offspring are healthy and not sterile, but they are all females. The curious thing is that these females are parthenogenic, they can produce healthy exact copies of themselves (more parthenogenic females) without any “inputs” from male whiptails. So where is the species dividing line here? Are the original males and females really the same species that just look different? Are they the same species as their parthenogenic female offspring? The standard definition does not apply. The field books list them as three different species. 

The problems with the Biological Species Concept are many. If the criteria for distinguishing between different species is their ability to breed and produce fertile young, how does one deal with most putative species pairs that never come in contact because they are separated by different habitats, rivers, mountains, or oceans? We can’t assess their breeding compatibility because they do not have the opportunity to be close enough to each other to give it a try. Captive breeding of different possible species is not practical, and the cues and choices that organisms use to choose mates may not be operating in captive situations. Then there are plants. It turns out many plants are not very discriminating as to which species’ pollen they will accept to create viable seeds. This is especially true in wind pollinated species such as oaks. Oaks are notorious for having their DNA being an amalgam of multiple recognized species. Sometimes the oak seedlings are sterile, but more often they are fertile.

An alternative species definition is the Evolutionary Species Concept. Within this definition species are distinguished based on whether they are on separate evolutionary paths. This addresses the problems presented by titmice, dune roaches, and whiptails, but creates a new one. How far along a diverging evolutionary path does one need to be to be considered a separate species? Species’ populations are defined by having some, but not necessarily 100%, breeding isolation from other populations. Within any given population, individuals are subjected to levels of predation, of parasitism, differing food supplies, pollinator types and densities, and differing climates. Over time, those individuals that are best at avoiding predators and parasites, at finding food, attracting mates and pollinators and completing all those tasks within the climate available to them will produce the most offspring. Depending on how different those stressors and resources differ between populations, those populations will develop unique genetic characteristics and so be on separate evolutionary paths. The unanswered question is how far down those paths do they need to be before being considered different species?

The answer to “what is a species?” is not simply an academic exercise. Laws enacted to prevent species from going extinct are based on our ability to distinguish species that are in peril. Where there is any room to question the veracity of a species, economic interests will use that space to argue that the population in peril is no different from other populations that are healthy and secure. Back in 1980, the Coachella Valley fringe-toed lizard was listed as threatened under federal law and endangered under state law. Land developers saw this as a threat to their economic futures and hired consultants to argue that this lizard was just a population of a more widespread and already somewhat protected fringe-toed lizard that occurred south of the Coachella Valley in the extensive dune systems there. Consultants even went so far as to place Coachella Valley and other fringe-toed lizards together in tubs to show that they could breed together. I was told that house cats ate the lizards before the viability of any offspring could be assessed. The state and federal agencies went forward, protecting the fringe-toed lizard and creating a conservation plan that also protected the economic interests of the developers and the region. Subsequent genetic research has confirmed that the Coachella Valley fringe-toed lizard is on a completely different evolutionary path from the more southern lizards (and has been for some 70,000 years), and that the southern fringe-toed lizards are not one species but are at least three different species.

I once believed that it would take the next “Darwin” to develop a universal criterion for distinguishing species. Now, I am increasingly comfortable with a non-binary, not black or white interpretation of the biological world, including that most basic biological unit of the species. Biology, especially ecology, is full of fuzzy shades of gray. Those shades of gray have created and continue to create the incredible diversity on our planet and should be embraced and celebrated.

Nullius in verba

Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe