“No one definition has satisfied all naturalists, yet every naturalist knows what they mean when they speak of a species” — Charles Darwin, The Origin of Species

By DR. CAMERON BARROWS 

What is a species? Species are the currency by which we measure biodiversity, how we make decisions about conservation priorities, and how we add to our life lists. We have laws that mandate that rare and endangered species deserve protection. As such, a clear definition of what is a species could then dictate what lands are set aside relatively undisturbed, and which will be destined to become houses, farms, playgrounds, schools, or warehouses. Yet a single definition for what is a species has been elusive. Augustin Pyramus De Candolle (1778–1841), who was the first to introduce the word taxonomy in his book Théorie Élémentaire de la Botanique (De Candolle, 1813), then defined species as "a collection of all the individuals which resemble each other more than they resemble anything else." Simple, but not measurable. How similar or how different do individuals within a population need to be before they cross a line to being identified as a different species?

One of the most used species definitions is referred to as the "biological species concept." That definition focuses on whether individuals in different populations can and will breed, and then produce healthy, viable offspring that can continue to breed. If they produce healthy fecund offspring, then they are deemed the same species. If not, then they are separate species. Horses and burros can be induced to breed and produce healthy mules, animals stronger than either parent. However, mules, with some very rare exceptions cannot produce offspring, so horses and burros are deemed separate species.

On the surface the biological species concept seems a straightforward, clear distinction for defining species. However, as they say, the "devil is in the details." The biological species concept seems to work well when two populations are connected in space and time, meaning that they have had the opportunities to interbreed but have demonstrated that between-population dalliances do not occur or are very rare or there is only a very narrow hybrid zone. Then species designations can be made. The challenge exists when those two populations are temporally and/or spatially separated, perhaps living on different islands, different ponds, different mountains, different sand dunes, or mature (such as with short-lived annual plants or many insects) at different times of the year. Such isolation physically keeps populations from being tempted to interbreed, but if those barriers disappeared could they breed? Without human intervention, who can say, and so how then to define species boundaries?

It gets even more complicated. Long-term studies examining species boundaries are exceedingly rare. They are expensive and require dedication to the pursuit of insights that may or may not be forthcoming except over decades of research. There are, however, very few on-going research efforts that shed light on the efficacy of the biological species concept. One is in the Galapagos Island archipelago, where Rosemary and Peter Grant, and their students, have looked at this question with Darwin’s finches. On some of the smaller islands they have been able to capture and mark every finch for decades. For the most part the finches follow De Candolle’s definition, only breeding with individuals that resemble themselves, thus staying within their own species. But sometimes there are not enough of the opposite sex within a given species for pairing up; then, sometimes, two different species will nest together and produce viable, healthy chicks that then grow up and successfully nest themselves. Either these morphologically distinct finches are not separate species, or the biological species concept is not, by itself, sufficient to define species boundaries.

A different species delineation is referred to as the "evolutionary species concept." This approach posits that isolated populations that are on different evolutionary trajectories, independently adapting to local conditions, should then be considered separate species. This criterion removes the need to ask whether two separate populations could, under artificial conditions, breed and produce fertile offspring. Here, modern genetic approaches are used to determine if the genomes of isolated populations are distinct, demonstrating both isolation and different genetic trajectories. This concept has been used to identify species of fringe-toed lizards occupying separate, isolated sand dunes in the Colorado Desert. Where once there was just a single species identified, today at least four different species have been named. Each species is morphologically distinct, but only subtly so since the need to be adapted to a loose, fine textured sand dune habitat has narrowly confined them into very similar body plans. Their genomes are, however, sufficiently distinct to warrant separate specific status.

Perhaps some combination of De Candolle’s approach with one or both biological/evolutionary species concepts is the best we’ll be able to use for identifying and distinguishing what is a species. Even then there are some challenging species groups. There are at least eight named species of native oaks growing in southern California. Oaks are wind pollinated and so indiscriminate as to where their pollen lands. I have heard botanists speculate that despite having named species, probably all share at least some genes with each other, that no “pure species” exist. I can attest that identifying live oaks based on leaf morphology and the stellate hairs on the underside of each leaf sometimes feels more like art than science. Occasionally the oak’s genetic combinations do render them infertile "mules," referred to as "oracle oaks." Otherwise there appear to be many that continue to be fecund and successful, and sometimes very difficult to identify.

It may be that there will never be just one species delineation criteria. Accepting that reality, and then getting back to enjoying life as a naturalist is the best approach.

Nullius in verba – Go outside, tip your hat to a chuckwalla (and a cactus), and think like a mountain.