"As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form." — Charles Darwin

"There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved." — Charles Darwin 

Beyond an ongoing love affair with nature, a fundamental currency for being a naturalist is the ability to recognize species. How else do we measure biodiversity or identify species’ ranges, how those ranges are shifting in response to environmental change? But what is a species? The truth is that a single definition of “what is a species" has eluded any real consensus. The following are common definitions and their shortcomings, and hopefully adding a modicum of clarity.

Common Definition: Species: a group of organisms in which any two individuals of appropriate sexes can produce fertile offspring.

This definition is what is referred to as the “Biological Species Concept,” which has held priority for naturalists ever since the days of Darwin. Except it does not work, arguably, for most species. One of many problems with this definition is that it cannot be applied to populations/species with disjunct/separate distributions because they, without intervention, would never be in contact and so we could never determine whether they would produce fertile or infertile offspring. How can we know that an east coast Connecticut warbler and a west coast MacGillivray’s warbler aren’t the same species? They look similar, but they never come into contact during the breeding season, so we cannot be certain that they could not have fertile offspring. Then there’s plants which are notorious for interbreeding, exchanging genetic material with other species via wind or insect pollination. Yet, plants are usually classified as separate species based on solely morphological differences. Oaks are an excellent example. I have heard some say that many, if not most oak species are a blend of surrounding oak “species.” Often these hybrids are very capable of continuing to reproduce, although some are infertile “mules.” Hybridization yielding fertile offspring is considered an additional means of speciation in plants, additional to mutation followed by natural selection. So, the Biological Species Concept as a guiding definition is of no help in countless cases.

An alternative definition is the “Evolutionary Species Concept,” which is often supported by genetic analyses, to determine if the two populations were on different or “blended” evolutionary trajectories (divergent trajectories would indicate separate species). Using a current example, consider the western and eastern populations of Joshua trees. There are clear genetic differences, and most naturalists would quickly identify morphological differences, and have identified that each population has its own, separate species of moth as its primary pollinator and so is on a different evolutionary trajectory. There is a very narrow zone of contact where there is minimal interbreeding. Based on those observations, taxonomists are leaning toward identifying these populations as separate species, Yucca brevifolia (western population) and Yucca jaegeriana (eastern population). Ultimately, what we should care about in terms of conservation objectives should be that biodiversity — including genetic diversity — is conserved and protected. With that as our objective, the Evolutionary Species Concept is currently the best definition, especially when it is coupled with morphological and/or physiological differences, indicating adaptations to a specific environment.

Common Definition: Subspecies: a grouping within a species used to describe geographically isolated variants.

If we employ the Evolutionary Species Concept, this designation of subspecies has a narrow meaning. It could be a way of identifying two populations that were on the cusp of diverging onto separate evolutionary trajectories, but the genetic and morphological differences had not yet become sufficiently distinct. This could apply to populations that had relatively recently become spatially disjunct, perhaps on a time scale of decades to a few centuries. An example is the Peninsular bighorn sheep versus the desert bighorn found elsewhere in the deserts of North America. Although they are not considered subspecies (yet) they fit the above definition. Complete isolation of these populations is relatively recent, probably coincident with our building freeways and agricultural development. Officially the Peninsular bighorn population is referred to as an Evolutionary Significant Unit (ESU), which receives the identical level of protection as a species or subspecies under the federal Endangered Species Act.

Common Definition: Variety: more or less recognizable entities within species that are not genetically isolated from each other, below the level of subspecies.

This is a term used exclusively with plants. I do not see this as being operationally different than a subspecies and so perhaps has little or no value. An example is the Coachella Valley milkvetch, Astragalus lentiginosus, variety coachellae. This federally endangered species/subspecies/variety is just one of 19 varieties identified for this species – all geographically isolated, many of which are quite common. Some of these varieties look nothing like each other (yellow flowers versus pale purple flowers versus bright magenta flowers). Others “look” fairly similar. Many occur on isolated sand dunes, but not all, so at least some have evolved to take advantage of very different ecological niches. I don’t know whether these “varieties” have undergone genetic scrutiny, but there is isolation that is at least on the order of many centuries, thousands of years or more. To me these varieties are screaming for a taxonomic revision, some of which would certainly rise to the levels of separate species under the Evolutionary Species Concept, while others may be reclassified as subspecies

I used to think that the next great scientist, the next Darwin, would cut through all this messiness and develop a clear, concise, and universal definition of what is a species. More and more I now think that this messiness is inherent in nature. Why should there be just one way to create genetic traits that offer individuals a “leg up” in surviving and thriving in a world that is always changing. Why then should our definition of what is a species put all such species under one classification.

Nullius in verba

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