Along Those Lines

The Boundaries That Create Our World

Read an excerpt

After years of crossing borders in search of new birds and new landscapes, Peter Cashwell's exploration of lines between states, between time zones, and between species led him to consider the lines that divide genders, seasons, musical genres, and just about every other aspect of human life. His conclusion: Most had something in common—they were largely imaginary.

An Excerpt from Along Those Lines by Peter Cashwell
(From Paul Dry Books)
©2014 by Peter Cashwell

It’s not as though we are lacking in examples of the potentially dire consequences of mistaking a border for a natural phenom­enon. Yet on every bird count and in every field guide, that caution is at best played down, and at worst ig­nored altogether, all in the name of science—and all too often, in the science of names.

The modern science of names—taxonomy—has its origins in Carl Linnaeus’ 1735 masterpiece Systema Naturae,in which he set down the basic ways in which all organisms were to be categorized, described, and named. It was an enormous proj­ect, and the presence of errors in something so vast should come as no surprise. Those errors have led to a number of de­partures from Linnaeus’ methods, as well as numerous changes to the categorizations he originally established; even Linnaeus himself made some adjustments in later editions, realizing for example that his initial classification of whales and manatees as fish was a mistake. Still, he was among other things the first scientist to recognize that bats are mammals, and the basics of his system remain in place after nearly three centuries.

Each type of organism in Linnaeus’ system is given a particular name—a Latin name, since Latin was the universal language of science in his day. The two-word name (or binomial) identifies the organism’s genus (always capitalized) and its species (typically in lowercase), as in Homo sapiens. These are hardly the only two categories to which an organism belongs, however; biologists now assign each to a particular domain, kingdom, phylum (or division, for plants), class, order, family, genus, and species, each respective category becoming more specific. Often, the former category will contain multiple examples of the latter categories. Thus, the phylum Chordata—animals with spinal cords—contains multiple classes: Mammalia, Aves, Reptilia,Amphibia, etc. Within each class there are typically multiple orders, within each or­der multiple families, and so on, until we get to the most familiar unit of taxonomy—the one that has been a source of uproar for centuries, and not merely because Charles Darwin wrote his most famous book about it: The Origin of Species.

A species is traditionally defined as a population of organisms that can interbreed and produce fertile offspring, but cannot breed with any other organisms—or at least, that’s how the Comte de Buffon de­fined the word in the eighteenth century. Thus, if two foxes mate and produce little foxes, those foxes can be said to belong to the same spe­cies when and if the little foxes grow up to produce little foxes of their own; basically, grandparenthood is what establishes a species. If the original litter is not able to produce its own little foxes, we’re dealing with interspecies romance—an occurrence far more common than some staid biologists would like to admit.

How common is it? Common enough that there are numerous words in English that refer to such relations between species—half-breed, crossbreed, mongrel, mutt. The technical term, however, is hybrid.

Clearly, we would not have such words if organisms practiced any kind of fidelity to their species, nor would we have another very par­ticular English word: mule. When a male donkey gets amorous with a female horse, the resultant creature is a mule, but a male and female mule can engage in all the sex they like without producing another generation of mules. (The offspring of a male horse and a female don­key, known as a hinny, is rarer than the mule, but just as infertile.) Though there have been recorded cases where female mules have been impregnated by horses or donkeys, male mules are unable to produce issue, a feature so well known that the word mule is applied to the in­fertile offspring of any two species, animal or plant.

With all this information, one might well suppose that fertility be­tween individuals—interfertility—would be a perfectly useful test for speciation. In truth, it’s not so great. For one thing, it’s slow. Infants take a while to reach puberty, after all, so often we can’t check quickly to determine whether an animal is a mule or not. It’s also worth noting that, two centuries after Buffon, biologist Ernst Mayr defined species a bit differently: “Species are groups of actually or potentially inter­breeding populations, which are reproductively isolated from other such groups.” The key changes there are the word potentially and the phrase reproductively isolated. To isolate something, you have to draw some kind of border around it, and what have we been saying about Mother Nature and lines?

Consider, for example, the phenomenon of “ring species.” Atop the world, there are many seagulls belonging to the genus Larus, and those gulls are spread around the Arctic zone in great numbers and multiple species. In Northern Europe, for example, you will discover two species within the genus: the Lesser Black-backed Gull (Larus fus­cus), which has a black back and yellow legs, and the Herring Gull (Larus argentatus), which has a grey back and pinkish legs. They can­not interbreed. But the Herring Gull can and does interbreed with the American Herring Gull, which itself can breed with the East Siberian Herring Gull. Go west a bit more and you’ll find the latter mating with Birula’s Gull, which interbreeds at the western edge of its own range with Heuglin’s Gull. And Heuglin’s Gull is interfertile with the Sibe­rian Lesser Black-backed Gull, the easternmost subspecies ofLarus fuscus. In other words, gulls interbreed in a continuous line all around the North Pole, suggesting the existence of a single species within the genus Larus, but the two gulls at the ends of that line cannot inter­breed—which indicates they’re not the same species.

This paradox is duplicated around other geographic features such as the Himalayan plateau, and it is but one of the many problems mod­ern science has in defining a species. Consider the issue of geographic isolation. There are two populations of Abert’s Squirrels in the Grand Canyon area, one on the north rim of the canyon (known as the Kai­bab Squirrel) and one on the south rim. Could they interbreed? Quite possibly. But until one of the groups develops either a) the determina­tion to climb more than a mile down into the canyon, swim the Col­orado River, and climb another mile up to the opposite plateau, or b) the ability to fly, they never will interbreed. They are geographically isolated, and over time, as each population is influenced by mutation, inbreeding, predation, and other factors affecting its gene pool, they will become less and less likely to remain interfertile, which would eventually make them separate species. But unless we either transplant squirrels across the canyon or breed them in captivity, we will never know when that particular Rubicon has been crossed.