Unnatural Selection (Princeton University Press, 2018), by Katrina van Grouw teaches readers about selective breeding, which happens to both captive and domestic animals. However, the distinction is drawn between selective breeding and domestication because they are not the same. Domestication is the first step towards selective breeding and it begins with the transition of wild animals into self-sustaining and tame populations. After this is achieved selective breeding begins with more new and continuous changes that occur within the now tame populations. These changes lead to a more productive, efficient, successful, beautiful, and different version from the original population. Readers can also learn the method behind the naming of different species while gaining access to knowledge about interspecies breeding that results in completely new species, such as the many species of geese.
Names are important. they’re a sort of code — an abbreviation, allowing us to communicate without ambiguity or the need for lengthy descriptions. Like any code, the only way a name can be useful is if it’s understood by everyone using it. In a small community, it makes no difference if everyone calls a Song thrush a Mavis or a Surf scoter a Skunk duck — one name is as good as another as long as the whole population can relate to it. However, a visiting alien or other explorer hearing the words “Bog bull,” “Butterbump,” “Mire drum,” or “under pumper” could be forgiven for assuming they belonged to four separate types of animal when in fact they’re all names for a single species: the American bittern. We all know that the single word “Robin” refers to two very different birds on either side of the Atlantic. A Butterfische in German is a Rock gunnel fish in English, while a Butter fish in English is a Medusenfische in German.
There’s nothing actually wrong with any of these names. They’ve simply been taken out of their geographical confines to realms where their meaning has been lost or changed. When the community in question is the international scientific community, misinterpretation is only avoided by the use of the most rigidly strict rules — rules to prevent duplication of names, rules to prevent the same name being used for more than one thing, and rules to dictate the form a name must take.
There’s only one problem with this: when rules are so inflexible, they can’t adapt to changes in understanding. They may make provision for advances in knowledge, but that’s another thing entirely.
Our system of zoological nomenclature belongs to a pre-Darwinian era. It was developed by Swedish botanist Carl Linnaeus in 1758 in the celebrated tenth edition of his Systema Naturae, though, as with most great achievements, Linnaeus was building on foundations laid by others before him, perfecting a system that was already in place. The same principles are still in use, upheld by the International Code of Zoological Nomenclature (ICZN), often referred to simply as “the Code.”
Living as we do, in the third century AD (Anno Darwin, that is), we’ve had a long time to come to grips with the idea of evolution and to be conditioned to expect classifications of living things to reflect actual relationships between and within species. It’s almost impossible for most of us to imagine the concept of living things not being related to one another. Linnaeus’s intention, however, was to group similar things together simply as a means of classification. Similar species were grouped into genera, similar genera into orders, orders into classes, and classes into kingdoms. Only species and genera were considered by him to reflect affinities in nature, which he described as “God-given”; the other levels he thought of as merely artificial groupings for the sake of convenience. Indeed, he adopted as his own personal motto “Deus creavit, Linnaeus disposuit” (God created, Linnaeus, organized). In pre-Darwinian thinking, classification was purely an attempt to organize God’s creations into groups, and the groups into ever larger groups of diminishing similarity. Classification, in its purest form, is an entirely separate discipline from phylogenetics — the organization of living things according to their evolutionary relationships. There’s actually no logical reason why it shouldn’t be simply an effort to make sense of the organic world and to categorize every animal and plant according to its usefulness or harmfulness, habits or habitat. And that’s precisely how it had been for the better part of history — no different from keeping pearl buttons, black buttons, and brown buttons in different compartments of a sewing box. It’s pure serendipity, then, that Linnaean classification, drawn in diagrammatic form, results in a familiar treelike structure with tiny branches sprouting from larger branches and so on, ultimately springing from a single trunk. It provided a ready-made framework on which biologists could eventually hang taxa according to their rightful place in evolutionary history — a phylogenetic tree of life.
Such a sublimely elegant taxonomic system appears at first glance to dovetail perfectly with our modern understanding of biology — so much so that we tend to accept it without question. In truth, our practical need to pigeonhole things into distinct categories is at odds with the natural world itself, and arbitrary lines of distinction hinder us in recognizing the presence of the intermediate forms — the “missing links”— we’re always seeking. Darwin himself saw the difference between species and race, race, and variety, variety, and individual as purely arbitrary. Whatever system is used, taxonomy — the science of naming and defining organisms — is an artificial constraint attempting to freeze in time a process that is ever changing; to separate the components of a process that has no truly distinct parts and that works only in unison with every other living thing and its environment.
With wild animals, the discipline works tolerably well. Evolutionary change usually works slowly enough for us to convince ourselves that species and races are genuinely divided. It’s when we turn to domesticated animals that the cracks appear. Domesticated animals are not separate from the rest of the animal kingdom. They’re subjected to evolutionary forces directly comparable with the forces that shape wild animals, and they too evolve to fill the niches presented by a world increasingly dominated by humans. The difference is that domesticated animals change fast.
A universal system of naming requires a universal language. Latin was chosen, though many names are Greek or Latinized versions of other words. (That’s why it’s better to refer to them as scientific names instead of Latin names.) Instead of using a descriptive passage of text, Linnaeus adopted a two-part name — a binomial. The first part of a scientific name is the generic name, always written with a capital letter. This name puts each species in the company of other, similar species. Most bears, for example, share the generic name Ursus and that name can’t be used for any other genus in the animal kingdom. This is followed by a specific name that gives the species, which is always written in lowercase — even if it refers to a country or person. Only the American black bear is called Ursus americanus although lots of animals of other genera have the specific name americanus.
In order to name something, you need to be able to define the parameters of that thing—to know categorically when it ceases to be one thing and becomes another. Species are defined as populations of animals that interbreed to produce fertile offspring so, superficially at least, it forms a clear-cut, standard unit. However, species with wide geographical ranges often show a marked degree of regional variation, and these differences give the option of a third name, after the specific name, designating the race or subspecies. The binomial now becomes a trinomial. American black bears from California, for example, are called Ursus americanus californiensis.
Deciding on whether races belong to their respective species or deserve to be considered species in their own right is more than enough to keep most taxonomists occupied for life. There are even historical trends for lumping and splitting species and races back and forth. One thing that different races will readily do, and different species generally won’t, is interbreed, which seems to present a fail-safe way of defining, once and for all, where the parameters lie. However, just because two animals don’t breed together in practice — for example, if two populations are physically separated — it doesn’t necessarily mean that they couldn’t interbreed if they were reunited. There’s the question of how they became separated, and of how long ago, so it’s impossible to know where to draw the line. And two species that may never cross paths under normal circumstances may easily do so in captivity.
By defining a species as something that can only interbreed (and produce fertile offspring) with others of the same species, you’re effectively denying any possibility that species can interbreed — otherwise, they wouldn’t be species. But animal species do hybridize, and they do produce fertile offspring. And for evidence, you only need to look, once again, to domesticated animals. Take geese, for example.
Geese are among the few domesticated animals that have not just one but two wild ancestors. I don’t just mean subtle genomic differences that suggest a hybridization event early on in their domestication history. No, purebred geese that derived from two totally separate species — the Swan goose, Anser cygnoides, from Central Asia and the Greylag goose, Anser anser, from Central Europe — hybridize readily and regularly. Out of any mixed farmyard flock, it’s normal to find a substantial number of hybrids between the two. Even several recognized breeds, like the Steinbacher from Germany, are hybrids between the two parent species. The domesticated forms of the Swan goose are the sublimely elegant Chinese goose and the more heavyweight African goose. Although Swan geese have a slender head and bill like a swan, with only a subtly raised “knob” at the base of the bill, both of the domesticated varieties have a deeper skull, and the bill knob is positively enormous. Both, however, share the Swan goose’s unusually smooth silky neck feathering and (unless they’re leucistic) the deep chocolate brown stripe running from the crown to the base of the neck. Greylag geese have a much deeper, more powerful bill than Swan geese and have the deeply furrowed feathering down the neck so typical of the majority of goose species. Straight crosses between the two are intermediate in every respect: a faintly two-tone, slightly furrowed neck and a medium-weight, moderately angled bill. Three-quarter crosses, either way, have more of the qualities of that species.
Interbreeding isn’t simply a matter of willingness. The fertility of the hybrid offspring is generally considered to be the real test of whether two animals belong to the same species. You can cross lions and tigers together to produce (depending on which parent is which) ligers and tigons, but only very rarely will one of these be capable of contributing to offspring of its own. However, fertility between two species isn’t necessarily all or nothing and, once again, it’s in domesticated animals that the exceptions are revealed. There are varying degrees of fertility between individuals — and between sexes. Occasionally failure to produce viable offspring is simply a boundary that can be crossed with perseverance.
Reprinted with permission from Unnatural Selection by Katrina van Grouw and published by Princeton University Press, 2018.