A koala bear isn’t actually a bear, it’s a marsupial. Whales aren’t fish, they’re mammals. Tomatoes aren’t vegetables, they’re fruit. Almost nothing is actually a nut. Peanuts, Brazil nuts, cashews, walnuts, pecans and almonds: none of them are really nuts (for the record, peanuts are legumes, Brazils and cashews are seeds, and the others are all drupes). Hazelnuts and chestnuts are the exception: they are the elite, the “true” nuts. We’ve all heard facts like this before. But they are more than just ammunition for pub conversation. They reflect an area of science known as biological taxonomy, the classification of organisms into different groups. At the core of this area lies the notion of the species. The basic idea is very simple: that certain groups of organisms have a special connection to each other. There is something that you and I have in common – we are both human beings. That is, we are members of the same species. Biological taxonomy’s core aim is to sort all of the organisms of the world into species. Of course, this job really matters, both inside biology and out. The task of evolutionary biology is to track the evolution and development (and eventual extinction) of species. Outside of biology, conservation programmes routinely put various species on “endangered” lists, and urge us to donate money to stop them dying out. In order for any of this to make sense, we need to know how many species there are, and what a species even is.  So, what even is a species? The truth is, we don’t really have any idea. What is a species?The most famous definition of a species comes from the 20th century German-born biologist Ernst Mayr, who emphasised the importance of interbreeding. The idea (roughly) is that two organisms are of the same species if they can breed with one another to produce fertile offspring. That is why a donkey and a horse aren’t the same species: they can breed and produce offspring, but not fertile offspring. Mayr’s way of thinking about species has some amazing consequences. Recently, due to rising temperatures in the Arctic, polar bears and grizzly bears have been coming into increased contact, and have been producing fertile offspring. The offspring are (adorably) called grolar or pizzly bears. What this suggests is that polars and grizzlies may actually be the same species after all, despite radical differences in size, appearance, hibernation behaviours, diet and so on. But it wasn’t long before the problems with Mayr’s approach became apparent. The definition makes use of the notion of interbreeding. This is all very well with horses and polar bears, but smaller organisms like bacteria do not interbreed at all. They reproduce entirely asexually, by simply splitting in two. So this definition of species can’t really apply to bacteria. Perhaps when we started thinking about species in terms of interbreeding, we were all just a bit too obsessed with sex. So maybe we should forget about sex and look for a different approach to species. In the 1960s, another German biologist, Willi Hennig, suggested thinking about species in terms of their ancestry. In simple terms, he suggested that we should find an organism, and then group it together with its children, and its children’s children, and its children’s children’s children. Eventually, you will have the original organism (the ancestor) and all of its descendents. These groups are called clades. Hennig’s insight was to suggest that this is how we should be thinking about species. But this approach faces its own problems. How far back should you go before you pick the ancestor in question? If you go back in history far enough, you’ll find that pretty much every animal on the planet shares an ancestor. But surely we don’t want to say that every single animal in the world, from the humble sea slug, to top-of-the-range apes like human beings, are all one big single species? Enough of species?This is only the tip of a deep and confusing iceberg. There is absolutely no agreement among biologists about how we should understand the species. One 2006 article on the subject listed 26 separate definitions of species, all with their advocates and detractors. Even this list is incomplete. The mystery surrounding species is well-known in biology, and commonly referred to as “the species problem”. Frustration with the idea of a species goes back at least as far as Darwin. In an 1856 letter to his friend Joseph Hooker, he wrote:
Darwin even dreamt of a time when a revolution would come about in biology. He proposed that one day, biologists could pursue their studies without ever worrying about what a species is, or which animals belong to which species. Indeed, some contemporary biologists and philosophers of biology have taken up this idea, and suggested that biology would be much better off if it didn’t think about life in terms of species at all. Scrapping the idea of a species is an extreme idea: it implies that pretty much all of biology, from Aristotle right up to the modern age, has been thinking about life in completely the wrong way. The upshots of this new approach would be enormous, both for our scientific and philosophical view of life. It suggests that we should give up thinking about life as neatly segmented into discrete groups. Rather, we should think of life as one immense interconnected web. This shift in thinking would fundamentally reorient our approach to a great many questions concerning our relation to the natural world, from the current biodiversity crisis to conservation. And, in a way, this kind of picture may be a natural progression in biological thought. One of the great discoveries of evolutionary biology is that the human species is not special or privileged in the grand scheme of things, and that humans have the same origins as all the other animals. This approach just takes the next step. It says that there is no such thing as “the human species” at all. Editorial note: This article was edited on July 17 to correct the spelling of the word “drupe”. If so, you’ll be interested in our free daily newsletter. It’s filled with the insights of academic experts, written so that everyone can understand what’s going on in the world. With the latest scientific discoveries, thoughtful analysis on political issues and research-based life tips, each email is filled with articles that will inform you and often intrigue you.
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species, in biology, classification comprising related organisms that share common characteristics and are capable of interbreeding. This biological species concept is widely used in biology and related fields of study. There are more than 20 other different species concepts, however. Some examples include the ecological species concept, which describes a species as a group of organisms framed by the resources they depend on (in other words, their ecological niche), and the genetic species concept, which considers all organisms capable of inheriting traits from one another within a common gene pool and the amount of genetic difference between populations of that species. Like the biological species concept, the genetic species concept considers which individuals are capable of interbreeding, as well as the amount of genetic difference between populations of that species, but it may also be used to estimate when the species originated. The designation of species originates in taxonomy, where the species is the fundamental unit of classification recognized by the International Commission of Zoological Nomenclature. Every species is assigned a standard two-part name of genus and species. The genus is the generic name that includes closely related species; the gray wolf, for example, is classified as Canis lupus and is a close relative of the coyote found in North America and designated as Canis latrans, their systematic relation indicated by their sharing the same genus name, Canis. Similarly, genera that have shared characters (or traits) are classified in the same taxonomic family; related families are placed in the same order; related orders are placed in the same class; and related classes are placed in the same phylum. This classification system is a hierarchy applied to all animals and plants, as originally set forth by the Swedish naturalist Carolus Linnaeus in the 18th century. Organisms are grouped into species partly according to their morphological, or external, similarities, but more important in classifying sexually reproducing organisms is the organisms’ ability to successfully interbreed. Individuals of a single species can mate and produce viable offspring with one another but almost never with members of other species. Separate species have been known to produce hybrid offspring (for example, the horse and the donkey producing the mule), but, because the offspring are almost always inviable or sterile, the interbreeding is not considered successful.
Coffee “beans” are not actually beans. They are seeds that have been dried and roasted. An unprocessed seed planted in the ground could grow into a new coffee tree. See All Good FactsInterbreeding only within the species is of great importance for evolution in that individuals of one species share a common gene pool that members of other species do not. Within a single pool there is always a certain amount of variation among individuals, and those whose genetic variations leave them at a disadvantage in a particular environment tend to be eliminated in favour of those with advantageous variations. This process of natural selection results in the gene pool’s evolving in such a way that the advantageous variations become the norm. Because genetic variations originate in individuals of a species and because those individuals pass on their variations only within the species, then it is at the species level that evolution takes place. The evolution of one species into others is called speciation. Get a Britannica Premium subscription and gain access to exclusive content. Subscribe Now |
Two organisms that interbreed and produce fertile offspring are said to be individuals of the same
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