What Is a Species? Exploring Nature’s Building Blocks

Species are the fundamental units of the living world, much like the bricks of a house, the elements of the periodic table, or the keys on a piano. Each species plays a crucial role in the tapestry of life, joining with others to form communities that are deeply connected to each other and to their physical environments. Yet, despite their vital importance, scientists have not reached complete agreement on what exactly defines a species.

The Classical Definition: Who Belongs with Whom?
The most familiar definition of a species is a group of individuals that can breed together and produce fertile offspring. For example, a poodle and a Labrador can have puppies that are as fertile as their parents—clearly, all dogs are the same species. Meanwhile, rare crosses like the liger (the offspring of a male lion and a female tiger) do not typically produce fertile offspring, so lions and tigers remain separate species.

Interestingly, even wolves and domestic dogs can interbreed and produce fertile pups, illustrating that dogs and wolves are technically the same species—Canis lupus. However, domestic dogs are recognized as a subspecies (Canis lupus familiaris) because they share common traits such as tameness and shorter teeth, subtly drifting from their wild ancestors over time.

Why Species Aren’t So Simple
This definition doesn’t always hold up. No one has tried to cross a hippopotamus and a whale—the hippo’s closest living relative—to see if they could produce offspring. In many cases, species need to be identified in other ways, especially when dealing with organisms discovered for the first time.

One modern approach uses genetics. Scientists collect tissue samples—like blood from animals or leaves from plants—and sequence their DNA. By comparing DNA differences, researchers can find out if a community is genetically distinct from other known species. If so, it’s strong evidence that genetic exchange isn’t happening with related organisms—a sure sign of a new species.

DNA: Solving Mysteries and Creating Questions
DNA has revolutionized how we discover and catalog life, making it easier to identify new species, especially among fungi. In the UK, for example, DNA is adding 50 or more new fungal species to the national list each year. Fungi often go unnoticed because what we see as mushrooms are only small outgrowths of huge organisms living within soil or wood. In Oregon, the world’s largest living organism—a honey mushroom from the Armillaria genus—spans vast distances and may weigh as much as 35,000 tons, aging over 2,500 years.

Fungal researchers now commonly sample soil to extract and sequence environmental DNA, uncovering rich hidden biodiversity. But to put a name to a DNA sequence, it must be matched to reference specimens from museums, a process called “DNA barcoding”—reminiscent of scanning groceries at a checkout.

But DNA isn’t a perfect solution. It breaks down over time, rarely surviving more than 1.5 million years even under optimal conditions. This makes the plot of Jurassic Park—where dinosaur DNA is extracted from ancient mosquitoes—impossible. The oldest DNA sequenced to date, from a mammoth, is just over one million years old. For most extinct species, we rely on fossil forms and careful assumptions about their relationships, especially when only fragments remain.

When One Species Hides Many
Sometimes, the problem flips: species look the same and have nearly identical DNA, but are isolated enough geographically that they never naturally interbreed. These are “cryptic species”—hidden diversity masquerading as a single name. This might mean we’re severely undercounting species, especially in remote areas like the Arctic.

Other mysteries arise when organisms maintain separate forms while occasionally exchanging genes. This is common among bacteria, where species boundaries are fuzzy. Certain plants, like Brazil’s bromeliads, also swap genes across visible differences and even between individuals with differing chromosome numbers. And remarkably, humans themselves still carry about 2% of their DNA from Neanderthals, the legacy of ancient encounters.

Why Species Live Where They Do
Not only what defines a species, but where and how many species exist, is a matter of ecology and geography. The “species-area relationship” shows that larger islands or habitats harbor more species, while isolation decreases diversity by limiting arrivals and exchanges. First presented by Robert MacArthur and Edward O. Wilson in 1967, this rule underpins the Theory of Island Biogeography—a key idea for understanding biodiversity.

Most species cluster near the equator: as you move closer to the tropics, the variety of life explodes, a pattern known as the latitudinal diversity gradient. Finally, most species—whether tropical or otherwise—are rare, their populations limited either by nature or by human activity.

In essence, species are the basic building blocks of life’s vast architecture. While modern science has offered powerful new ways to define, discover, and even debate what counts as a species, the living world remains wonderfully diverse and, at times, deeply mysterious. That’s what makes the study of species—and the quest to understand them—so endlessly fascinating.

Source : The Hidden Universe: Adventures in Biodiversity by Alexandre Antonelli

Goodreads : https://www.goodreads.com/book/show/56669607-the-hidden-universe

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