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Biology · Glossary

What is Species?

Definition 30.1 Primary & Middle School Biology · Chapter 30 — Biodiversity

A species is one particular kind of living thing: all the individuals built on the same model, which reproduce among themselves. The house sparrow is a species; so is the common daisy, the red fox, the garden snail. Chapter 21’s classes are boxes of many species: “mammals” holds thousands of them.

Examples

Example 30.3 (Counting the planet)

Scientists have named about two million species so far — and are sure that is only part of the total, since new species are found on every serious expedition: beetles above all (no group has more named species), but also fish, frogs, plants, even occasional mammals. Earth’s full species count is still unknown — most estimates run into the millions beyond what is named.

Example 30.6 (The orchard test)

An orchard with hedges, wildflower strips and old trees hosts dozens of helper species: bees and hoverflies pollinate the blossom (Remark 16.5), ladybugs eat the plant lice, birds patrol for caterpillars, worms keep the soil open. Grub out the hedges and mow the flowers, and the helpers leave — the fruit grower must then do their jobs alone, jobs the web used to do free.

Example 30.7 (People live from the web)

Everything humans eat traces back to living species (Remark 22.8); most of our medicines were first found in plants, molds and other living things; cotton, wool, timber and leather are species’ products. The web’s threads run straight through every kitchen, pharmacy and wardrobe.

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Definition 5.2 High School Biology · Chapter 5 — Biodiversity at Every Scale

A species is a group of living things that resemble one another, can breed together, and produce fertile offspring; two populations that cannot interbreed, or whose hybrids are sterile, are distinct species. In practice — for fossils, for bacteria, for organisms never seen breeding — species are recognised by their shared characteristics, and Chapter 25 will show that the boundary is not always sharp.

Examples

Example 5.3 (Horse, donkey, mule)

A horse and a donkey resemble each other and can breed: their offspring is the mule. But the mule is sterile, so horse and donkey are two species. A poodle and a wolfhound look nothing alike, breed readily, and give fertile puppies: one species, the dog — which can also breed with the wolf, its ancestor.

Example 5.6 (Two lawns)

Ten quadrats on a mown school lawn: 6 plant species, of which grass covers 90%. Ten quadrats on an unmown verge beside it: 23 species, none above 30%. The verge is richer and more even; the difference is not the soil or the climate, which are the same, but the mowing, which removes every plant that cannot flower close to the ground.

Example 5.8 (Blood-group alleles around the world)

The gene of the ABO blood groups has three common alleles, A, B and O. In western Europe their frequencies are about 0.27, 0.07 and 0.66; in eastern Asia about 0.20, 0.20 and 0.60; in some native populations of South America the O allele alone reaches 0.98. Same species, same gene, different distributions of its alleles: the genetic diversity of humans is real, and it is spread across populations rather than splitting them — most of the diversity of the species is found within any one population.

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Definition 29.1 University Biology — Year 1 · Chapter 29 — Classifying the Living World

A species is, under the biological species concept, a group of populations whose members can interbreed and produce fertile offspring and are reproductively isolated from other such groups. The concept cannot be applied to organisms that do not reproduce sexually (most bacteria, many protists and some plants and animals), to fossils, or to populations that never meet; there a species is recognised by its morphology (the morphological concept), by its distinct lineage on a tree (the phylogenetic concept), or by a threshold of genetic divergence. All the concepts try to name the same thing: a lineage evolving separately from others.

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Definition 23.1 University Biology — Year 2 · Chapter 23 — Speciation and Macroevolution

Under the biological species concept, a species is a group of populations whose members can interbreed with one another and produce fertile offspring, and are reproductively isolated from other such groups: what unites a species is gene flow, and what separates species is its absence. The concept works for the sexual organisms of one time and place, which is most of what a biologist meets; it fails for asexual lineages (which are named by resemblance), for fossils (which cannot be crossed), and for the many cases where the boundary is leaky — oaks and willows that hybridise across species, ducks whose species interbreed in captivity, ring species in which neighbouring populations interbreed all the way round a barrier until the ends meet and do not. Other concepts fill the gaps: the morphological species (a cluster of like forms), the phylogenetic (the smallest diagnosable lineage), the ecological (a lineage occupying its own niche). None is wrong; each measures something real, and speciation is the process by which they come to agree.

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