A species is a group of organisms that share so many characteristics they can interbreed to produce fertile offspring. Biologists classify all living things into a hierarchy of groups — from broad (domain, kingdom) to narrow (genus, species) — to make sense of the enormous diversity of life on Earth.
What is a species?
The most widely used definition of a species is a group of organisms that can interbreed with one another to produce fertile offspring, but cannot do so with members of other species. This is the biological species concept.
A key word is fertile: a horse and a donkey can mate and produce a mule, but mules are almost always sterile. Horses and donkeys are therefore different species despite being able to hybridise. The biological species concept works well for sexually reproducing animals, but is harder to apply to bacteria (which reproduce asexually) and to plants (which hybridise freely). Biologists use it as a guide rather than an absolute rule.
Across all ecosystems, scientists have described approximately 8–9 million species, though estimates of the true number (including undescribed species) range up to 1 trillion. Classification systems are the tools that make this diversity navigable.
Who developed the classification system we use today?
The system of classifying organisms into a strict hierarchy was developed by the Swedish botanist Carl Linnaeus (1707–1778). His major work Systema Naturae (first edition 1735) introduced the hierarchical groupings and the binomial (two-name) naming system that scientists still use today.
Before Linnaeus, organisms had long, inconsistent Latin descriptions; two scientists studying the same animal might call it by entirely different names. Linnaeus provided a single, universal system that could be used by scientists of any language — a revolutionary advance for communication across the emerging global scientific community.
What are the levels of the Linnaean hierarchy?
The Linnaean hierarchy organises living things into nested groups from the broadest (domain) to the most specific (species). A useful mnemonic is Daring King Philip Came Over For Good Soup:
| Level | Example (domestic dog) |
|---|---|
| Domain | Eukarya |
| Kingdom | Animalia |
| Phylum | Chordata |
| Class | Mammalia |
| Order | Carnivora |
| Family | Canidae |
| Genus | Canis |
| Species | lupus familiaris |
The full scientific name of the domestic dog is therefore Canis lupus familiaris. Scientific names are always written in Latin or Latinised form; the genus name is capitalised and the species name is in lower case; both are italicised in print (or underlined in handwriting).
What are the five kingdoms?
The original Linnaean system used two kingdoms (animals and plants). Modern biology now recognises five kingdoms, based on cell type, cell structure, and mode of nutrition:
| Kingdom | Cell type | Key features | Examples |
|---|---|---|---|
| Animalia | Eukaryotic | Multicellular; heterotrophic; no cell wall | Mammals, fish, insects |
| Plantae | Eukaryotic | Multicellular; autotrophic (photosynthesis); cell wall of cellulose | Flowering plants, ferns, mosses |
| Fungi | Eukaryotic | Multicellular (mostly); heterotrophic; cell wall of chitin; digest externally | Mushrooms, moulds, yeasts |
| Protista | Eukaryotic | Mostly unicellular; varied nutrition | Amoeba, algae, Plasmodium |
| Prokaryotae (Monera) | Prokaryotic | Unicellular; no membrane-bound nucleus | Bacteria |
The key division is between eukaryotic cells (with a true membrane-bound nucleus) and prokaryotic cells (no membrane-bound nucleus). All kingdoms except Prokaryotae are eukaryotic.
What is the three-domain system?
Modern molecular biology (particularly comparison of ribosomal RNA sequences) revealed that the five-kingdom system misses a fundamental division. Carl Woese proposed the three-domain system in 1990, which places all life into three domains that reflect deeper evolutionary divisions:
- Bacteria: prokaryotes with no nucleus; familiar disease-causing and beneficial bacteria
- Archaea: prokaryotes that look similar to bacteria but have profoundly different biochemistry; often found in extreme environments (hot springs, salt lakes)
- Eukarya: all organisms with a true nucleus — includes all five-kingdom groups except Prokaryotae
The surprising finding was that Archaea and Eukarya are more closely related to each other than either is to Bacteria. This is invisible from structure alone and was only revealed by DNA analysis.
How is classification based on evolutionary relationships?
Modern classification aims to reflect evolutionary history — a system called phylogenetics. Organisms are grouped together if they share a common ancestor, not just because they look similar. For example:
- Bats, whales, and humans are all classified as mammals because they all descended from the same ancestral mammal group, even though their lifestyles are completely different.
- Birds are classified as a group within the reptiles (archosaurs) because they descended from theropod dinosaurs — making "reptile" a group that technically includes birds.
- Dolphins were once placed with fish because they look similar and live in water; DNA analysis showed they are far more closely related to hippos and even-toed ungulates.
Looks can be misleading; shared DNA sequences and molecular evidence have repeatedly overturned groupings that were based on superficial similarity.
Frequently asked questions
What is the definition of a species in KS3 biology?
A species is a group of organisms that can interbreed with each other to produce fertile offspring. Members of the same species share many physical and genetic characteristics. They cannot produce fertile offspring with members of other species — which is what makes species distinct biological units. There are around 8–9 million named species on Earth, and scientists use the Linnaean classification hierarchy to organise and name them all.
What is binomial nomenclature?
Binomial nomenclature is the two-name system for naming species, introduced by Linnaeus. Every species gets a unique scientific name consisting of its genus (capitalised) followed by its species name (lower case), both written in italic (e.g. Homo sapiens for humans). This universal Latin-based system means that scientists worldwide use the same name for the same organism, regardless of their language. Common names vary by country and language; scientific names do not.
Why did scientists move from the five-kingdom to the three-domain system?
The five-kingdom system was based primarily on visible features of organisms. Molecular biology, especially comparisons of ribosomal RNA gene sequences, revealed that the single "Prokaryotae" kingdom actually contains two profoundly different lineages — Bacteria and Archaea — that are as different from each other as either is from eukaryotes. The three-domain system reflects these evolutionary relationships more accurately. It also revealed that Archaea are more closely related to Eukarya than to Bacteria.
How does classification help scientists?
Classification provides a shared language that allows scientists worldwide to communicate about specific organisms without ambiguity. It also organises biodiversity in a way that reflects evolutionary relationships, making it easier to predict the properties of organisms based on their relatives. Medical researchers, for example, use classification to identify which organisms are likely to respond similarly to drugs, or to trace the origin of a disease outbreak to a related species in a wildlife reservoir.
For Socratic KS3 biology with Professor Darwin — asking first what two species share in common before deciding whether to lump them in the same genus or split them, then checking against the DNA evidence — visit aitutors.me.