IB Biology SLTopic 3 — Classifying Living DiversityPaper 1 & 2Core idea~13 min read
The Biological Species Concept
“What is a species?” sounds like a question with an easy answer. It is not. Biologists have several definitions, each of which works brilliantly in some cases and falls apart in others — and the exam wants you to know exactly where each one breaks.
📚 What you need to know
Variation means the differences between organisms — in appearance, behaviour and biochemistry.
Variation comes from a combination of genetic and environmental factors.
Discontinuous variation falls into distinct categories; continuous variation is measured on a scale.
The morphological species concept defines a species by its observable features. It often misleads.
The biological species concept defines a species as a group that can interbreed to produce fertile offspring.
It cannot be applied to asexual organisms, extinct species, or cases of fertile hybrids.
Other evidence used to classify: morphology, DNA, biochemistry, ecology, evolutionary lineage.
Linnaeus gave us the binomial system: a two-part Latin name, genus then species.
Speciation happens when populations become isolated and stop exchanging genes.
Variation, and where it comes from
No two individuals are identical when you compare every characteristic — not even identical twins, because tiny environmental differences act on them.
Variation has two sources working together:
Genetic. Genes decide which proteins an organism can make. New genetic variation arises through mutation, and sexual reproduction shuffles alleles into new combinations.
Environmental. The surroundings decide whether an organism has what it needs to actually produce a protein, so they affect gene expression. Temperature, nutrient supply and oxygen availability all matter.
The test is simple: could an individual sit halfway between two of the values? If not, the variation is discontinuous.
Naming species: Linnaeus and the binomial system
Before Linnaeus, species names were long descriptions in Latin. The tomato was called Solanum caule inermi herbaceo, foliis pinnatis incises, racemis simplicibus — a whole sentence describing the plant.
Carl Linnaeus, an 18th century Swedish botanist, replaced this with a two-part Latin name: the genus first, then the species. The tomato became Solanum lycopersicum. Because the names are the same everywhere in the world, a scientist in any country knows exactly which organism is meant.
🧩 Writing a binomial name correctly
Genus gets a capital letter, species gets a lower-case one: Apis mellifera, the honey bee.
Typed names are italicised. Handwritten names are underlined instead.
Write it in full the first time it appears: Triticum aestivum.
After that you may abbreviate the genus: T. aestivum.
Species in the same genus are closely related — Canis lupus (wolf), Canis latrans (coyote) and Canis familiaris (domestic dog) all share the genus Canis.
Linnaeus shaped how we name species, but he did not always get the grouping right. He classified using appearance alone, and species that look alike are not always related. Plenty of his names have since been changed.
Two ways of defining a species
Concept
Definition
Main weakness
Morphological
A group of organisms that are morphologically unique
Similar-looking organisms are not always closely related
Biological
A group of organisms that can interbreed to produce fertile offspring
Cannot be applied to asexual, extinct, or hybridising organisms
Learn this wording exactly
A species is a group of organisms that can interbreed to produce fertile offspring
Where the biological species concept breaks down
Asexual reproduction. Bacteria reproduce asexually, so asking whether two of them can breed together is meaningless.
Fertile hybrids. Occasionally two species produce fertile offspring — the “wholphin” from a melon-headed whale crossed with a bottlenose dolphin. By the strict definition that would make a new species, but hybridisation only creates species when it happens frequently, and wholphins are very rare.
Extinct species. You cannot breed a woolly mammoth with a modern elephant to check. They look similar, but the test is impossible to run.
So what do biologists actually use? A combination, chosen to suit the organism. Morphology for visible features; DNA sequence similarity; biochemistry, such as which gases a bacterium produces; ecology, because each species has its own niche; and evolutionary lineage from the fossil record. Bacteria all look alike, so they are classified by biochemistry and ecology. A long-extinct animal is classified by lineage and morphology.
Speciation, and why the boundary is blurry
Species do not stay the same. Change over time is evolution; the process by which one species becomes two or more is speciation.
Natural selection acting differently on each side drives them apart. Genetic drift can do the same job by chance in small populations.
The important consequence is that the ability to interbreed fades gradually. There is no moment when a switch flips. So deciding when two populations count as two species is partly a judgement call — it is subjective.
A live example. Killer whales (Orcinus orca) vary a great deal between populations. They are currently treated as one species with several “ecotypes”, but some scientists argue there is more than one species of orca. The evidence has not settled the question.
Worked examples
WORKED EXAMPLE
Explain why the biological species concept cannot be used to classify bacteria. [2]
Step 1: state the definition being used
The biological species concept relies on whether organisms can interbreed to produce fertile offspring.
Step 2: explain the mismatch
Bacteria reproduce asexually, so they never interbreed and the test cannot be applied.
Other evidence, such as biochemistry or ecology, must be used insteadthe third line is worth adding even in a 2-mark question – it shows you know the fix
WORKED EXAMPLE
A horse and a donkey can breed to produce a mule, but mules are sterile. Using the biological species concept, state whether horses and donkeys are the same species. Explain your answer. [2]
Step 1: apply the definition
A species is a group that can interbreed to produce fertile offspring.
Step 2: check the case
Horses and donkeys do interbreed, but the mule cannot reproduce.
Different species — the offspring is not fertilethe word “fertile” is doing all the work here – never leave it out
WORKED EXAMPLE
State whether each is continuous or discontinuous variation: (a) human eye colour categories, (b) leaf surface area, (c) ability to roll the tongue, (d) body mass. [4]
Ask each time: could a value fall between the categories?(a) discontinuous — separate named categories(b) continuous — can be any area within a range(c) discontinuous — you either can or you cannot(d) continuous — measured on a scaleCategories = discontinuous; measurements = continuous
💡 Exam tip
Learn the biological species definition word for word, including fertile.
Know all three limitations: asexual reproduction, fertile hybrids, extinct species.
Binomial names: capital genus, lower-case species, italics or underline. Marks are lost on formatting.
If a question asks how bacteria are classified, name biochemistry and ecology specifically.
For speciation, the key phrases are isolation, no gene flow, and reproductively isolated.
Say that species boundaries are subjective where the evidence is gradual — it is a genuine mark point.
⚠ Common mix-up
Leaving “fertile” out of the definition. Without it, horses and donkeys become one species.
Writing binomial names with two capitals or with the species first.
Saying morphology is never used. It is still useful, just not on its own.
Confusing continuous and discontinuous. Continuous is measured; discontinuous is counted into categories.
Thinking variation is purely genetic. The environment affects gene expression too.
Saying speciation happens suddenly. The ability to interbreed declines gradually.
Up next: Chromosome Number — why every species has its own fixed number of chromosomes, and why that number is always even in a body cell.
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