What you need to know
- 4.14(i)classificationSorting organisms into groups based on shared features and evolutionary relationships. is built around the speciesA group of organisms that can interbreed to produce fertile offspring. concept, using phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment. and genotypeThe alleles an organism has for a gene..
Reproductive isolation as a process that can form new species is developed in 4C (4.19–4.20). Here the focus is what a species is, and why one definition does not cover every organism. Do not treat a single definition as a law of nature.
By the end, you should be able to…
- State a biological species definition and its purpose.
- Explain why phenotype alone can split or merge groups incorrectly.
- Give situations where breeding evidence is unavailable or unhelpful.
Build your understanding
A working biological definition
A widely used biological species conceptDefining a species as a group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other groups. treats a speciesA group of organisms that can interbreed to produce fertile offspring. as a group of organisms that can interbreed to produce fertile offspring, and that is reproductively isolated from other such groups. The point is a shared gene poolAll the alleles of all the genes in a population.: alleleA different version of the same gene, found at the same locus. can move among members under natural conditions, but not freely into other species.
Fertile offspring matter. A mule is a living hybridA mix of two different sources: the offspring of two different species (usually infertile), or a DNA molecule with one strand from each source. of horse and donkey, but mules are typically sterile, so horses and donkeys remain separate species on this definition. Potential interbreeding in nature is the criterion, not forced mating in a zoo.
When appearance misleads
Members of one species can look different because of sex, life stage, season or environment. Different species can look almost identical. classificationSorting organisms into groups based on shared features and evolutionary relationships. that used only a stripe pattern or leaf shape can therefore split one species or lump two.
genotypeThe alleles an organism has for a gene. helps. If two populationAll the individuals of one species living in one area at the same time. share similar sequences across many loci and there is evidence of gene flowMovement of alleles between populations when individuals migrate and interbreed., they may be one species despite colour differences. If they look alike but their genomes form two distinct clusters with little mixing, they may be two species. Molecular evidence still needs sampling, alignment and critique; a single geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. can mislead.
Where the breeding test fails
Asexual organisms, including many bacteria, do not form species by interbreeding. Fossils cannot be tested for fertility. Populations separated by oceans might interbreed if brought together, or they might not; the experiment may be impossible. Plants and some animals hybridise in nature, blurring boundaries.
In those cases taxonomists combine phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment., ecological role and molecular data. The species remains the unit of classification, but the evidence used to recognise it is stated. Disagreeing papers are expected; that is the scientific community evaluating data, not a reason to abandon classification.
NOT: producing any offspring is not enough. The offspring must be fertile.
- The biological idea is about a shared gene pool
- A sterile hybrid is evidence of a barrier
- Asexual populations need morphology or genes
- How isolation arises is the next topic, not this definition
Two traps this diagram blocks
- Do not say a mule proves horses and donkeys are the same species.
- Do not force the biological definition onto bacteria or fossils without other evidence.
From the diagram alone
Question
Use the biological species concept to explain why a horse and a donkey are classified as different species. [2]
Show a model answer
They can produce a mule, but the mule is usually sterile. Because they do not produce fertile offspring, they are not the same species under the biological concept.
| Approach | Main evidence | Works well when | Struggles when |
|---|---|---|---|
| Biological | Actual or potential interbreeding and fertile offspring | Sexual populations that can be observed | Asexual organisms, fossils, many geographically isolated populations |
| Morphological | Phenotypic similarity | Distinct anatomy, including many fossils | Cryptic species or highly variable phenotypes |
| Molecular / phylogenetic | Sequence similarity and shared ancestry | Microbes and groups with little visible structure | Sparse sampling, incomplete sequences, or conflict among genes |
Make your reasoning visible
Include fertility
“They can breed” is incomplete. State fertile offspring, or explain why that test cannot be used.
Name the missing evidence
For a fossil, say that breeding cannot be observed, then use morphology and context rather than inventing a mating test.
Avoid this misconception
The claim: If two organisms look different, they must be different speciesA group of organisms that can interbreed to produce fertile offspring..
The correction: phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment. can vary within a species. Breeding and genetic evidence may show they still share a gene poolAll the alleles of all the genes in a population..
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
State the biological species concept.
Show suggested answer
A species is a group of organisms that can interbreed to produce fertile offspring and is reproductively isolated from other groups.
Practice question
Retrieval and explanation · suggested answer
Why are horses and donkeys still classified as separate species even though they can produce a mule?
Show suggested answer
The mule is typically sterile, so the populations do not share a freely mixing gene pool of fertile offspring.
Practice question
Retrieval and explanation · suggested answer
Give two reasons the biological species concept is hard to apply to bacteria.
Show suggested answer
Many bacteria reproduce asexually, so interbreeding is not how they form groups. Horizontal gene transfer can also move DNA between distant lineages, so a simple breeding test does not map cleanly onto their diversity.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two bird populations look identical, live on separate islands, and have not been tested for interbreeding. Sequence data form two clusters with little shared variation. What can you conclude, and what remains uncertain?
Show a hint
Separate a genomic pattern from a completed breeding test.
Explore the reasoning
The molecular pattern supports two distinct groups and weakens a claim that they are one freely mixing species. You still lack a direct breeding test, so a biological-species conclusion is an inference. More loci, more individuals and, if ethical and possible, breeding or field gene-flow data would test it.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare classification by phenotype with classification by genotype.
Classification and three domains
Practice question
Retrieval and explanation · suggested answer
Compare classification by phenotype with classification by genotype.
Lesson: 4B 1: Principles of Classification
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A species as a gene pool
4B · What Is a Species?NHGRI describes a gene pool as all the alleles present in a reproducing population or species. That genomic idea matches the biological species concept: the species is the…Specification 4.14(i)Read & use the scienceClose article
Why this matters
NHGRI describes a gene pool as all the alleles present in a reproducing population or species. That genomic idea matches the biological species concept: the species is the largest group that shares alleles under natural conditions. A large gene pool can hold more diversity; it is not automatically a different species from a small one. The biological species concept focuses on gene flow, but it is difficult to apply directly to fossils, asexual organisms or populations that are geographically separated. Scientists may also use phenotype and molecular evidence. A gene pool is relevant because breeding can move alleles between individuals, yet a definition alone does not settle every boundary. This is a good stretch question: what observations would actually support or challenge the proposed species grouping? The specification rewards reasoning with evidence rather than memorising a single absolute test.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 4.14(i) understand that classification is a means of organising the variety of life based on relationships between organisms using differences and similarities in phenotypes and in genotypes, and is built around the species concept
Use the science
Why does a large gene pool not by itself prove that two named species should be merged?
Explore a possible response
Diversity within one species can be high. Merger requires evidence of actual or potential gene flow producing fertile offspring, or equivalent molecular support, not merely lots of alleles.
Explore the source
- NHGRI: gene poolNational Human Genome Research Institute
Look-alike bees, different species?
4B · What Is a Species?The Natural History Museum describes cryptic bumblebee species: organisms that look similar enough to be grouped under one name until other evidence reveals differences. DNA barcodes can help,…Specification 4.14(i)Read & use the scienceClose article
Why this matters
The Natural History Museum describes cryptic bumblebee species: organisms that look similar enough to be grouped under one name until other evidence reveals differences. DNA barcodes can help, but taxonomists compare several individuals and traits before redrawing a boundary. The biological species concept raises another question: do the groups exchange genes in nature? Conservation priorities may change when one apparent widespread species turns out to include several narrower populations. This illustrates why species is a scientific hypothesis supported by phenotype, genotype and reproductive evidence, not a label decided by colour alone.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 4.14(i) understand that classification is a means of organising the variety of life based on relationships between organisms using differences and similarities in phenotypes and in genotypes, and is built around the species concept
Use the science
If two bee populations look alike but have different DNA barcodes, what more would strengthen a claim that they are separate species?
Explore a possible response
Consistent differences in multiple samples, reproductive isolation or lack of gene flow, and relevant ecological or phenotypic evidence would strengthen it.
Explore the source
- Bumblebee specialist group and cryptic speciesNatural History Museum