Species and taxonomy
AQA
A-Level
The biological species concept defines a species by reproductive compatibility, not by appearance. Two organisms belong to the same species only if they can interbreed and the offspring are themselves fertile, so that alleles keep flowing between them and they share a single gene pool. A horse and a donkey interbreed and produce a mule, but the mule is sterile because its odd chromosome number prevents homologous chromosomes pairing in meiosis; horse and donkey are therefore separate species. The word fertile is what carries the definition, because offspring that cannot themselves reproduce are an evolutionary dead end and the two parent populations continue to evolve apart.
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Objectives
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Exam Tips
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Pitfalls
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Key Terms
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Mark Points
Subtopics in this area
Two organisms belong to the same species if they are able to produce fertile offspring.
Courtship behaviour as a necessary precursor to successful mating.
The role of courtship in species recognition.
A phylogenetic classification system attempts to arrange species into groups based on their evolutionary origins and relationships.
It uses a hierarchy in which smaller groups are placed within larger groups, with no overlap between groups.
Each group is called a taxon (plural taxa).
One hierarchy comprises the taxa: domain, kingdom, phylum, class, order, family, genus and species.
Each species is universally identified by a binomial consisting of the name of its genus and species, eg, Homo sapiens.
Students should be able to appreciate that advances in immunology and genome sequencing help to clarify evolutionary relationships between organisms.
Species and taxonomy Revision Guide
Learning Objectives
What you need to know and understand
- Decide, from a description of two organisms and the offspring they produce, whether one species or two is present.
- Explain why the mule places the horse and the donkey in separate species, referring to the failure of chromosome pairing in meiosis.
- Apply the definition at the end of a speciation answer to justify that two isolated populations have become separate species.
- Distinguish between organisms that look alike and organisms that share a gene pool when deciding how many species a sample contains.
- Describe courtship as a chain of stimulus and response in which each behaviour triggers the partner's reply.
- Explain three separate ways in which courtship raises the chance of successful mating, including synchronisation of gamete release.
- Suggest, for an unfamiliar animal described in a question, why its courtship must be completed before mating can occur.
- Explain why a species-specific display prevents a member of another species from completing the courtship chain.
- Justify species recognition as an advantage by naming the cost of mating with a different species (e.g., wasted gametes).
- Use differences in courtship to account for reproductive isolation between two closely related species that share a habitat.
- Explain how a phylogenetic classification differs from one based on observable similarity, using common ancestry in your answer.
- Interpret a branching diagram of taxa to state which two groups are most closely related and why.
- Evaluate a claim made about a set of phylogenetic data by giving evidence both for and against it.
- Describe the hierarchical arrangement of taxa using nesting and the absence of overlap between groups.
- Decide which of two pairs of organisms is more closely related from the smallest taxon each pair shares.
- Explain why a single organism can occupy only one group at each level of the hierarchy.
- Define a taxon as any group within the classification hierarchy and give an example at two different levels.
- Use the singular and plural forms correctly when writing about one group and several groups.
- Write a comparison between named taxa taken from a figure, keeping the statement at the level of the group.
- Write the eight taxa in order from domain to species without prompting.
- Complete a classification table for a named organism, keeping each column to one organism and each row to one level.
- Identify which level of the hierarchy a given group name belongs to, and name the levels immediately above and below it.
- Write the binomial of a named species as the genus name followed by the species name, using the conventional capitalisation.
- Explain why a universal naming system is more useful to scientists than common names.
- Deduce from two binomials whether the species share a genus, and say what that shows about their relatedness.
- Explain how comparing base sequences allows the evolutionary relationship between two species to be judged.
- Describe how the binding of antibodies to proteins from another species provides evidence of relatedness.
- Suggest why classifications based on observable features such as flagella have been revised since sequencing became available.
Marking Points
Key points examiners look for in your answers
- one mark for stating that members of the same species can interbreed to produce fertile offspring, with the word fertile present
- one mark for using the sterility of a hybrid such as a mule to place its two parents in different species
- one mark, at the end of a speciation answer, for saying the two populations can no longer interbreed to produce fertile offspring
- one mark for linking interbreeding to gene flow between populations or to a shared gene pool
- one mark for describing courtship as a chain of stimulus and response in which each action triggers the partner's next behaviour
- one mark for recognition of a member of the same species, so that mating can produce fertile offspring
- one mark for identifying a mate that is sexually mature, fertile or receptive
- one mark for synchronising mating or triggering the release of gametes so that fertilisation is more likely
- one mark for forming a pair bond that supports the rearing of the young
- One mark for stating that a courtship display or behaviour is specific to one species.
- One mark for explaining that only a member of the same species gives the correct response, so the stimulus-response chain is not completed with another species.
- One mark for stating that mating with a different species often produces no offspring or infertile offspring.
- One mark for concluding that species recognition prevents the waste of gametes, time, or energy.
- One mark for linking failed recognition to reproductive isolation and the maintenance of separate gene pools.
- State that groups are based on evolutionary origins and common ancestry rather than on similarity of appearance.
- State that the more recently two species shared a common ancestor, the more closely related they are.
- When reading a phylogenetic diagram, identify the most recent common ancestor shared by specific taxa to determine relatedness.
- Identify a piece of data that does not fit a suggested trend, such as a taxon with an unexpected value compared to its close relatives.
- Name the comparison of DNA base sequences, mRNA, or amino acid sequences as the evidence used to place groups.
- Describe a hierarchy as smaller groups contained within larger groups.
- State that there is no overlap between groups at the same level.
- Explain that each organism belongs to exactly one group at every level of the hierarchy.
- Link a smaller shared group to a more recent common ancestor and a closer relationship.
- Define a taxon as a group of organisms at any level of the classification hierarchy.
- Use the correct plural, taxa, when referring to more than one group.
- Name a level of the hierarchy as an example of a taxon, such as domain, genus or species.
- In a data question, write a comparison between named taxa rather than between individual organisms.
- Recognise that a value given for a taxon may be an aggregate or mean for the group and need not apply to every species it contains.
- Give the levels in the correct sequence from the largest group down to the species: domain, kingdom, phylum, class, order, family, genus, species.
- Complete a classification table by reading across the row to identify the level and down the column to keep the organism consistent.
- Give the genus and species names together as the binomial in the species row.
- Keep every entry within a column referring to the same organism.
- Identify which level of the hierarchy a given group name belongs to, and name the levels immediately above and below it.
- Give the binomial as two words in the order genus then species, since the species word on its own is not the binomial.
- When completing a classification table, put both words of the binomial in the species row.
- Explain that the name is universal, so the same species is identified by the same name everywhere.
- Use a shared genus name as evidence that two species are closely related.
- Distinguish between common names and scientific names, explaining why the binomial system is more useful to scientists.
- one mark for naming DNA or genome base sequencing as the evidence used to clarify evolutionary relationships
- one mark for naming amino acid sequencing of proteins as an alternative line of evidence
- one mark for stating that the more similar the sequences, the more closely related the species or the more recent the common ancestor
- one mark for an immunological comparison in which antibodies bind to similar proteins from another species, with more precipitate indicating a closer relationship
- one mark for explaining that new evidence contradicted a grouping based on observable features, so species were renamed or moved
Examiner Tips
Expert advice for maximising your marks
- 💡Write the full phrase 'interbreed to produce fertile offspring' every time; the single word fertile is often the difference between the mark and no mark.
- 💡In a five-mark speciation question, save this definition for your final sentence so it scores as the separate 'now two species' point.
- 💡If a question gives you a hybrid, state whether it is fertile before you decide how many species are involved.
- 💡Turn every behaviour you describe into a consequence, using the words 'so that' to force the benefit out.
- 💡Treat courtship as a self-contained paragraph you can deploy in any essay on reproduction or on interactions between organisms, rather than relying on a published topic list.
- 💡Two- and three-mark courtship questions want separate benefits, not one benefit explained three ways; count distinct ideas before you write.
- 💡Answer in three steps: the display is species-specific, only that species responds correctly, so gametes are not wasted on an infertile mating.
- 💡If a question features two closely related species in the same habitat, use differences in courtship behaviour as the reproductive isolation mechanism keeping their gene pools separate.
- 💡Always use the phrase 'members of the same species'; vague terms like 'the right partner' or 'compatible mate' rarely score marks.
- 💡In a justify question about a phylogenetic data set, write at least one point for and one point against, because a balanced answer is more likely to access all the available marks.
- 💡Quote the named taxa and the figures from the diagram rather than writing 'the earlier group' and 'the later group'.
- 💡Learn the distinction in one sentence: a phylogenetic system groups by ancestry, and the evidence for ancestry is sequence data.
- 💡If a question asks what a hierarchy means, you need both halves: nested groups and no overlap.
- 💡Sketch the levels as nested rectangles in rough work; it stops you inverting the order under pressure.
- 💡Compare relatedness by naming the smallest taxon two organisms share, because that is what fixes how closely related they are.
- 💡When a question says 'six taxa', check the levels shown before you compare them; they are often not all the same rank.
- 💡Use taxon and taxa correctly in written answers, since the specification uses both and examiners expect the vocabulary.
- 💡If a data question is about taxa, keep comparisons at group level: 'the ferns produce less than the asterids', not 'this plant produces less'.
- 💡Write the eight levels down the margin as soon as a classification table appears, then fill the table from your list.
- 💡In a completion table, check each entry against both its row and its column before moving on.
- 💡If you have to write the binomial in the species row, give both words; the species word alone is not the binomial.
- 💡The capital letter and the underlining are conventions of scientific writing; what is credited is both words, in the order genus then species.
- 💡If a table gives a genus in one column and a species in another, join them in that order to build the binomial.
- 💡Papers may shorten the genus to its initial once a species has been named in full, as in C. difficile and E. coli, so read those as binomials and write both words out whenever a question asks you to name the species.
- 💡When asked why species have been renamed, name a technique such as base sequencing or amino acid sequencing and link it to the evidence it provides.
- 💡Use the word resolution, not magnification, whenever you explain what a better microscope provided.
- 💡Say what is being compared in full: the base sequence of DNA, or the amino acid sequence of a named protein.
Common Mistakes
Pitfalls to avoid in your exam answers
- writing that the two organisms can produce offspring and leaving out fertile, which is the word the mark scheme requires
- writing 'no inbreeding' instead of 'no interbreeding', which examiners reject
- defining a species by how similar the organisms look rather than by whether their offspring are fertile
- merging reproductive isolation and the species definition into one point in a speciation answer, when isolation belongs at the start of the process and fertile offspring at the end
- assuming any hybrid shows the parents are one species, when only a fertile hybrid does
- describing what the animal does, such as dancing or singing, without saying what the behaviour achieves
- writing that courtship 'attracts a mate' and stopping there, which does not name a benefit the mark scheme lists
- claiming courtship guarantees fertilisation rather than raising its probability
- giving the same benefit twice in different words, such as species recognition and avoiding the wrong species
- writing about the survival of the individual when the marks are for reproduction and offspring
- Stating courtship allows an animal to recognise 'the right mate' without explicitly specifying 'a member of the same species'. Always use the exact terminology.
- Claiming cross-species mating always produces no offspring. Correct this by stating it often produces no offspring, or produces infertile hybrids (such as the mule, which is the infertile offspring of a horse and a donkey).
- Confusing species recognition with recognising the opposite sex or sexual maturity; these are distinct functions of courtship and should be treated as separate marking points.
- Anthropomorphising by writing that animals 'know' they belong to the same species. Correct this by describing it as an innate response to a specific stimulus.
- Describing a phylogenetic system as grouping organisms that look alike, which is the definition of a system based on observable characteristics.
- Assuming that convergent features, such as the streamlined body of a fish and of a dolphin, show close relatedness.
- Reading a tree from left to right and calling the right-hand taxon the most advanced or most evolved.
- In a 'do the data support this?' question, giving only supporting points and losing the marks reserved for evidence against.
- Saying one living species evolved from another living species rather than from a shared common ancestor.
- Listing the levels without saying that each level is contained within the one above.
- Forgetting the no-overlap rule and suggesting a species could be placed in two different genera.
- Inverting the order, so that a domain is described as sitting inside a kingdom.
- Saying two organisms in the same class are closely related without noting that sharing a genus would make them more closely related still.
- Treating the hierarchy as a ranking of how advanced organisms are rather than as nested groups.
- Believing a taxon is one particular level, usually species, rather than any group in the hierarchy. Correction: a taxon can be any rank, from domain down to species.
- Writing 'taxa' for a single group, or inventing the plural 'taxons'. Correction: one group is a taxon; several groups are taxa.
- Confusing taxon with taxonomy, which is the study of classification itself. Correction: taxon is the group; taxonomy is the study of how organisms are classified.
- Treating a value given for a taxon in a table as though it applied to a single organism. Correction: the value may be an aggregate or mean for the group.
- Assuming all the taxa shown in one figure must be at the same level of the hierarchy. Correction: taxa in a figure may sit at different levels.
- Swapping class and order, the two levels most often transposed. Correction: the sequence is class then order.
- Putting both the genus name and the species name in the genus row of a table. Correction: the binomial belongs in the species row.
- Omitting domain and starting the hierarchy at kingdom. Correction: domain is the largest group and comes first.
- Inventing extra ranks such as sub-phylum, which are not required by this specification. Correction: use only the eight levels listed.
- Filling gaps by copying the pattern of a neighbouring column instead of reading the level from the row. Correction: read across the row to identify the level.
- Capitalising the species word of the name, or capitalising neither word. Correction: the genus begins with a capital letter; the species word does not.
- Giving only the species word when the question asks for the binomial. Correction: the binomial requires both genus and species words.
- Giving the common name when the question asks for the scientific name. Correction: use the binomial, not a common name.
- Assuming two organisms that share the species word, such as two unrelated species called officinalis, must be related. Correction: only a shared genus shows close relatedness.
- Treating the abbreviation in E. coli as part of the species name rather than as the shortened genus. Correction: E. stands for the genus Escherichia.
- writing that improved microscopes gave greater magnification when the mark scheme requires greater resolution
- saying that DNA is compared without saying that it is the base sequence being compared
- claiming similar DNA proves organisms are the same species, rather than indicating how closely related they are
- describing the immunological method as a test for disease rather than as a comparison of proteins between species
- presenting renaming as a change of opinion rather than as a response to new sequence evidence