Defining Species & Courtship Behaviour Revision Notes | A Level AQA Biology

1. What Makes Two Organisms the Same Species?

Here is a question that sounds obvious until you think about it carefully: how do we decide whether a lion and a tiger are different species? They look similar, they can even produce offspring (ligers), so what is the deciding factor? The answer lies not in appearance, but in the fertility of those offspring.

The biological species concept gives us a precise, exam-ready answer. Two organisms belong to the same species if they are able to interbreed and produce fertile offspring. That word — fertile — is doing all the work here.

The liger example is instructive. A male lion and a female tiger can mate and produce a liger, but ligers are sterile — they cannot reproduce. Because the offspring are infertile, lions and tigers are classified as separate species. The criterion is not whether offspring can be produced at all, but whether those offspring can go on to reproduce themselves.

This distinction matters enormously in exam answers. Examiners specifically look for the word fertile when awarding marks for the species definition. Writing that organisms "can breed together" or "can reproduce" is not sufficient — you must state that the offspring produced are fertile.

It is also worth recognising that this definition has practical limitations. Organisms that reproduce asexually, or species known only from fossils, cannot be tested against this criterion. AQA expects you to know and apply the definition, but being aware of its boundaries shows higher-order thinking in extended-response questions.

2. Courtship Behaviour — The Necessary Precursor

Now that we have a clear definition of species, a new question emerges: how do organisms of the same species find and select each other in the first place? This is where courtship behaviour becomes essential.

Courtship behaviour refers to the innate behavioural sequences that occur before mating. AQA describes courtship as a necessary precursor to successful mating — meaning that without it, mating either does not occur or is far less likely to succeed. Think of it as the biological equivalent of a handshake that confirms both parties are ready and compatible before any commitment is made.

The phrase "necessary precursor" is precise AQA language. In exam answers, using this phrase signals to the examiner that you understand courtship is not optional decoration — it is a functional requirement for reproductive success in most sexually reproducing species.

2a. The Functions of Courtship Behaviour

Courtship behaviour serves several interconnected functions, all of which ultimately increase the probability of successful reproduction. These functions are worth knowing individually because exam questions often ask you to explain the role of courtship, which requires linking each function to a biological outcome.

• Species recognition: courtship signals are species-specific, ensuring that mating only occurs between members of the same species
• Mate assessment: courtship allows individuals to assess whether a potential mate is healthy, sexually mature, and genetically fit
• Synchronisation of reproductive physiology: courtship stimulates hormonal changes that prepare both partners for mating and fertilisation
• Pair bond formation: in species with biparental care, courtship helps establish a lasting bond between mates

Of these functions, species recognition is the one most directly linked to the species concept you learned in Section 1. If courtship signals were not species-specific, organisms might attempt to mate with members of a different species — wasting reproductive effort and potentially producing infertile hybrid offspring. Courtship acts as a biological filter.

3. The Role of Courtship in Species Recognition

Species recognition is arguably the most important function of courtship from an evolutionary perspective. Here is why: if an organism mates with a member of a different species, the resulting offspring are likely to be infertile (as with the liger). That represents a complete loss of reproductive investment — no genes are passed on to future generations.

Courtship signals solve this problem by being highly species-specific. Each species has its own unique combination of visual displays, vocalisations, chemical signals (pheromones), or behavioural sequences. A potential mate that does not produce the correct species-specific signal will not trigger the mating response in the other individual.

This is where it gets particularly interesting from an exam perspective. The species-specificity of courtship signals means that courtship acts as a pre-zygotic reproductive barrier — it prevents the formation of a zygote between members of different species in the first place, rather than relying on post-zygotic mechanisms such as hybrid infertility.

3a. Examples of Species-Specific Courtship Signals

Concrete examples help anchor this concept and are useful in exam answers when asked to describe or explain courtship. The table below summarises well-known examples across different signal types.

Notice that in every case, the signal is not merely attractive — it is a species-specific code. The receiving individual has an innate response mechanism tuned to that exact signal. A stickleback female does not respond to a red belly on a different species; a moth male does not follow the wrong pheromone blend. This selectivity is what makes courtship effective as a species recognition mechanism.

4. Linking Courtship Back to the Species Concept

Earlier, we established that two organisms belong to the same species if they can produce fertile offspring. Courtship behaviour is the mechanism that makes this possible in practice — it is the bridge between the theoretical definition of a species and the biological reality of reproduction.

Without species-specific courtship, organisms could not reliably identify compatible mates. Mating between different species would be more frequent, producing infertile offspring and reducing the reproductive success of both individuals. Courtship therefore reinforces the species boundary that the biological species concept describes.

This connection also has evolutionary significance. Natural selection favours individuals whose courtship signals are most effective at attracting same-species mates and repelling different-species individuals. Over time, this drives courtship signals to become increasingly species-specific — a process that can itself contribute to speciation when populations become reproductively isolated.

5. Exam Technique and Mark-Winning Strategies

These two topics — defining species and courtship behaviour — appear regularly in AQA papers, and the marks are very achievable if you use precise language. The following guidance consolidates the key exam strategies for both spec points.

Worked example: Answering an 'Explain' Question on Courtship and Species Recognition

Explain the role of courtship behaviour in species recognition. [4 marks]

1. Courtship behaviour consists of innate behavioural sequences that occur before mating. — This earns the first mark by correctly identifying courtship as innate and pre-mating. Avoid vague phrases like 'things animals do to attract mates' — be precise.
2. Courtship signals (such as visual displays, vocalisations, or pheromones) are species-specific — unique to each species. — This is the key mechanistic point. Naming a signal type (visual, chemical, acoustic) adds biological detail that can earn a mark.
3. Only members of the same species produce and respond to the correct courtship signal, so mating attempts between different species are prevented. — This links the mechanism to the outcome. The phrase 'prevented' is important — it shows you understand courtship as a pre-zygotic barrier.
4. This ensures reproductive effort is not wasted on producing infertile hybrid offspring, maximising the chance of producing fertile offspring of the same species. — This final point connects courtship back to the species concept and shows higher-order reasoning — exactly what examiners reward in the final mark of a multi-mark question.

Answer: Courtship behaviour consists of innate, species-specific signals. Only same-species individuals respond to the correct signal, preventing cross-species mating. This ensures reproductive effort produces fertile offspring rather than infertile hybrids.

Notice how each step builds on the previous one. The examiner is following a logical chain: what courtship is → why signals are species-specific → what this prevents → why this matters for reproduction. Answers that jump straight to "it prevents mating with other species" without explaining how (species-specific signals) will miss marks.

6. Retrieval and Exam-Style Practice

The best way to consolidate these two spec points is to practise applying them to unfamiliar scenarios — exactly what AQA 'suggest' and 'evaluate' questions require. Work through the questions below before checking the sample answers.

That recall question models the kind of evaluative reasoning AQA rewards in longer-mark questions. The key move is to identify the precise criterion being violated (fertility of offspring), apply it to the specific example (mules are sterile), and reach a clear conclusion. Vague answers that say only "they are different species" without explaining why will not score highly.

Together, the biological species concept and courtship behaviour form a coherent picture: the species concept defines what it means to be reproductively compatible, and courtship is the behavioural mechanism that makes same-species mating — and therefore fertile offspring — reliably achievable in practice.