Species and Courtship Behaviour — AQA A-level Biology

SPEC 3.4.5 • GENETIC INFORMATION — SPECIES & TAXONOMY

Species, Taxonomy & Phylogenetic Classification

What makes a species? How courtship prevents the wrong pairings. How scientists group organisms by their evolutionary relationships.

What is a Species?

The question "are these two organisms the same species?" can be answered with a simple reproductive test.

Species (AQA)

Two organisms belong to the same species if they are able to produce fertile offspring.

Why "Fertile"?

The word "fertile" is crucial. A horse (Equus caballus) and a donkey (Equus asinus) can mate and produce offspring — a mule — but mules are sterile. Horses and donkeys are therefore different species.

Examiner Alert

⚠️ Always include the word "fertile". Writing "produce offspring" alone loses the mark — many cross-species pairings produce offspring; only same-species pairings produce fertile offspring.

Courtship Behaviour

Before mating, most animals perform a species-specific ritual — a courtship display. AQA spec states this is a "necessary precursor to successful mating".

Courtship Displays — Examples

Peacock: male fans iridescent tail feathers and performs a shaking display

Fireflies: species-specific patterns of bioluminescent flashes

Frogs: species-specific mating calls (tested in AQA AS Paper 2, 2024)

Sticklebacks: male's red belly + zig-zag dance; female's head-up posture

Birds of paradise: elaborate dances with colourful plumage

Courtship Behaviour

A species-specific behavioural sequence that must be successfully performed before mating occurs. Both partners must recognise and respond correctly to each other's signals.

Five Roles of Courtship

Courtship isn't just decoration — every element serves a specific reproductive function.

1Species recognition — ensures both partners are members of the same species (so offspring will be fertile).

2Sex recognition — identifies the individual as male or female (especially important where sexes look similar).

3Indicates sexual maturity — only mature individuals display the correct courtship signals.

4Indicates willingness to mate — synchronises pair-bonding and readiness.

5Forms a pair bond — strengthens cooperation needed for raising young (in some species).

Why Species Recognition Matters

Getting species recognition wrong is biologically expensive — it wastes gametes and reproductive effort on offspring that won't reproduce.

Natural selection favours animals that can identify the correct species to mate with — this is why courtship behaviours have evolved.

Classifying Life — Hierarchy

Biologists arrange species into nested groups called taxa (singular: taxon). Each taxon sits inside a larger one — like Russian dolls.

Taxon

A named group at any level of the classification hierarchy. Plural: taxa.

Two Key Rules of the Hierarchy (AQA)

Smaller groups are placed within larger groups — e.g. the genus Homo sits inside the family Hominidae, which sits inside the order Primates.

No overlap between groups — an organism belongs to exactly one group at each level (you cannot be in two families at the same time).

The Eight Taxa (Domain to Species)

AQA specifies one hierarchy with eight levels, from largest (most inclusive) to smallest (most specific):

Mnemonic

Dear King Philip Came Over For Good Soup

Domain · Kingdom · Phylum · Class · Order · Family · Genus · Species

Binomial Nomenclature

Every species has a unique two-part Latin name. This system lets scientists worldwide identify the same organism regardless of the local language.

Binomial Nomenclature

A universal two-word naming system using Latin: the genus name (capitalised) + the species name (lowercase). Both italicised when printed.

Writing Rules

RuleExampleGenus name capitalisedH omoSpecies name lowercaseHomo s apiens (not "Sapiens")Both italicised (or underlined when handwritten)Homo sapiensGenus may be abbreviated after first useH. sapiens

Examples

Panthera leo — lion

Panthera tigris — tiger

Escherichia coli — a gut bacterium

Reading Relatedness

Panthera leo and P. tigris share the same genus — so they are more closely related to each other than either is to the domestic cat (Felis catus).

Phylogenetic Classification

A classification system that attempts to arrange species into groups based on their evolutionary origins and relationships (their common ancestry).

Key Principle

Organisms that share a more recent common ancestor are more closely related. Phylogenetic classification aims to match the taxonomic hierarchy to this evolutionary tree — so members of the same taxon genuinely share a common ancestor.

Examiner Alert

⚠️ Phylogenetic classification uses evolutionary relationships — NOT appearance. Two organisms may look very similar (e.g. dolphins and sharks) but occupy very different taxa. Similarity of appearance alone is called "phenetic" classification and is not what AQA is asking about.

Reading a Phylogenetic Tree

A phylogenetic tree shows how species are related through common ancestors. The closer together two branch-tips are, the more recent their shared ancestor.

Reading the Tree

Species C and D are the most closely related — they share the most recent common ancestor.

Species A is the most distantly related to the others — its lineage split off first.

All four species share one ultimate common ancestor (the root).

Advances I — Genome Sequencing

AQA requires you to appreciate how two modern techniques — genome sequencing and immunology — have improved classification.

Genome Sequencing

The DNA base sequences of different species are compared directly.

More closely related species have more similar DNA base sequences

Differences accumulate over time as mutations occur in separated lineages

Number of differences can estimate when two species last shared a common ancestor (molecular clock)

Why It's Powerful

Objective: DNA sequences are numerical data, not subjective judgements of appearance

Works for very different organisms: can compare bacteria with humans because all life uses DNA

Has re-organised the tree of life: some species once grouped by appearance have been re-classified based on DNA evidence

Advances II — Immunological Comparison

Principle

When a protein (e.g. serum albumin) from Species A is injected into Species B (e.g. a rabbit), the rabbit produces antibodies against that protein. These antibodies will also bind (cross-react) with the equivalent protein from other species — but only to the extent that those proteins are similar in shape.

1Inject purified protein from Species A into a rabbit. Rabbit produces antibodies against Species A's protein.

2Collect the rabbit's serum (containing these antibodies).

3Mix the serum with the same protein from Species B, C, D, etc.

4Measure precipitate formed — more precipitate = more antibody binding = more similar protein structure.

5More similar protein → more recent common ancestor with Species A.

Synoptic Link

Links to Topic 2.6/2.7 (Immunity): antibody–antigen binding depends on complementary shape. The same specificity that protects us from disease is exploited to compare proteins between species.

DEEP Answer — Species & Classification

Typical exam question: "Explain why two organisms that look almost identical may be classified as different species." [6 marks]

DDefine: Two organisms belong to the same species only if they can produce fertile offspring. Phylogenetic classification groups organisms by their evolutionary relationships, not their appearance.

EExplain: Similar appearance may be the result of convergent evolution — two unrelated species evolving similar traits because they face similar selection pressures. Their DNA base sequences and protein structures will still differ significantly, revealing separate evolutionary origins.

EEvidence: Dolphins (Mammalia) and sharks (Chondrichthyes) have streamlined body shapes and fins but share very different DNA sequences — they are in different classes with distantly related common ancestors.

PPractical: Genome sequencing or immunological comparison would show many more base/protein differences between a dolphin and a shark than between a dolphin and another mammal — confirming they are in different groups despite superficial similarity.

Practice Question 1

Q1. A horse (Equus caballus) and a donkey (Equus asinus) can mate to produce a mule.

(a) Explain why horses and donkeys are classified as different species. [2 marks]

(b) Name the taxon that contains both horses and donkeys but not humans. [1 mark]

(c) Suggest one type of molecular evidence a biologist could use to confirm the relationship between horses and donkeys. [1 mark]

(a)

(1) Although horses and donkeys can mate and produce offspring (a mule), the mule is sterile / cannot produce fertile offspring. (2) Two organisms belong to the same species only if they can produce fertile offspring, so horses and donkeys must be different species.

(b)

Genus (Equus). Both species share the first part of the binomial name.

(c)

Comparing DNA base sequences (genome sequencing) OR comparing amino acid sequences of a shared protein OR immunological cross-reaction against a common protein.

Practice Question 2 — Courtship

Q2. In a population of two closely related frog species that share a pond, males produce species-specific mating calls. Females respond only to calls of their own species.

(a) Explain the role of these species-specific mating calls. [3 marks]

(b) Scientists observe that when a pollutant disrupts frog hearing, the proportion of infertile hybrid offspring rises sharply. Explain this observation. [3 marks]

(a)

(1) The mating call is a form of courtship behaviour used for species recognition. (2) Females recognise males of their own species by their specific call. (3) This ensures mating occurs only between members of the same species, so offspring will be fertile (rather than sterile hybrids).

(b)

(1) Hearing damage means females can no longer reliably distinguish between the calls of their own species and those of the other species. (2) Some females therefore mate with males of the wrong species. (3) Because the two species are different, their offspring (hybrids) are sterile / infertile — explaining the rise in infertile offspring.

Practice Question 3 — Trees

Q3. A simplified phylogenetic tree shows four primate species: humans (Homo sapiens), chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla) and orangutans (Pongo pygmaeus). Humans and chimpanzees are on adjacent tips; gorillas branch off next; orangutans branch off earliest.

(a) Which species is most closely related to humans, and how can you tell from the tree? [2 marks]

(b) All four species share the same family (Hominidae). Explain what this means in terms of evolutionary relationship. [2 marks]

(c) Describe one piece of molecular evidence that would support this arrangement. [2 marks]

(a)

Chimpanzees (Pan troglodytes) are most closely related to humans because their lineage shares the most recent common ancestor with humans on the tree.

(b)

Being in the same family means all four species share a common ancestor at the family level of the hierarchy. They are more closely related to each other than to species in any different family.

(c)

Comparing DNA base sequences: humans and chimpanzees share ~98–99% of their DNA, higher than humans share with gorillas or orangutans. OR immunological cross-reaction would give a stronger precipitate between human and chimp proteins than between human and orangutan proteins.

Practice Question 4 — Synoptic

Q4. Historically, dolphins and sharks were sometimes grouped together because of their similar streamlined body shape and fins. Modern classification places them in different classes: Mammalia (dolphins) and Chondrichthyes (sharks).

(a) Explain why appearance alone is not a reliable basis for classification. [2 marks]

(b) Describe how genome sequencing would provide stronger evidence for classifying these two animals in different groups. [3 marks]

(a)

(1) Similar appearance can result from similar selection pressures in similar environments (convergent evolution) and does not necessarily indicate close evolutionary relationship. (2) Phylogenetic classification groups organisms by common ancestry, not by appearance.

(b)

(1) Genome sequencing allows direct comparison of DNA base sequences between species. (2) More closely related species show more similar DNA sequences; more distantly related species show more differences. (3) Dolphin DNA sequences would be much more similar to those of other mammals than to shark DNA — confirming dolphins and sharks share only a very distant common ancestor and belong in different classes.

Summary

Must-Know Definitions

Species — two organisms belong to the same species if they can produce fertile offspring.

Courtship behaviour — species-specific behaviour that acts as a necessary precursor to mating; key role is species recognition.

Taxon — a named group at any level of the hierarchy.

Phylogenetic classification — grouping organisms by their evolutionary origins and relationships.

Binomial nomenclature — two-part Latin name: Genus (capital) + species (lowercase), both italicised.

Eight Taxa

Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species

Dear King Philip Came Over For Good Soup

Modern Evidence

Genome sequencing — compares DNA base sequences. Immunology — measures antibody cross-reaction against shared proteins.

Top Exam Tip

When defining species, always include the word FERTILE. When explaining courtship, always mention species recognition and the consequence of mating with the wrong species (sterile hybrids, wasted reproductive effort).