Classification – The Term “Species”
Definition (syllabus wording): A species is the fundamental unit of classification – a group of organisms that share a set of defining characteristics and are distinct from other such groups.
The Cambridge International AS & A Level Biology (9700) expects students to:
- Define a species and describe the four main species concepts (biological, morphological/phenetic, phylogenetic, ecological).
- State the strengths and limitations of each concept.
- Explain how each concept relates to the four recognised speciation mechanisms.
- Use classification tools (dichotomous keys, cladograms, molecular phylogenetics).
- Discuss the relevance of species concepts to biodiversity conservation and genetic technology.
1. Species Concepts
1.1 Biological Species Concept (BSC)
“A group of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” – Ernst Mayr
- Key idea: reproductive isolation (pre‑zygotic & post‑zygotic barriers) prevents gene flow between groups.
Strengths
- Directly linked to the evolutionary process of speciation.
- Works well for sexually reproducing animals where breeding behaviour can be observed.
Limitations
- Not applicable to asexual organisms, fossils, or organisms that cannot be bred in the laboratory.
- Hybrid zones and occasional back‑crossing can blur boundaries.
Typical isolation mechanisms
- Temporal – different breeding seasons (e.g. Rana temporaria vs. R. arvalis).
- Behavioural – distinct courtship songs in crickets.
- Mechanical – incompatible genitalia in many insects.
- Hybrid inviability/sterility – mules, horse‑donkey hybrids.
1.2 Morphological (Phenetic) Species Concept
- Key idea: the smallest group of organisms that is consistently and distinctly different in overall morphology.
- When it is used: fossils, plants, microorganisms, or any taxa where breeding or genetic data are unavailable.
Strengths
- Simple and practical for field work and paleontology.
- Requires only observable characters.
Limitations
- Subjective – depends on which traits are chosen and how they are weighted.
- Phenotypic plasticity can mask true genetic relationships.
- Cryptic species (genetically distinct but morphologically similar) may be missed.
Example
The fossil trilobite Paradoxides species are distinguished by the shape of the glabella and the pattern of pygidial spines.
1.3 Phylogenetic Species Concept (PSC)
“A species is the smallest monophyletic group of organisms that share a common ancestor and can be distinguished by unique, derived characters (often DNA sequences).” – Avise & Ball
- Key idea: monophyly + diagnosable characters (morphological or molecular).
- Typical data: mitochondrial DNA, nuclear gene sequences, or fixed morphological traits.
Strengths
- Objective and repeatable when robust phylogenies are available.
- Applicable to asexual taxa, extinct groups, and organisms with limited morphological variation.
- Reveals cryptic diversity.
Limitations
- Requires high‑quality molecular or morphological data and rigorous phylogenetic analysis.
- Can lead to “taxonomic inflation” by splitting populations into many narrowly defined species.
Example
DNA barcoding split the African elephant into two species: Loxodonta africana (savanna) and L. cyclotis (forest).
1.4 Ecological Species Concept (ESC)
- Key idea: a species is the smallest group of organisms that occupies a unique ecological niche.
- Emphasis: adaptation to particular resources, habitats, or functional roles within an ecosystem.
Strengths
- Links speciation directly to adaptive divergence and resource partitioning.
- Useful for understanding how environmental factors drive the emergence of new taxa.
Limitations
- Niches can overlap, making boundaries fuzzy.
- Quantifying niche differences often requires extensive ecological data.
Example
The three species of Darwin’s finches on the Galápagos each exploit different seed sizes (ground finch, cactus finch, woodpecker finch), representing distinct ecological niches.
2. Comparison of the Four Species Concepts
| Aspect | Biological | Morphological | Phylogenetic | Ecological |
|---|
| Basis of definition | Reproductive isolation | Overall similarity of form | Monophyly + diagnosable characters | Unique ecological niche |
| Applicable groups | Sexually reproducing animals | Fossils, plants, asexual microbes | All organisms with phylogenetic data | Organisms with well‑defined niches |
| Strengths | Reflects evolutionary process | Simple, field‑friendly | Objective, works for cryptic species | Highlights adaptation & resource use |
| Weaknesses | Fails for asexual/extinct taxa | Subjective; ignores genetics | Requires robust phylogenies; may over‑split | Hard to quantify niche differences |
| Typical A‑Level use | Speciation mechanisms | Fossil classification, plant IDs | DNA barcoding, cryptic species studies | Adaptive radiation, conservation case‑studies |
3. Linking Species Concepts to the Four Recognised Speciation Mechanisms
The four major speciation mechanisms are best understood when the underlying species concept is considered. The table below shows the primary conceptual link for each mechanism.
| Speciation Mechanism | Definition | Illustrative Example | Species Concept(s) Most Relevant |
|---|
| Allopatric | Geographic isolation splits a population; divergence occurs in separate areas. | Hawaiian silversword alliance after island emergence. | Biological (reproductive isolation evolves) & Phylogenetic (monophyly of each island lineage). |
| Peripatric | A small peripheral population becomes isolated; genetic drift and selection act strongly. | Dwarf elephants on Mediterranean islands. | Biological & Phylogenetic (founder‑effect lineages become monophyletic). |
| Parapatric | Adjacent populations experience divergent selection across a gradient; limited gene flow. | Grass Anthoxanthum odoratum on metal‑contaminated vs. normal soils. | Ecological (niche divergence) and Biological (partial reproductive barriers). |
| Sympatric | New species arise within the same geographic area, usually via ecological or behavioural shifts. | Apple maggot fly Rhagoletis pomonella shifting from hawthorn to apple hosts. | Ecological (host‑plant niche) and Biological (behavioural isolation); Phylogenetic (distinct lineages). |
Explicit link paragraph: The Biological Species Concept underpins allopatric and peripatric speciation because geographic separation allows reproductive barriers to evolve, eventually producing reproductively isolated (and therefore biologically distinct) lineages. In parapatric and sympatric scenarios, ecological divergence is the primary driver; the Ecological Species Concept therefore provides the most intuitive framework for interpreting how niche differentiation can lead to reproductive isolation. The Phylogenetic Species Concept complements both views by offering a genetic test of monophyly, confirming that the diverging populations have become independent evolutionary lineages.
4. Classification Tools (AO3 – Practical Skills)
- Dichotomous key – a series of paired, mutually exclusive statements that lead the user to an identification.
- Skill tip: Construct a key for five local tree species using leaf shape, bark texture, and fruit type.
- Cladogram – a branching diagram that shows hypothesised relationships based on shared derived characters (synapomorphies).
- Skill tip: Draw a simple cladogram for the mammals human, rabbit, dolphin, platypus using dental formula, presence of a placenta, and mode of reproduction.
- Molecular phylogenetics – DNA sequencing (e.g., COI barcoding, 16S rRNA) used to infer monophyletic groups and estimate genetic distances.
- Skill tip: Interpret a short mitochondrial DNA alignment to decide whether two bird populations belong to the same species under the Phylogenetic Species Concept.
5. Relevance to Biodiversity Conservation
- Accurate species delimitation is essential for assessing extinction risk (IUCN Red List) and for allocating legal protection.
- Different concepts can lead to different conservation priorities; for example, cryptic species recognised by the PSC may each require separate management plans.
- Understanding reproductive isolation (BSC) helps design captive‑breeding programmes that avoid unwanted hybridisation.
- Ecological niche information (ESC) guides habitat restoration, reserve design, and re‑introduction strategies.
- Genetic technologies (CRISPR, gene drives) raise ethical questions that hinge on how we define a “species” and on what level of genetic distinctiveness we consider intervention acceptable.
6. Practical/Skill Box (AO3)
Task: Using the data set below, complete the three activities.
- Dichotomous key – Write a key to separate the four insects (grasshopper, beetle, butterfly, dragonfly) based on observable traits.
- Cladogram construction – From the character matrix (wing type, mouthpart, metamorphosis), draw a cladogram indicating the most parsimonious relationships.
- Phylogenetic analysis – Given the short COI sequences, identify which two specimens share a recent common ancestor and state whether they would be considered the same species under the PSC.
Data set (simplified)
| Specimen | Wing type | Mouthpart | Metamorphosis | COI (partial) |
|---|
| Grasshopper | None | Chewing | Incomplete | ATG‑CCT‑GAA‑… |
| Beetle | Hard elytra | Chewing | Complete | ATG‑CCT‑GAG‑… |
| Butterfly | Scaled | Sucking | Complete | ATG‑CCT‑GAA‑… |
| Dragonfly | Transparent | Chewing | Incomplete | ATG‑CCT‑GAC‑… |
7. Key Take‑away Points
- Four major species concepts (Biological, Morphological, Phylogenetic, Ecological) each use a different criterion to delimit species.
- Each concept has clear strengths and limitations; in practice taxonomists often combine evidence from several concepts.
- The four recognised speciation mechanisms (allopatric, peripatric, parapatric, sympatric) illustrate how reproductive isolation, niche divergence, or genetic drift generate new species, and each mechanism aligns naturally with one or more species concepts.
- Classification tools – dichotomous keys, cladograms, and molecular phylogenetics – are essential practical skills for A‑Level examinations (AO3).
- Accurate species delimitation underpins biodiversity conservation, informs management of threatened taxa, and frames ethical discussions surrounding modern genetic technologies.