Cambridge A-Level Biology – Classification: Species Concepts
Classification – The Term “Species”
In biology, a species is the fundamental unit of classification. Different scientists have proposed various ways to define what constitutes a species. For the A‑Level syllabus we focus on three major concepts:
1. Biological Species Concept (BSC)
Proposed by Ernst Mayr, the BSC defines a species as:
A group of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.
Key idea: Reproductive isolation (pre‑zygotic and post‑zygotic barriers) prevents gene flow between species.
Examples of isolation mechanisms:
Temporal isolation – different breeding seasons.
Behavioural isolation – distinct courtship rituals.
Mechanical isolation – incompatible genitalia.
Hybrid inviability or sterility – e.g., mules.
Advantages: Reflects evolutionary processes; useful for sexually reproducing animals.
Limitations: Not applicable to asexual organisms, fossils, or hybrids that can back‑cross.
2. Morphological (Phenetic) Species Concept
This concept groups organisms based on overall similarity of observable traits.
Key idea: Species are the smallest groups that are consistently and distinctly different in morphology.
Often used when genetic or breeding data are unavailable (e.g., fossils, plants, microorganisms).
Advantages: Simple, practical for field identification and paleontology.
Limitations:
Subjective – depends on which traits are considered.
Phenotypic plasticity can blur boundaries.
Cryptic species may be overlooked.
3. Ecological Species Concept
Defines a species as a set of organisms adapted to a particular niche.
Key idea: Species occupy a unique ecological role; they are the smallest group of organisms that can maintain a distinct niche in an ecosystem.
Emphasises the relationship between organisms and their environment.
Advantages: Highlights adaptive divergence and resource partitioning.
Limitations: Niche overlap can occur; difficult to delineate when ecological data are limited.
Comparison of the Three Concepts
Aspect
Biological Species Concept
Morphological Species Concept
Ecological Species Concept
Basis of definition
Reproductive isolation
Overall similarity of form
Unique ecological niche
Applicable groups
Sexually reproducing organisms
Fossils, asexual organisms, plants
Organisms with well‑defined niches
Strengths
Reflects evolutionary processes
Practical for identification
Links species to environment
Weaknesses
Fails for asexual or extinct taxa
Subjective; ignores genetics
Hard to quantify niche differences
Typical use in A‑Level
Understanding speciation mechanisms
Classifying fossils and plants
Discussing adaptive radiation
Suggested diagram: Venn diagram showing overlap and differences between the three species concepts.
Key Take‑away Points
All three concepts aim to delineate groups of organisms that are evolutionarily distinct, but they use different criteria.
The Biological Species Concept is central to understanding reproductive isolation and speciation in animals.
The Morphological Species Concept is valuable when genetic or breeding data are unavailable, especially for fossils.
The Ecological Species Concept highlights the role of environment and niche in maintaining species boundaries.
In practice, taxonomists often combine elements of each concept to reach a robust classification.