Difficulties of Defining Species (AQA A-Level Biology): Revision Notes
Difficulties of Defining Species
Defining a species appears straightforward at first - organisms that share observable similarities and can produce fertile offspring when they reproduce. However, this biological species concept faces several significant challenges that make classification more complex than it initially seems.
The biological species concept defines species as groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups. While widely used, this definition has important limitations in practice.
Evolution and time-based changes
Species are not static entities but constantly evolve over time. Through natural selection and genetic drift, populations gradually accumulate changes that can eventually lead to the development of entirely new species. This evolutionary process creates difficulties because:
- Determining the exact point when one species becomes another is nearly impossible
- Transitional forms in the fossil record blur the boundaries between species
- Contemporary populations may be in the process of speciation but haven't yet completed the divergence
The continuous nature of evolutionary change means that species boundaries are often more fluid than discrete, making precise classification challenging even for living organisms.
Variation within species
Significant variation can exist within a single species, which sometimes makes individuals appear very different from one another. The classic example is domestic dogs - from Chihuahuas to Great Danes - all belonging to the same species despite dramatic differences in size, appearance, and behaviour.
Worked Example: Domestic Dog Variation
All domestic dogs (Canis lupus familiaris) belong to the same species despite extreme variation:
Size range: Chihuahuas (2-6 pounds) to Great Danes (110-175 pounds)
Appearance: From flat-faced pugs to long-snouted greyhounds
Behaviour: From aggressive guard dogs to gentle lap dogs
Key point: All these breeds can successfully interbreed and produce fertile offspring, confirming their shared species status.
Artificial selection has accelerated this variation, creating breeds that might seem like separate species based on appearance alone. This demonstrates how morphological (physical) differences don't always indicate separate species status.
Extinct species and fossil records
Most species that have ever existed are now extinct, leaving only fossil evidence behind. This creates substantial problems for classification because:
- Fossils rarely preserve soft tissues needed for complete morphological analysis
- Behavioural characteristics and reproductive compatibility cannot be assessed
- Fragmentary remains make it difficult to distinguish between closely related species
- The fossil record is incomplete, with many transitional forms missing
Critical Limitation: Since we cannot test reproductive compatibility in extinct organisms, palaeontologists must rely solely on morphological features, which may not accurately reflect species boundaries.
Asexual reproduction
Some organisms reproduce asexually, which completely bypasses the "fertile offspring" criterion for species definition.
Examples of Asexual Reproducers:
- Many bacteria and archaea (binary fission)
- Some plants and fungi (vegetative propagation, spore formation)
- Certain animal species (parthenogenesis in some insects and reptiles)
Since these organisms don't interbreed at all, the biological species concept becomes irrelevant for their classification.
Geographic isolation
Members of the same species may become separated by geographical barriers such as oceans, mountain ranges, or other obstacles. These isolated populations never have the opportunity to interbreed, making it impossible to test their reproductive compatibility.
Over time, these separated populations may:
- Remain capable of interbreeding if brought together
- Diverge sufficiently to become reproductively incompatible
- Be mistakenly classified as different species due to lack of gene flow
The Testing Problem: Without the ability to bring geographically isolated populations together for breeding tests, scientists cannot definitively determine whether they represent the same species or have diverged into separate species.
Misclassification of isolated populations
Populations that appear morphologically distinct may be classified as separate species when they're actually capable of interbreeding. This error occurs because researchers cannot easily test reproductive compatibility between geographically separated groups.
When such populations are eventually brought together, they may successfully produce fertile offspring, revealing their shared species status.
Sterile organisms and hybrids
Some organisms are naturally sterile or produce sterile offspring when mating with closely related species.
Worked Example: Horse-Donkey Hybridisation
Parent species:
- Horses possess 64 chromosomes (32 pairs)
- Donkeys have 62 chromosomes (31 pairs)
Hybrid offspring:
- Mules inherit 63 chromosomes (32 from horse + 31 from donkey)
The sterility problem: During meiosis, mules cannot form proper chromosome pairs due to the odd number, preventing normal gamete formation. This chromosomal incompatibility renders most mules infertile.
Exception: Rare cases of fertile female mules have been documented, though male mules remain universally sterile.
This example demonstrates how closely related species can produce viable but reproductively unsuccessful offspring, challenging simple definitions of species boundaries.
Implications for biological classification
These difficulties highlight the complexity of biological classification and explain why:
- Multiple species concepts exist beyond the biological species concept
- Taxonomists must consider various criteria when classifying organisms
- Modern classification increasingly relies on genetic analysis and phylogenetic relationships
- Species boundaries may sometimes be somewhat arbitrary human constructs imposed on naturally continuous variation
Modern Approaches: DNA sequencing and molecular phylogenetics have provided new tools for species classification, but they have also revealed that the boundaries between species are often more complex than traditional methods suggested.
Key Points to Remember:
- Species definition faces challenges because organisms evolve continuously over time
- Significant variation can exist within species due to natural and artificial selection
- Geographic isolation prevents testing of reproductive compatibility between populations
- Extinct species can only be classified using incomplete fossil evidence
- Asexually reproducing organisms bypass the interbreeding criterion entirely
- Hybrid sterility (like in mules) results from chromosomal incompatibilities during meiosis
- Modern classification uses multiple approaches including genetic analysis to overcome these limitations