Why the Biological Species Concept Faces Challenges
Biological Species Concept (BSC)
The biological species concept (BSC) defines a species as a group of organisms capable of interbreeding and producing fertile offspring, emphasizing reproductive isolation.
- BSC works well for sexually reproducing organisms.
- But, it faces challenges when applied to organisms that reproduce asexually or exchange genetic material across species boundaries.
Asexual Reproduction Challenges The BSC
- Many organisms reproduce entirely asexually, meaning there's no interbreeding so the BSC cannot be applied.
- Offspring are genetically identical clones of the parent, produced by mitosis rather than fertilization.
- Without recombination of genetic material, species boundaries are difficult to define because all individuals are essentially genetic copies.
Clone
A group of genetically identical organisms or cells derived from a single parent.
- Plants:
- Dandelions (Taraxacum officinale) reproduce asexually by forming seeds without fertilization.
- Many other plants reproduce vegetatively (e.g., runners, tubers).
- Animals:
- Some reproduce by parthenogenesis, where females produce offspring from unfertilized eggs.
- Seen in aphids, some reptiles (e.g., Komodo dragons), and even sharks.
- Parthenogenesis: A form of asexual reproduction where an egg develops into an individual without fertilization.
If asked in exams, highlight that parthenogenesis challenges the BSC because no interbreeding takes place, yet viable lineages persist.
Horizontal Gene Transfer in Bacteria
Horizontal Gene Transfer
Horizontal gene transfer involves the direct exchange of genetic material between unrelated organisms, bypassing traditional parent-offspring inheritance. It is especially common in prokaryotes (bacteria and archaea).
- Since bacteria reproduce by binary fission (strictly asexual), the BSC cannot be applied.
- This is because bacteria frequently exchange genes across strains and even species through HGT.
Antibiotic resistance genes can spread between bacterial species, altering their genetic makeup independently of reproduction.
Why The BSC Fails in These Cases
- Asexual Organisms
- No interbreeding occurs, so the core definition of species under the BSC cannot be applied.
- Leads to arbitrary classification into many clones or microspecies.
- Horizontal Gene Transfer
- Species boundaries are blurred as genetic material flows across unrelated taxa.
- In bacteria, "species" boundaries are almost meaningless because of widespread gene sharing.
Always state both asexuality and HGT as reasons why the BSC does not work universally.
Note- The biological species concept is useful for sexually reproducing organisms but has significant limitations when applied to asexual organisms and bacteria.
- Competing species definitions exist to address these challenges.
- Define the biological species concept and explain its key limitation in organisms that reproduce asexually.
- Why are Taraxacum officinale (dandelions) and Rubus fruticosus (blackberries) problematic for the BSC?
- Define parthenogenesis and give one animal example.
- What is horizontal gene transfer, and why does it prevent the BSC from being applied to bacteria?
