IB Biology HLClassifying Living DiversityPaper 1 & 2~11 min read
Species Concept: Challenges
The biological species concept asks one question: can these two interbreed and produce fertile offspring? For a great many organisms the honest answer is “they never interbreed with anything” — and bacteria go further, handing genes sideways to species they are barely related to.
📚 What you need to know
The biological species concept: a group of organisms that can interbreed to produce fertile offspring.
It assumes certain reproductive characteristics are unique to each species — mating rituals and seasons, sex organs, gametes that fuse to give a viable zygote, and gametes with the same haploid number forming homologous pairs.
It cannot be applied to organisms that do not reproduce sexually.
Parthenogenesis: a new individual forms without a male gamete. Seen in sharks, Komodo dragons and aphids.
Binary fission: bacterial cells grow, replicate their contents and divide into two, producing clones.
Vegetative propagation: plants clone themselves, for example strawberry runners and daffodil bulbs.
Horizontal gene transfer passes genetic information between cells within the same generation, unlike normal vertical transfer.
It occurs in bacteria by conjugation, and DNA can also move between viruses, bacteria and eukaryotes.
What the concept assumes
The biological species concept rests on the idea that certain reproductive characteristics are unique to each species, and so stop members of different species from breeding successfully:
Compatible mating rituals and seasons — if two animals court differently or breed at different times of year, they never get as far as mating.
Compatible sex organs.
Gametes that can fuse to produce a viable zygote.
Gametes with the same haploid chromosome number, so the chromosomes can match up into homologous pairs.
Every one of those assumes sexual reproduction. Take that away and the whole test collapses.
Organisms that reproduce without a partner
Binary fission and vegetative propagation produce clones. Parthenogenesis sometimes does, and sometimes shuffles the mother’s alleles instead.
Parthenogenesis
Occurs in some fish, some lizards and some invertebrates — sharks, Komodo dragons and aphids are the standard examples.
A new individual forms without the need for a male gamete; the female reproduces on her own. It is sometimes described as a virgin birth.
Offspring are usually all female, and are either clones of the mother or carry shuffled combinations of the mother’s alleles.
Binary fission
Occurs in bacterial cells.
The cell grows, replicates its contents, then divides into two daughter cells.
Clones are produced.
Vegetative propagation
Occurs in some plants.
A new individual develops from a runner growing out from the parent plant, or when a bulb splits into several new bulbs. Strawberries form runners; daffodils develop new bulbs from one parent bulb.
The offspring are clones.
Notice the shape of the argument. The biological species concept is not wrong about these organisms — it simply has nothing to say. The test needs two individuals interbreeding, and there is no interbreeding to observe. That distinction is worth making explicitly in an answer.
Horizontal gene transfer
Gene transfer is normally vertical: genetic information passes from one generation to the next. Horizontal gene transfer passes genetic information from one cell to another within the same generation.
Vertical transfer keeps genetic information inside a lineage. Horizontal transfer punches holes in the idea that a species is a closed genetic box.
It occurs in bacteria, where two cells join and DNA is passed from one to the other. This event is called conjugation.
Genes can be transferred within a species and even between different species.
Although the term usually describes DNA moving between bacterial cells, it is also possible between other organisms — viruses to bacteria, viruses to eukaryotes, and bacteria to eukaryotes.
The consequence is awkward for classification. Many organisms contain genetic information from species that are, in evolutionary terms, only very distantly related. If you are trying to group organisms by shared ancestry, genes arriving sideways make the picture considerably messier.
Link this back to earlier pages. Plasmid transfer through a pilus is exactly how antibiotic resistance spreads through a bacterial population. The same mechanism that makes resistance so hard to contain is the one that makes bacterial species so hard to define.
🧠
Down or across
Vertical transfer goes down a family tree. Horizontal transfer goes across between neighbours. Picture the tree and the direction tells you which is which.
Worked examples
WE 1
Explain why the concept fails for a plant
Explain why the biological species concept is difficult to apply to a strawberry plant that reproduces using runners. (3 marks)
Point 1: what the plant does
It reproduces by vegetative propagation, growing a new plant from a runner, so the offspring are clones.
Point 2: no interbreeding
No gametes fuse and no second individual is involved, so there is no interbreeding to observe.
Point 3: the consequence
The concept defines a species by the ability to interbreed and produce fertile offspring, so it cannot be applied to this method of reproduction.
Clones, no interbreeding, so the test has nothing to measuresay the concept cannot be applied, rather than that it is wrong
WE 2
Distinguish two forms of gene transfer
Distinguish between vertical and horizontal gene transfer, and state one organism group in which the latter occurs. (3 marks)
Vertical
Genetic information passes from one generation to the next, from parent to offspring.
Horizontal
Genetic information passes from one cell to another within the same generation.
Where it happens
It occurs in bacteria, by conjugation, and can also move DNA between viruses, bacteria and eukaryotes.
Down the generations, against sideways within one“within the same generation” is the exact phrase being tested
WE 3
Explain a problem for classification
Suggest why horizontal gene transfer makes classifying bacteria difficult. (3 marks)
Point 1: genes cross boundaries
Genes can be transferred not only within a species but between different species.
Point 2: the effect on the evidence
An organism may therefore carry genetic information from species it is only distantly related to, so DNA similarity does not always mean shared ancestry.
Point 3: the wider problem
Bacteria also reproduce asexually, so the biological species concept cannot be applied either, leaving classification dependent on biochemistry and ecology.
Shared genes may come from a neighbour rather than an ancestorthis is a “suggest” question, so build the argument rather than reciting definitions
💡 Exam tips
Name the reproduction type — parthenogenesis, binary fission, vegetative propagation — not just “asexual”.
Give a named organism for each: sharks or aphids, bacteria, strawberries.
Define horizontal gene transfer as within the same generation.
Use the word conjugation for the bacterial process.
Say the concept cannot be applied, rather than that it is disproved.
Mention that asexual reproduction produces clones.
⚠ Common mistakes
Saying parthenogenesis needs a male gamete. It is reproduction without one.
Calling binary fission mitosis. Bacteria have no nucleus and no mitosis.
Describing horizontal transfer as inheritance. It happens within one generation.
Saying horizontal transfer only occurs in bacteria. DNA can move between viruses, bacteria and eukaryotes.
Writing that asexual offspring are always identical. Parthenogenesis can shuffle the mother’s alleles.
Concluding the species concept is useless. It works well for most sexually reproducing organisms.
Up next: Chromosome Number: Cross-Breeding — what actually goes wrong when two different species do manage to mate.
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