IB Biology HLClassifying Living DiversityPaper 1 & 2~13 min read
The Biological Species Concept
Everyone thinks they know what a species is until they try to write it down. Look the same? Plenty of near-identical species are not related. Can breed together? Then bacteria have no species at all. Biology’s neatest-sounding definition turns out to be the one with the most holes in it.
📚 What you need to know
Variation is the differences between organisms. It comes from a combination of genetic and environmental factors.
Variation is discontinuous (distinct categories, such as blood group) or continuous (measured on a scale, such as height).
Classification is sorting organisms into groups; the science of it is taxonomy, and each group is a taxon.
The morphological species concept defines a species by observable features, but it often leads to mistakes.
Linnaeus introduced the binomial system: a two-part Latin name made of the genus and the species.
The biological species concept defines a species as a group of organisms that can interbreed to produce fertile offspring.
It cannot be applied to asexual organisms, fertile hybrids or extinct species.
Speciation produces new species when populations become isolated and gene flow stops. Deciding when two populations have become two species is often subjective.
Variation
Differences between organisms are called variation. It shows up in more than just appearance:
Visual appearance — fur colour, body length.
Behaviour — mating rituals, how aggressive an animal is.
Biochemistry — antibiotic resistance, the products of metabolism.
Where it comes from
Variation is the result of genetic and environmental factors together, and the two are not separate stories.
Genes determine which proteins an organism can make, and the proteins shape its characteristics. Genetic variation is generated by mutation, and by alleles being combined in different ways during sexual reproduction.
The environment decides whether an organism has the resources to make a particular protein, so it can affect gene expression. Temperature, nutrient availability and oxygen concentration all do this.
Variation exists between species, which is what lets us classify them into groups, and within a species, because individuals carry different combinations of alleles. Compare enough characteristics and no two individuals are identical — even identical twins, who share their alleles, differ because of small environmental differences.
Discontinuous and continuous
The gap between the bars on the left is the whole point. Nobody has a blood group halfway between A and B, but people come in every height between the extremes.
Discontinuous
Continuous
Categories
Distinct classes, with nothing in between
No distinct classes
Measuring it
Cannot be measured over a range
Measured incrementally on a scale
Individuals
Cannot fall between categories
Fall anywhere between two extremes
Example
Human blood group, black or brown fur
Height
Naming and classifying
There are far too many species to make sense of without logical groups. Sorting organisms into groups is classification; the science of doing it is taxonomy, and the people who do it are taxonomists. Each group is a taxon (plural taxa), and the smallest one is the species.
Historically, an organism’s species was decided from its observable characteristics. That is morphological classification, and its species concept says:
Morphological species concept
A species is a group of organisms that are morphologically unique
Morphology is still a useful guide, but classification is no longer done on morphology alone, because species that look alike are not always closely related.
Linnaeus and the binomial system
Carl Linnaeus was an 18th-century Swedish botanist, and he is called the father of taxonomy because he invented the naming system still in use today. Before him, names were long descriptions — the tomato’s name was a full sentence of Latin describing its stem and leaves. Linnaeus replaced that with a two-part Latin name, the same everywhere in the world. The tomato became Solanum lycopersicum.
Both words are Latin, or a latinised version of another word — which is how a spider ended up named after a fictional school house.
Convention
Example
Genus starts with a capital, species with lower case
The honey bee is Apis mellifera
Typed names are in italics; handwritten names are underlined
A limpet is Patella vulgata
Write the name in full the first time, then abbreviate the genus
Wheat is Triticum aestivum, then T. aestivum
Linnaeus shaped modern naming, but he did not always get the classification right. He was working from morphology alone, and many of the species he named have since been reclassified. That is a useful example if a question asks how scientific knowledge changes as new evidence arrives.
The biological species concept
Because morphology misleads, biologists rely on other definitions. The most widely used one is the biological species concept:
Biological species concept
A group of organisms that can interbreed to produce fertile offspring
Note the word fertile. Two organisms producing offspring is not enough — those offspring must be able to reproduce themselves.
Where it falls apart
Problem
Why the concept cannot cope
Asexual reproduction
Bacteria reproduce asexually, so asking whether two of them can breed together is meaningless
Fertile hybrids
Rarely, two different species produce fertile offspring. The ‘wholphin’, from a melon-headed whale crossed with a bottlenose dolphin, is fertile. Strictly the concept would make it a new species, but hybridisation only creates species if it is a frequent event, and wholphins are rare
Extinction
You cannot breed a woolly mammoth with an elephant to test the fertility of the offspring, so the concept cannot be applied to extinct species at all
Because the concept is imperfect, other characteristics are used as well, and which ones depends on the organism:
Morphology — members of a species share similar form.
DNA — sequences can be compared, and a certain level of similarity indicates the same species.
Biochemistry — species may produce different molecules as products of metabolism. Some bacteria release carbon dioxide, others methane.
Ecology — a species’ precise ecological niche is usually distinctive.
Evolutionary lineage — fossils of extinct species can be compared with similar living ones and placed in a lineage.
Match the tool to the organism. Bacteria often look alike under a microscope, so they are separated by biochemistry or ecology. We know almost nothing about the biochemistry of a long-extinct animal, so it is classified by morphology or evolutionary lineage. If a question gives you an awkward organism, say which characteristic you would use and why.
Populations, species and speciation
Species do not stay the same. Change over time is evolution, and the process by which one species gives rise to two or more is speciation.
A population becomes isolated from other populations of the same species, usually by living in a different area.
Isolation means the populations cannot breed together, so gene flow cannot take place between them.
If conditions differ, natural selection acts differently on each population. Genetic drift can also push them apart.
Eventually the two can no longer breed to produce fertile offspring. They are reproductively isolated, and are separate species.
The awkward part is the middle. Speciation happens over very long periods, and the ability to interbreed declines gradually rather than stopping on a particular day. So deciding when two populations have become two species can seem arbitrary, and often comes down to the opinions of scientists — it is subjective.
Killer whales are the standard example. Orcinus orca shows big differences between populations, which are currently described as several ‘ecotypes’. Some scientists think there is more than one species of orca hiding in there. Nobody is wrong yet, because the evidence does not force a single answer.
Worked examples
WE 1
Distinguish two types of variation
Distinguish between continuous and discontinuous variation, giving one example of each. (3 marks)
Point 1: discontinuous
Characteristics fall into distinct categories with nothing in between, for example human blood group.
Point 2: continuous
Characteristics can be measured incrementally on a scale and fall anywhere between two extremes, for example height.
Point 3: the consequence
An individual cannot fall between discontinuous categories, but can take any value within a continuous range.
Separate categories versus a measurable rangegive an example for each — the definitions alone rarely reach full marks
WE 2
Explain a limitation of the biological species concept
Explain why the biological species concept cannot be applied to bacteria. (2 marks)
Point 1: the definition
The concept defines a species as organisms that can interbreed to produce fertile offspring.
Point 2: why bacteria break it
Bacteria reproduce asexually by binary fission, so they never interbreed and the test simply cannot be applied to them.
No interbreeding, so the definition has nothing to measuresay what is used instead — biochemistry or ecology — if the question allows
WE 3
Discuss where to draw the line
Two isolated populations of a bird differ in song and plumage but can still produce a few fertile offspring. Discuss whether they should be classed as separate species. (3 marks)
Point 1: the case against separating them
They can still produce fertile offspring, so under the biological species concept they remain one species.
Point 2: the case for
They are isolated with little gene flow, and differences in song and plumage may already prevent most interbreeding.
Point 3: the judgement
Speciation is gradual, so the ability to interbreed declines slowly and the decision is partly subjective; more evidence, such as DNA comparison, would help.
Probably one species for now, but the boundary is genuinely blurred“discuss” wants both sides and a stated judgement, not a single verdict
💡 Exam tips
Learn the biological species concept word for word, including fertile.
Give all three limitations: asexual reproduction, fertile hybrids, extinct species.
Use the correct terms: taxonomy, taxon, binomial, genus, species.
Get the writing conventions right — capital genus, lower-case species, italics or underlined.
Say variation comes from genetic and environmental factors together.
For speciation, mention isolation, no gene flow, and reproductive isolation.
⚠ Common mistakes
Leaving out “fertile”. A mule exists, but horses and donkeys are still separate species.
Capitalising the species name.Canis Lupus is wrong; Canis lupus is right.
Saying variation is either genetic or environmental. It is nearly always both.
Treating morphology as reliable. Similar-looking species are not always related.
Calling identical twins truly identical. Environmental differences still separate them.
Presenting species boundaries as exact. Where to draw the line is often subjective.
Up next: Chromosome Number — the counting rule that quietly decides which organisms can breed with which.
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