IB Biology HLPopulations & CommunitiesPaper 1 & 2~7 min read
Populations in Ecosystems
A population is not just “a lot of one animal”. The definition has three parts, and every one of them is doing work. Get them straight now and the rest of this unit — sampling, carrying capacity, competition — sits on solid ground.
📘 What you need to know
A population is a group of organisms of the same species, living in an area, at one time.
Members of a population interact with each other and can breed together.
A population can be isolated from other populations of the same species by living in a different area.
Isolation means members of separate populations cannot breed together, so gene exchange cannot take place between them.
The IB calls this reproductive isolation. Be aware that biologists more often use that term for the point in speciation where two populations have diverged into two different species.
The three parts of the definition
Examiners write mark schemes straight from definitions like this one, so it pays to know why each phrase is there.
Gannets nesting together on a sea cliff are a textbook population — one species, one area, all present in the same breeding season.
Isolation and gene exchange
Two groups of the same species can be split up simply by living in different areas. A mountain range, a stretch of ocean, or just a very long distance is enough. Once that happens, the two groups are separate populations.
The consequence matters more than the fact. If the members of two populations cannot breed together, then gene exchange cannot take place between them. Genes arising in one population stay in that population.
Both groups are still the same species. They are two populations because they live in different areas and no longer swap genes.
Watch the wording in the syllabus here. The IB uses reproductive isolation to mean two populations of the same species that are separate and not interbreeding. Most biologists save that phrase for the moment in speciation when two populations have diverged so far that they have become two different species. Both uses make sense in context — just read the question carefully before you answer.
Worked examples
WE 1
Define a population
Define the term population. (2 marks)
Point 1: the group
A group of organisms of the same species.
Point 2: place and time
Living in the same area at the same time, so that they can interact and breed together.
Same species, same area, same time“a group of animals in a habitat” earns nothing — the species and the time are the marks
WE 2
Two islands, one species
A species of lizard lives on two islands separated by 60 km of ocean. Explain why these are described as two populations rather than one. (3 marks)
Point 1: they are isolated
The lizards on each island live in a different area, so the two groups are isolated from one another.
Point 2: they cannot breed together
Members of the two groups cannot meet and interbreed, because they cannot cross the ocean.
Point 3: the consequence
Gene exchange cannot take place between them, so each group breeds only within itself and is treated as a separate population.
Different areas, no interbreeding, no gene exchangethey are still one species — do not claim they have become two
💡 Exam tips
Learn the definition word for word: same species, same area, one time.
Add that members interact and can breed together — it is often the second mark.
For isolation questions, always finish with no gene exchange.
Isolated populations are still the same species unless the question tells you speciation has happened.
A named example (gannets nesting on a sea cliff) makes an answer concrete and costs you one sentence.
⚠ Common mistakes
Leaving out “same species”. Without it you have described a community.
Leaving out “at one time”. Organisms from different eras are not one population.
Saying isolated populations are different species. Isolation is the start of speciation, not the finish.
Confusing population with habitat. The habitat is the place; the population is the organisms.
Writing “they cannot mix” and stopping. Explain what that stops: gene exchange.
Up next: Estimating Population Size. Now that you know what a population is, the obvious question is how anyone counts one — and the honest answer is that mostly they do not count it, they estimate it.
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