IB Biology HLNatural SelectionPaper 1 & 2~10 min read
Gene Pools
Individuals are temporary. Alleles are not. Zoom out from the single organism and a population looks like a shared pot of alleles that gets shuffled and dealt again every generation — and evolution is simply that pot slowly changing its contents.
📚 What you need to know
A gene pool is all the genes and their different alleles present in an interbreeding population.
The gene pool governs the genomes of the next generation.
Populations of the same species that are geographically isolated have separate gene pools.
A gene pool is stable when the population is large, mating is random, every individual has an equal chance of mating, and there are no selection pressures.
A stable gene pool means the population is not evolving.
Allele frequencies change through natural selection, sexual selection and genetic drift.
Darwin’s theory combined with genetics is called neo-Darwinism.
What a gene pool is
Definition
A gene pool consists of all the genes, and all their different alleles, present in an interbreeding population
The word interbreeding is doing a lot of work in that sentence. Two organisms only share a gene pool if their alleles can end up in the same offspring. Squirrels on opposite sides of an ocean belong to the same species but not to the same gene pool.
Thinking about all the alleles in a population at once matters because those alleles are exactly what the next generation will be built from. Nothing else is available. If an allele is missing from the pool, no offspring can inherit it, no matter how useful it would have been.
One species can have several gene pools
Some populations of the same species are geographically isolated from each other — separated by a mountain range, a stretch of sea, a motorway. Because they cannot interbreed, multiple gene pools can exist for a single species, and those pools can drift apart in composition over time.
Twelve allele copies in each pool here. In a real population it would be thousands, but the idea is identical.
When is a gene pool stable?
A population retains a stable gene pool — meaning allele frequencies stay the same from generation to generation — under four conditions.
The population is large.
Every individual has an equal chance of mating.
Matings are random.
There are no selection pressures acting on individuals because of their phenotype.
Here is the sentence that ties this whole topic together:
The link to remember
A stable gene pool means the population is not evolving. If allele frequencies are changing, the population is evolving.
Read those four conditions again and notice how unrealistic they are. Real populations are not infinitely large, mating is rarely random, and something is always acting as a selection pressure. That is the point — stability is the theoretical baseline you compare real populations against, which is exactly how the Hardy-Weinberg principle is used later in this topic.
What changes allele frequencies
Darwin came up with the theory of evolution by natural selection without any knowledge of genetics or DNA. His theory has since been developed by modern scientists who added genetics to it, and the combined version is called neo-Darwinism. That is what lets us describe evolution at the molecular level, as a change in allele frequency in a gene pool.
Three processes do the changing.
Process
What drives it
Is it random?
Natural selection
Some phenotypes survive and reproduce more because they are better adapted
No — the environment decides which alleles do well
Sexual selection
Some phenotypes are chosen as mates more often
No — the opposite sex decides
Genetic drift
Chance events change which individuals happen to breed or survive
Yes — entirely down to luck
Genetic drift in plain words
Imagine a population of twenty beetles, half green and half brown, in a habitat where colour makes no difference at all. A falling rock crushes six green beetles. Nothing about being green caused that — they were simply standing in the wrong place. The next generation is still built from whoever is left, so the frequency of the green allele drops anyway.
That is genetic drift: a change in allele frequency caused purely by chance. It matters most in small populations, where one unlucky event affects a large fraction of the pool. In a population of a million beetles, six deaths change nothing measurable.
You cannot see evolution by looking at one organism. You see it by counting alleles in a population across generations.
Careful with the word “random”. Mutation is random. Genetic drift is random. Natural selection is not. It is the one step in the whole process that is directed, because the environment consistently favours the same phenotypes.
Worked examples
WE 1
Gene pools of an isolated population
A river changes course and divides a population of insects into two groups that can no longer meet. Explain why the two groups may develop different allele frequencies. (3 marks)
Point 1: two gene pools
The groups no longer interbreed, so alleles cannot be exchanged and there are now two separate gene pools.
Point 2: different pressures
The two sides of the river may have different selection pressures, so different phenotypes survive and reproduce on each side.
Point 3: chance
Genetic drift also acts independently in each group, especially if either population is small.
No gene flow, so the pools change independentlyif the isolation lasts long enough this can lead to speciation, which is worth naming
WE 2
Deciding whether a population is evolving
The frequency of an allele in a beetle population is 0.42 in one generation and 0.41 in the next. State, with a reason, whether the population is evolving. (2 marks)
Statement
The allele frequency has changed, so by definition the population is evolving.
Reason with a caution
The change is very small, so it could be caused by genetic drift or by sampling error rather than by selection. More generations of data would be needed to identify a trend.
Any change in allele frequency is evolution — but not all change is selectionthe second mark comes from being cautious, not from being certain
WE 3
Why small populations are vulnerable
A conservation team is worried that a rare bird population has fallen to 30 individuals. Explain why a small gene pool is a problem. (3 marks)
Point 1: drift is strong
In a small population, chance events change allele frequencies quickly, so alleles can be lost from the gene pool entirely.
Point 2: less variation
A smaller gene pool contains fewer different alleles, so there is less variation for selection to act on.
Point 3: the consequence
If the environment changes, no individual may carry a useful allele, so the population cannot adapt and may become extinct.
Small pool, fast drift, low variation, poor chance of adaptingthis is why conservation programmes move individuals between reserves — to keep the gene pools mixed
💡 Exam tips
Use the word interbreeding in your gene pool definition. It is often the difference between one mark and none.
Learn the four stability conditions as a list: large, equal chance, random mating, no selection.
Say “the frequency of the allele increased”, not “the allele increased”.
Be precise about genetic drift: a change in allele frequency caused by chance, strongest in small populations.
Know the term neo-Darwinism — Darwin’s theory plus modern genetics.
If a question asks whether a population is evolving, check whether allele frequencies have changed. That is the whole test.
⚠ Common mistakes
Defining a gene pool as “all the genes in an organism”. It is all the genes and alleles in a whole interbreeding population.
Saying genetic drift is a type of natural selection. Drift is random; selection is not.
Assuming a species has one gene pool. Isolated populations have their own.
Confusing gene and allele. A gene is the instruction; an allele is one version of it.
Thinking a stable gene pool means a healthy population. It means a population that is not changing, which can be dangerous if the environment does.
Forgetting that “large population” is a condition. Small populations drift even with no selection at all.
Up next: Allele Frequencies (Skills). Now that you know what a gene pool is, the next step is counting one — working out actual frequencies from a list of genotypes, and seeing what happens when a handful of individuals start a brand new population.
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