IB ESS HL3.1 Biodiversity & EvolutionHL only~11 min read
Genetic Diversity Within Species
Natural selection can only act on variation that already exists. This page is about where that variation comes from — and why a species with too little of it, like the cheetah, is in trouble long before anything else goes wrong.
📘 What you need to know
DNA carries genetic information. Genes are sequences of DNA coding for particular traits or proteins.
Chromosomes are long strands of tightly coiled DNA. Humans have 46, in 23 pairs.
The genome is the complete set of genetic material in an organism.
Genetic diversity is the variation in genes between individuals within a species.
It arises through mutation (random changes creating new alleles) and sexual reproduction (genetic recombination).
Most mutations are neutral; some are beneficial and some harmful.
High genetic diversity aids survival and adaptation and reduces the risk of inbreeding.
The building blocks
Each pair of human chromosomes carries genes inherited from both parents, which is where the mixing on the next diagram comes from.
Genetic diversity refers to the variation in genes between individuals within a species. It is critical for the survival and adaptation of populations in changing environments.
How genetic diversity arises
This is the distinction examiners look for: only mutation creates alleles that did not previously exist anywhere in the population.
Mutation
Mutations are random changes in the DNA sequence. They occur naturally during cell division, or from environmental factors such as radiation.
They create new versions of genes known as alleles, introducing new traits into a population — resistance to a disease, or the ability to digest a new food source.
Most mutations are neutral, with no effect on the organism. Some are beneficial (increasing survival chances) and some are harmful (leading to genetic disorders).
Named example: in some human populations, a mutation in the CCR5 gene provides greater resistance to HIV infection.
Sexual reproduction
Sexual reproduction combines genetic material from two parents. An individual’s gametes are genetically distinct from each other, so which gametes fuse at fertilisation determines the offspring’s unique set of genes.
Genetic recombination — the mixing of DNA from both parents — produces new combinations of alleles.
Named example: human siblings, other than identical twins, carry different combinations of their parents’ genes, so genetic variation exists even within one family.
Why genetic diversity matters
What happens
Example
Survival and adaptation
High genetic diversity gives populations a greater ability to adapt to change. If a new disease, predator, food source or climate shift arrives, some individuals may carry the traits needed to survive
Some corals carry genes coding for proteins that help them tolerate higher temperatures, so they survive warming that bleaches and kills corals without them
Agricultural resilience
Maintaining genetic diversity in crops keeps them resilient to pests and diseases
Crop varieties with diverse genetics resist a new pest that would destroy a uniform planting
Reduced inbreeding risk
In populations with low genetic diversity, individuals are more likely to inherit harmful mutations through inbreeding — mating between closely related individuals — leading to genetic disorders or reduced survival
Cheetahs have low genetic diversity, making them more vulnerable to disease and reproductive problems
Notice how this page underpins everything in 3.1. Natural selection can only act on variation that already exists in a population — it cannot invent a useful trait on demand. That is why a species with low genetic diversity, like the cheetah, is fragile even when its numbers look adequate: it has fewer options if conditions change. Genetic diversity is the third and least visible level of biodiversity from the first note in this sub-topic.
Worked examples
WE 1
Sources of genetic diversity
Explain two ways in which genetic diversity arises within a species. (4 marks)
Source 1: mutation
Mutations are random changes in the DNA sequence, occurring during cell division or from factors such as radiation.
What mutation does
It creates new versions of genes, called alleles, introducing traits that did not previously exist in the population — such as the CCR5 mutation giving resistance to HIV.
Source 2: sexual reproduction
Sexual reproduction combines genetic material from two parents. Because an individual’s gametes are genetically distinct, which gametes fuse determines a unique set of genes in the offspring.
What recombination does
Genetic recombination produces new combinations of existing alleles, which is why siblings differ from one another.
Mutation creates new alleles; reproduction creates new combinations of themthe distinction between creating and recombining is the marking point
WE 2
Diversity and survival
Explain why a population with high genetic diversity is more likely to survive environmental change. (3 marks)
Step 1: the variation exists first
High genetic diversity means individuals carry many different forms of each gene, so the population contains a wide range of traits.
Step 2: change creates a new pressure
If a new disease, predator, food source or climate shift occurs, some individuals may already carry the right genetic traits to survive.
Step 3: the example
Some corals carry genes for heat-tolerance proteins, so they survive rising ocean temperatures while corals lacking those genes bleach and die.
The population survives because the useful variation was already therestress that the variation pre-exists the change; nothing adapts on demand
WE 3
Low diversity
Suggest why cheetahs are considered particularly vulnerable despite conservation efforts. (3 marks)
Step 1: the problem
Cheetahs have low genetic diversity, so individuals across the population are genetically very similar.
Step 2: inbreeding
Individuals are more likely to inherit harmful mutations through inbreeding, which can cause genetic disorders and reduced survival rates.
Step 3: the consequence
This makes them more vulnerable to disease and to reproductive problems, and leaves the population with little capacity to adapt if conditions change.
Increasing numbers does not, by itself, restore lost genetic diversitythat final point is a strong evaluation line for conservation questions
💡 Exam tips
Keep DNA, gene, chromosome and genome straight. They are examined as separate terms.
Know the human figures: 46 chromosomes in 23 pairs.
Use the word allele for a new version of a gene created by mutation.
Say most mutations are neutral. Students often assume all are harmful.
Have named examples ready: CCR5, heat-tolerant corals, cheetahs.
Define inbreeding as mating between closely related individuals.
⚠ Common mistakes
Saying mutations are always harmful. Most are neutral; some are beneficial.
Claiming sexual reproduction creates new alleles. It recombines existing ones.
Confusing gene with allele. A gene codes for a trait; an allele is one version of that gene.
Saying organisms mutate in response to a need. Mutations are random and undirected.
Treating a large population as automatically diverse. Cheetah numbers are not the issue; their genetics are.
Forgetting the agricultural angle. Crop genetic diversity is named in the syllabus.
Up next: Reproductive Isolation and Speciation. You know how variation builds up within a population. The next question is what stops two populations exchanging it — because that is how one species becomes two.
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