Variation — differences within one species
Variation means differences between individuals of the same species — both halves are needed for the mark: it is about differences, and the comparison sits within one species, never between two. Individuals differ because no two are genetically identical (identical twins excepted), and because the environment they grow in also shapes their characteristics. Variation is then described using two contrasting patterns: continuous and discontinuous.
Continuous vs discontinuous variation
Continuous variation gives a range of phenotypes between two extremes with every value in between — body length/height, hand span, body mass. Discontinuous variation gives a limited number of phenotypes with no intermediates — the four ABO blood groups, tongue-rolling ability. Continuous traits are controlled by many genes plus the environment and are measured; discontinuous traits are controlled by a single gene, are largely environment-independent, and are counted.
Mutation — a genetic change that forms new alleles
Mutation is a genetic change — a change in the amount or structure of an organism's DNA (deoxyribonucleic acid); within a single gene, a change in its sequence of DNA bases. Mutation is the way in which new alleles are formed. An allele is one version of a gene, so when a gene mutates, a version that never existed before can appear in a population — and it may produce a version of the characteristic (a phenotype) nobody has seen before.
Drawn from real examiner reports.
Continuous ≠ discontinuous
These two terms are swapped more often than any other pair on this topic. Continuous = a smooth range between two extremes, every value in between possible (height, hand span). Discontinuous = a fixed, limited set of separate categories with no intermediates (blood group, tongue-rolling). Fix the term from the definition, not from a half-remembered example.
Flagged Jun 2023 P11 Q11
Gaps between bars vs touching bars
Read the graph before naming the term. Bars drawn with gaps between distinct labelled categories show discontinuous variation. Bars touching on a continuous numerical axis — a histogram — show continuous variation. Candidates who choose the term first and then force the chart to fit it misread the display and lose the mark.
Flagged Jun 2022 P43 Q10aiii
Two extremes, not six
The mark-scheme phrase is a range of phenotypes between two extremes — one minimum and one maximum, with every value in between. Candidates instead copied six separate readings out of the data table and offered them all as extremes. From a data set, quote only the smallest and the largest, then state that all values in between also occur.
Flagged Jun 2023 P32 Q7b
Phenotype, not genotype
Variation is described in terms of the phenotype — the observable characteristic, such as an actual height or an actual blood group. The genotype is the combination of alleles an individual carries. Answering a variation question with allele letters such as AA or Aa describes the genotype and leaves the question that was actually asked unanswered.
Flagged Nov 2022 P33 Q7
Mutation is a cause, not variation itself
Mutation and variation are not two words for one idea. A mutation is a single genetic event — a change in DNA that creates a new allele. Variation is the observed differences between individuals that result from it. Mutation is one underlying cause; variation is the outcome you can see and measure.
Not every trait responds to environment
Diet, light and nutrients change continuous characteristics such as height and body mass, because those are controlled by many genes and are environmentally sensitive. Single-gene discontinuous characteristics such as ABO blood group are not — no amount of feeding changes a blood group. Do not attach the environment point to every characteristic.
Check the graph shape first
Variation questions are usually graph-based. Look at the axis and the bars before writing: gaps between named categories means discontinuous; a continuous numerical scale with touching bars means continuous. This one check prevents the single most repeated error on this topic.
Describe a range by its two ends
For continuous data, quote the minimum and the maximum from the figures given, then add that every value in between also occurs. Naming the two extremes and the values between them is what earns the description mark — a list of individual readings does not.
Run the mutation reasoning chain
For "how could a new characteristic first appear", run the chain: a mutation changes the DNA base sequence, forming a new allele, which produces a new phenotype when expressed, which adds to the variation within the species. Never write that the organism adapted on purpose.
Compare causes, not just labels
When asked how two characteristics differ, go beyond continuous versus discontinuous. State how many genes are involved and whether the environment affects the phenotype — that comparison is normally where the discriminating mark sits.
Variation — differences between individuals of the same species.
| Feature | Continuous variation | Discontinuous variation |
|---|---|---|
| Definition | A range of phenotypes between two extremes, with every value possible in between | A limited number of distinct phenotypes with no intermediates |
| Examples | Body length/height, hand span, body mass | ABO blood groups, tongue-rolling ability |
| Cause | Usually many genes acting together, PLUS the environment | Usually a single gene; largely unaffected by the environment |
| How measured | Measured on a numerical scale | Counted into separate categories |
| Graph used | Histogram — bars touching (no gaps) | Bar chart — bars drawn with gaps between them |
Mutation — described as a genetic change: a change in the amount or structure of the deoxyribonucleic acid (DNA) of an organism. A gene mutation is a change in the sequence of DNA bases that make up a gene. Mutation is the way in which new alleles are formed — it is the ultimate source of all new genetic variety in a species.
Define variation.
State two features that distinguish continuous variation from discontinuous variation. (2 marks)
