18.1 Variation
- Syllabus
- 0610–2026–2027
- Topic
- 18.1
- Level
- —
Learning objectives
18.1.1Variation as differences between• Describe variation as differences between individuals of the same species18.1.2Continuous variation results• State that continuous variation results in a range of phenotypes between two extremes; examples include body length and body mass18.1.3Discontinuous variation results• State that discontinuous variation results in a limited number of phenotypes with no intermediates; examples include ABO blood groups, seed shape in peas and seed colour in peas18.1.4Discontinuous variation is usually• State that discontinuous variation is usually caused by genes only and continuous variation is caused by both genes and the environment18.1.5Examples of continuous• Investigate and describe examples of continuous and discontinuous variation18.1.6Mutation as genetic change• Describe mutation as genetic change18.1.7Mutation is the way in which new• State that mutation is the way in which new alleles are formed18.1.8Ionising radiation and some chemicals• State that ionising radiation and some chemicals increase the rate of mutation18.1.9Gene mutation as a random change• Describe gene mutation as a random change in the base sequence of DNA18.1.10Mutation, meiosis, random mating• State that mutation, meiosis, random mating and random fertilisation are sources of genetic variation in populations
Define variation within a species
Variation means differences between individuals of the same species.
Individuals can differ in measured features such as body mass or in categories such as blood group. These differences may arise from genes, the environment, or both.
Differences between two different species are not variation within a species; the comparison must be among members of the same species.
Recognise continuous variation
Continuous variation produces a range of phenotypes between two extremes, with many possible intermediate values.
Body length and body mass are continuous: measurements can take many values, such as masses throughout a range rather than only a few fixed categories.
When many individuals are measured, adjacent measurement intervals form a continuous distribution. The extremes describe the lowest and highest observed values.
A measurement being rounded into groups does not make the underlying variation discontinuous; intermediate values still exist.
Recognise discontinuous variation
Discontinuous variation produces a limited number of distinct phenotypes with no intermediates between the categories.
| Characteristic | Separate phenotypes |
|---|---|
| ABO blood group | A, B, AB or O |
| pea seed shape | round or wrinkled |
| pea seed colour | distinct colour categories |
Different frequencies do not create intermediates: even if one blood group is much more common, every individual still belongs to one category.
Link variation type to genes and environment
| Variation type | Usual cause | Why |
|---|---|---|
| discontinuous | genes only | allele combinations place individuals into distinct categories |
| continuous | genes and environment | many genetic and environmental influences contribute to a measured range |
ABO blood group is genetically determined and discontinuous. Body mass varies continuously because inherited features and environmental factors such as nutrition can both affect it.
These are usual patterns: do not claim that the environment changes an individual's ABO blood-group category or that continuous variation is environmental only.
Investigate and describe variation
- Choose one clearly defined feature and a representative sample of the same species.
- Measure continuous features consistently, or record discontinuous categories using an unambiguous key.
- Tally every individual and present frequencies.
- Describe the pattern using the range and distribution for continuous data, or the number and frequency of categories for discontinuous data.
Use adjacent measurement intervals for a continuous distribution; use separate bars for discontinuous categories. State units for measurements and keep sampling and measurement methods consistent.
A graph alone does not establish cause. It describes the observed variation; genetic and environmental explanations require additional evidence or controlled comparison.
Define mutation as genetic change
A mutation is a genetic change.
Because genetic information is carried in DNA, a mutation changes genetic material and can create a difference that was not present in the original DNA.
A change in appearance caused only by the environment is not automatically a mutation; the change must be genetic.
Explain how mutation forms new alleles
Mutation is the way new alleles are formed. When the genetic information in a gene changes, the changed version is a new allele of that gene.
mutation in a gene → changed genetic information → new allele → possible new inherited variation
Selection does not create the new allele needed by an organism. Mutation creates alleles; selection can later change their frequencies in a population.
Identify factors that increase mutation rate
Ionising radiation and some chemicals increase the rate of mutation.
They increase the probability that genetic material will change, so mutations occur more frequently in an exposed group than they otherwise would.
They increase mutation rate but do not direct a specific useful mutation, and not every exposed cell necessarily mutates.
Define a gene mutation precisely
A gene mutation is a random change in the base sequence of DNA.
Changing the order of bases can create a new allele. If that gene codes for a protein, the changed base sequence may alter the amino-acid sequence and therefore the protein's shape or function.
Random means the change does not occur because the organism needs it. A gene mutation changes the DNA base sequence, not an amino-acid sequence inside DNA.
Identify four sources of genetic variation
| Source | Contribution to genetic variation |
|---|---|
| mutation | creates new alleles |
| meiosis | produces genetically different gametes |
| random mating | combines alleles from different parents |
| random fertilisation | one of many possible sperm and eggs fuse |
Together these processes create and reshuffle allele combinations, so individuals in a population can have different genotypes.
Mitosis and asexual reproduction normally produce genetically identical cells or offspring, so they do not reshuffle alleles in the same way.