Variation and Selection
Summary: Variation within a species can be continuous (full range, e.g. height, controlled by many genes and environment) or discontinuous (distinct categories, e.g. blood group, controlled by single genes). Variation arises from genes, environment, or both. Mutations are random changes in DNA that can create new alleles. Natural selection acts on variation — individuals with advantageous traits are more likely to survive, reproduce, and pass on their alleles (e.g. antibiotic resistance in bacteria). Selective breeding (artificial selection) is when humans choose organisms with desired traits to breed. Tags: igcse biology variation natural-selection antibiotic-resistance selective-breeding mutations Created: 2026-07-16 Last Updated: 2026-07-16
1. Continuous vs Discontinuous Variation
| Feature | Continuous Variation | Discontinuous Variation |
|---|---|---|
| Range of phenotypes | A complete gradation from one extreme to another (spectrum) | Distinct, separate categories (no intermediates) |
| Control | Usually many genes (polygenic) and influenced by environment | Usually a single gene (or few genes) — little environmental influence |
| Data type | Can be any value within a range | Only specific discrete values |
| Graph | Bell-shaped curve (normal distribution) | Bar chart |
| Examples | Height, body mass, skin colour, hand span, leaf length, milk yield | Blood group (A/B/AB/O), tongue rolling (can/cannot), ear lobe type (attached/free), sex |
| How to identify | Ask “Can it be any value?” Yes → continuous. “Are there clear categories?” Yes → discontinuous |
2. Causes of Variation
Variation between individuals of the same species can be caused by:
| Cause | Description | Examples |
|---|---|---|
| Genetic variation | Differences in DNA/genes/alleles inherited from parents | Eye colour, blood group, natural hair colour, genetic diseases |
| Environmental variation | Differences caused by the environment in which the organism lives | Language spoken, scar from injury, body mass (partly), height (partly) |
| Both genetic and environmental | Many characteristics are influenced by a combination of genes and environment | Height (genes determine potential range, nutrition determines where in that range), body mass, intelligence, skin colour (genes determine baseline, sun exposure affects tanning) |
Sources of genetic variation:
- Sexual reproduction: Meiosis (crossing over, independent assortment) and random fertilisation create new combinations of alleles
- Mutations: Random changes in DNA — the ultimate source of new alleles
3. Mutations
A mutation is a random, permanent change in the DNA base sequence of a gene or chromosome.
| Type of Mutation | Description | Possible Effects |
|---|---|---|
| Gene mutation | Change in a single gene (substitution, insertion, or deletion of bases) | May change the amino acid sequence of the protein coded by that gene → altered or non-functional protein |
| Chromosome mutation | Change in the structure or number of chromosomes (e.g. during meiosis) | Can cause significant effects. Example: Down syndrome is caused by an extra copy of chromosome 21 (trisomy 21) |
Mutation facts:
- Most mutations are neutral (no detectable effect on the phenotype)
- Some mutations are harmful (e.g. cystic fibrosis, sickle cell anaemia)
- Rarely, mutations are beneficial — providing an advantage for survival (these are the raw material for evolution by natural selection)
- The rate of mutation can be increased by mutagens: ionising radiation (X-rays, UV light, gamma rays) and certain chemicals (e.g. those in tobacco smoke)
4. Natural Selection
Natural selection is the process by which organisms better adapted to their environment tend to survive, reproduce, and pass on their advantageous alleles to the next generation. Over many generations, the frequency of advantageous alleles increases in the population.
The mechanism of natural selection (step by step):
- Variation exists within a population (caused by mutation and sexual reproduction)
- Individuals with characteristics that are better suited (better adapted) to the environment are more likely to survive
- These survivors are more likely to reproduce and pass on their advantageous alleles to their offspring
- Over many generations, the frequency of these alleles increases in the population
- The population becomes better adapted to its environment — this is evolution
Example: Antibiotic Resistance in Bacteria
Antibiotic resistance is a classic example of natural selection observable within a short time frame.
- In a population of bacteria, a few individuals have a mutation that gives them resistance to a particular antibiotic (this may happen randomly and rarely)
- When the population is exposed to the antibiotic, most (non-resistant) bacteria are killed
- The resistant bacteria survive and are not affected by the antibiotic
- These resistant bacteria reproduce rapidly (by binary fission) — there is now less competition from non-resistant bacteria
- They pass on the resistance allele to their offspring
- The new population is mostly or entirely antibiotic-resistant — the antibiotic is no longer effective
How to reduce antibiotic resistance:
- Only use antibiotics when necessary (not for viral infections like flu)
- Complete the full prescribed course (do not stop early — ensures all bacteria are killed)
- Reduce use of antibiotics in agriculture (animal feed)
- Develop new antibiotics
- Good hygiene in hospitals to prevent spread of resistant strains
5. Selective Breeding (Artificial Selection)
Selective breeding (artificial selection) is the process by which humans choose organisms with desired characteristics and breed them together. Over many generations, the desired trait becomes more pronounced.
How selective breeding works (step by step):
- Identify individuals in a population with the desired characteristic (e.g. high milk yield in cows)
- Breed only these selected individuals together
- From the offspring, again select those showing the strongest expression of the desired trait
- Repeat over many generations
- The desired characteristic becomes more extreme/pronounced in the population
Examples of selective breeding:
| Organism | Desired Characteristic | Result |
|---|---|---|
| Dairy cattle | High milk yield | Modern dairy cows produce far more milk than wild cattle |
| Beef cattle | Fast growth, large muscle mass | Beef cattle grow faster and produce more meat |
| Crops (wheat, rice) | High yield, disease resistance, drought tolerance | Modern high-yield crop varieties (Green Revolution) |
| Dogs | Various traits (size, temperament, coat type) | All modern dog breeds (from Chihuahua to Great Dane) were selectively bred from wolves |
| Horses | Speed (racehorses), strength (draught horses) | Specialised breeds for different purposes |
Problems with selective breeding:
- Reduced genetic diversity — breeding from a small number of individuals reduces the gene pool
- Inbreeding — increases the chance of harmful recessive alleles being expressed (inbreeding depression)
- Increased susceptibility to disease — if all individuals are genetically similar, one disease can wipe out the entire population
- Unintended health problems — e.g. some dog breeds have breathing difficulties due to selectively bred flat faces
Related Notes
- Inheritance — Genes, alleles, meiosis, and how variation is passed on
- Reproduction — Sexual reproduction creates genetic variation; asexual reproduction produces clones
- Diseases and Immunity — Antibiotic resistance as a consequence of natural selection
- Characteristics and Classification of Living Organisms — Classification reflects evolutionary relationships
- IGCSE-Bio-Index — Full IGCSE Biology index
Sources
- BBC Bitesize GCSE Biology — Variation / Natural selection and evolution, BBC (free educational resource)
- OpenStax Biology 2e — Chapter 18: Evolution and the Origin of Species, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Topic 18: Variation and Selection, Cambridge Assessment International Education
- CK-12 Biology for High School — Evolution chapter, CK-12 Foundation (free, CC BY-NC 3.0)
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Natural selection is about organisms ‘trying’ to adapt” | Natural selection is not purposeful — it is the result of differential survival. Organisms do not consciously adapt or develop traits because they “need” them |
| ”Individuals evolve during their lifetime” | Populations evolve, not individuals. An individual cannot change its genes during its lifetime in response to the environment (except by mutation, which is random) |
| “Antibiotic resistance develops because bacteria get ‘used to’ the antibiotic” | Resistance arises from random mutations that pre-exist in the population. The antibiotic does not cause the mutation — it selects for bacteria that already have it |
| ”Selective breeding and natural selection are the same” | In selective breeding, humans choose which organisms reproduce. In natural selection, the environment determines survival and reproduction |
| ”All mutations are harmful” | Most mutations are neutral; some are beneficial. Mutations are essential for evolution — they create the variation on which natural selection acts |
| ”Continuous and discontinuous variation are caused by the same things” | Continuous variation is usually polygenic and influenced by environment. Discontinuous variation is usually controlled by a single gene with little environmental effect |