Continuous and discontinuous variationOxford AQA IGCSE Biology: Revision notes
Section 1
What is variation and how does it arise?
Variation refers to differences between individuals of the same species. These differences can be caused by:
- Genetic factors – differences in DNA inherited from parents
- Environmental factors – differences caused by external conditions (e.g. diet, temperature, sunlight)
- Both genetic and environmental – most traits result from a combination of both (e.g. human height is influenced by genes and nutrition)
For example, two plants of the same species may differ in height due to different soil nutrients (environmental), or humans may have different eye colours due to inherited genes (genetic).
Examiners expect you to distinguish between the causes of variation clearly. Always name the factor causing the difference, not just describe the difference itself. For example, say 'different alleles' for genetic variation, not just 'they look different'.
Section 2
How do continuous and discontinuous variation differ?
Variation can be classified into two types based on how traits are distributed in a population:
| Feature | Continuous Variation | Discontinuous Variation |
|---|---|---|
| Pattern | Forms a smooth range of values | Forms distinct categories with no in-between |
| Inheritance | Controlled by many genes (polygenic) | Controlled by one or few genes |
| Environmental influence | Strongly affected by environment | Little or no environmental influence |
| Examples | Height, skin colour, mass, intelligence | ABO blood groups, pea plant seed colour, ability to roll tongue |
| Graph shape | Bell curve (normal distribution) | Bar chart with separate bars |
Key distinction: Continuous traits show a gradual spectrum of variation (e.g. heights range from 140 cm to 190 cm), whilst discontinuous traits fall into separate categories (e.g. blood type A, B, AB, or O).
Students often confuse 'continuous' with 'genetic' and 'discontinuous' with 'environmental'. This is wrong. Continuous traits can be affected by environment (e.g. malnutrition reduces height), whilst some discontinuous traits are purely genetic (e.g. blood type).
If asked 'Is human height continuous or discontinuous variation?', answer: 'Continuous, because it is controlled by many genes (polygenic inheritance) and shows a range of values. It is also influenced by environmental factors such as diet.'
Section 3
How does natural selection drive evolution?
Natural selection is the mechanism by which evolution occurs. It works through a four-step process:
- Variation exists – individuals within a population have different alleles and traits due to genetic variation
- Selection pressure acts – environmental factors (e.g. predators, disease, food scarcity, climate) create a challenge for survival
- Survival of the fittest – individuals with traits best suited to the environment are more likely to survive and reproduce (this is differential reproduction)
- Inheritance of advantageous traits – survivors pass on their beneficial alleles to offspring, increasing the frequency of these alleles in the population
- Evolution occurs – over many generations, the frequency of advantageous alleles increases and disadvantageous alleles decrease, causing the population to evolve
Example: In a population of moths, if predators hunt moths visually, darker moths are better camouaged. More dark moths survive to breed, so the next generation contains more dark alleles. Over many generations, the population becomes predominantly dark.
Key point: Natural selection is non-random (selection happens based on fitness) and requires heritable variation (traits must be genetic to be inherited).
Examiners mark 'the four-step process' explicitly. Always structure your answer as: variation exists → selection pressure → some individuals survive better → advantageous alleles become more frequent. This shows full understanding of the mechanism.
Think of natural selection like a filter: variation is the range of objects trying to pass through, selection pressure is the filter, and only the objects that fit through (fittest) survive to reproduce. Each generation, more 'fit' objects pass through, changing the overall population.
Section 4
What evidence supports the theory of evolution?
Several independent lines of evidence demonstrate that evolution has occurred:
The fossil record
- Fossils show that organisms have changed over time
- Transitional fossils show intermediate forms between species (e.g. Archaeopteryx between dinosaurs and birds)
- Fossils in deeper rock layers are generally older and show simpler organisms; younger fossils show more complex organisms
- Fossil sequences show gradual changes in species over millions of years
Antibiotic resistance in bacteria
- Most rapid observable example of evolution
- Bacteria show genetic variation in antibiotic resistance
- Selection pressure: antibiotics kill non-resistant bacteria
- Resistant bacteria survive and reproduce, increasing resistance alleles in the population
- Over just a few generations, the bacterial population becomes resistant
- This demonstrates natural selection in real-time
Selective breeding
- Humans have deliberately bred organisms with desired traits (e.g. larger dogs, sweeter apples)
- Demonstrates that heritable variation and selection can rapidly change a population
- Artificial selection shows the same principle as natural selection, but human-directed
Homologous structures (additional evidence)
- Similar bone structures in different species (e.g. human arm, bat wing, whale flipper) suggest common ancestry
- Organs adapted for different purposes but with similar underlying structure
When answering about antibiotic resistance, always explain the four-step process of natural selection: variation exists → selection pressure from antibiotics → resistant bacteria survive → alleles for resistance increase. Name it as 'observable/rapid evolution' to show understanding.
If asked 'How does antibiotic resistance provide evidence for evolution?', answer: 'Bacteria have genetic variation in antibiotic resistance. Antibiotics are a selection pressure that kills non-resistant bacteria. Resistant bacteria survive and reproduce, passing resistance alleles to offspring. The frequency of resistance alleles increases rapidly in the population, showing evolution by natural selection.'
Section 5
What was Darwin's theory and how was it received?
Darwin's theory of evolution by natural selection (published 1859 in 'On the Origin of Species') proposed that:
- All species share common ancestors
- Species evolve through natural selection acting on variation
- Over long periods, natural selection creates new species
- This explained both the diversity and the similarities of organisms
Reception of Darwin's theory:
| Factor | Details |
|---|---|
| Scientific support | Initially controversial; gradually accepted as evidence accumulated (fossil record, genetics) |
| Religious opposition | Conflicted with religious beliefs about creation; many religious groups opposed it |
| Initial acceptance | Some scientists accepted it quickly; others were sceptical due to lack of explanation for variation |
| Modern acceptance | Now the foundation of modern biology; supported by genetics, DNA evidence, and observed examples |
| Key missing piece | Darwin had no mechanism to explain how variation arose or was inherited; Mendel's laws of inheritance (discovered later) provided this explanation |
Why was it controversial?
- Challenged the belief that God created all species separately
- Suggested humans evolved from other animals
- Implied no special creation or purpose
- Took time to gather enough evidence to convince skeptics
Examiners often ask why the theory was not immediately accepted. Key answer: lack of genetic mechanism (how variation arose and was inherited) and religious opposition. Mention that Mendel's work later provided the missing explanation.
Section 6
How do genetic engineering, selective breeding, and cloning differ? (Higher Tier)
Selective breeding
- Method: Humans choose organisms with desirable traits and breed them together over many generations
- Mechanism: Uses natural genetic variation; no genetic material is modified
- Speed: Slow; takes many generations to achieve desired traits
- Examples: Larger cattle, dogs with specific features, sweeter fruits
- Limitation: Cannot introduce traits not already in the gene pool
Genetic engineering
- Method: Direct manipulation of an organism's DNA by inserting, deleting, or modifying genes
- Process:
- Restriction enzymes cut DNA at specific sequences
- The desired gene is inserted into a vector (plasmid or virus)
- DNA ligase seals the new DNA into the vector
- The vector carries the gene into a host cell (the transformed organism)
- Host cell replicates, expressing the new trait
- Speed: Very rapid; can achieve desired traits in one generation
- Examples: Insulin-producing bacteria, herbicide-resistant crops, disease-resistant plants
- Advantages: Traits from other species can be introduced; precise; faster than selective breeding
- Risks: Unknown long-term effects, potential for allergic reactions, environmental impact if modified organisms escape, ethical concerns
Cloning
- Method: Creating genetically identical copies of an organism
- Embryo splitting: Dividing an embryo into separate parts; each develops into an identical organism
- Nuclear transfer (somatic cell nuclear transfer): Nucleus from a body cell is placed into an enucleated egg cell; the egg develops into an organism genetically identical to the nucleus donor
- Speed: Moderate; can produce organisms quickly
- Examples: Dolly the sheep (nuclear transfer), identical twins from embryo splitting
- Advantage: Produces organisms with desired traits guaranteed
- Risks: Reduced genetic variation in a population; potential health problems in cloned organisms; ethical concerns
Comparison table:
| Feature | Selective Breeding | Genetic Engineering | Cloning |
|---|---|---|---|
| Speed | Slow (many generations) | Very fast (1 generation) | Moderate |
| Genetic modification | No; uses natural variation | Yes; DNA directly altered | No; identical copy |
| New traits from | Within species gene pool | Other species or designed sequences | Exact replica of parent |
| Main limitation | Limited to existing variation | Potential unknown effects | Lack of genetic diversity |
Benefits of genetic engineering:
- Agriculture: Crops resistant to pests, disease, or drought; increased yield; reduced pesticide use
- Medicine: Production of medicines (insulin, growth hormone); gene therapy for genetic diseases; disease-resistant crops reduce malnutrition
Risks of genetic engineering:
- Agriculture: Unknown effects on non-target organisms; gene flow to wild relatives; reduced biodiversity; antibiotic-resistant marker genes
- Medicine: Off-target gene editing effects; unknown long-term consequences; ethical issues; potential for misuse
- General: Environmental impact if modified organisms escape into nature; public concern about 'unnatural' organisms
Examiners ask about genetic engineering process steps frequently. Remember the order: restriction enzyme cuts → insert into vector → DNA ligase seals → vector enters cell → cell replicates the new gene. Always name the enzymes and their roles.
Question: 'Explain how genetic engineering could produce insulin-producing bacteria.' Answer: 'A restriction enzyme cuts the human insulin gene and bacterial plasmid DNA. The insulin gene is inserted into the plasmid. DNA ligase seals the plasmid, creating recombinant DNA. The plasmid is taken up by bacterial cells (transformation). The bacteria replicate, expressing the insulin gene and producing insulin.'
Students often confuse genetic engineering with selective breeding. Remember: genetic engineering is direct DNA modification (fast, precise); selective breeding is choosing organisms to breed (slow, uses natural variation). Cloning produces identical copies with no genetic modification at all.
Section 7
What is speciation and how does it occur? (Higher Tier)
Speciation is the evolutionary process by which new species develop from existing species.
Mechanism of speciation:
Step 1: Reproductive isolation
- A population becomes separated from other populations of the same species
- Geographic isolation is common: physical barriers (mountains, rivers, distance) prevent gene flow between groups
- After isolation, the separated populations cannot interbreed
Step 2: Different selection pressures
- Each isolated population experiences different environmental conditions
- Different selection pressures act on each population
- Natural selection favours different traits in each population
Step 3: Genetic divergence
- Over many generations, each population accumulates different alleles
- Allele frequencies change differently in each population
- The populations evolve along different paths
Step 4: Reproductive incompatibility
- After sufficient genetic divergence, the populations can no longer produce fertile offspring even if they reunite
- This reproductive isolation indicates they are now separate species
- Key point: A new species is defined as a population that cannot interbreed with other populations to produce fertile offspring
Example: Darwin's finches in the Galápagos Islands evolved from a common ancestor. Different islands had different food sources (seeds of different sizes). Finches on islands with large seeds evolved larger beaks; those on islands with small seeds evolved smaller beaks. After many generations, they could no longer interbreed and became separate species.
Why populations must be reproductively isolated:
- Without isolation, gene flow between populations would homogenise allele frequencies
- Isolated populations can accumulate different mutations and experience different selection
- Only through reproductive isolation can enough genetic difference accumulate for speciation to occur
Examiners expect you to explain speciation as a process over time, not an instant event. Always mention: geographic isolation → different selection pressures → genetic divergence → reproductive isolation develops → new species. This shows understanding of the mechanism.
Question: 'Explain how two populations of the same species can become separate species.' Answer: 'Geographic barriers isolate populations, preventing gene flow. Different selection pressures act on each isolated population. Over many generations, the populations accumulate different alleles. Eventually, they diverge so much genetically that they can no longer produce fertile offspring even if reunited. At this point, they are separate species.'
Must Know
- Variation is differences between individuals; it can be caused by genetic factors, environmental factors, or both
- Continuous variation shows a range of values (e.g. height; controlled by many genes); discontinuous variation shows distinct categories (e.g. blood type; controlled by few genes)
- Natural selection works through: variation exists → selection pressure → survival of fittest → advantageous alleles increase → evolution occurs over generations
- Evidence for evolution: fossil record (shows change over time and transitional forms), antibiotic resistance in bacteria (rapid, observable evolution), selective breeding (shows heritable variation can change populations), homologous structures (indicate common ancestry)
- Selective breeding: humans choose organisms with desirable traits and breed them; slow, uses natural variation only
- Genetic engineering: direct DNA modification using restriction enzymes (cut DNA), ligase (seal DNA), vectors (carry genes in), produces transformed organisms; very fast, can introduce new traits
- Cloning: produces genetically identical organisms via embryo splitting or nuclear transfer; no genetic modification, just exact copies
- Speciation: occurs when reproductive isolation separates populations → different selection pressures → genetic divergence → eventually cannot interbreed → new species; requires geographic isolation to prevent gene flow
- Higher Tier: Understand restriction enzyme and ligase roles; evaluate benefits (medicine production, disease resistance) and risks (unknown effects, ethical concerns) of genetic engineering
That's the notes covered.
Carry on to the next subtopic.