AQA GCSE Biology: Inheritance, Variation, Evolution and Ecology Revision Guide

Inheritance, Variation and Evolution alongside Ecology make up two of the three major topics examined on AQA GCSE Biology Paper 2. Together, they account for a significant proportion of the marks available and span some of the most conceptually demanding -- and most fascinating -- content in the entire specification. From the molecular structure of DNA to the vast interconnections within ecosystems, these topics require you to think across scales, connect ideas, and apply your understanding to unfamiliar contexts.

This guide covers everything you need to know for both topics, organised in the order the specification presents them. Use it as a structured revision resource, and make sure you actively test yourself as you work through each section.

AQA GCSE Biology Exam Structure

Before diving into the content, it helps to understand exactly where these topics sit within the exam. AQA GCSE Biology (8461) is assessed through two written papers:

• Paper 1 covers Cell Biology, Organisation, Infection and Response, and Bioenergetics. It lasts 1 hour 45 minutes, is worth 100 marks, and accounts for 50% of the final grade.
• Paper 2 covers Homeostasis and Response, Inheritance, Variation and Evolution, and Ecology. It also lasts 1 hour 45 minutes, is worth 100 marks, and accounts for the remaining 50%.

Both papers include a mix of multiple-choice, structured, closed short-answer, and open-response questions. Inheritance, Variation and Evolution and Ecology together form the bulk of Paper 2 alongside Homeostasis and Response, so strong performance in these areas can make a decisive difference to your overall grade.

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Inheritance, Variation and Evolution

Sexual and Asexual Reproduction

Organisms reproduce in two fundamentally different ways. Sexual reproduction involves the fusion of gametes (sex cells) from two parents, producing offspring that are genetically different from either parent. This genetic variation is a major advantage because it increases the chances that some offspring will survive if environmental conditions change.

Asexual reproduction involves only one parent and produces offspring that are genetically identical to the parent -- clones. There is no fusion of gametes and no mixing of genetic material. The advantage of asexual reproduction is that it is faster and requires only one organism, which is useful when conditions are stable and favourable. The disadvantage is a complete lack of genetic variation, which makes an entire population vulnerable to the same disease or environmental shift.

Many organisms can reproduce both sexually and asexually. For example, strawberry plants send out runners (asexual) but also produce flowers for sexual reproduction. Fungi can reproduce using spores both sexually and asexually. Malaria parasites reproduce sexually in the mosquito but asexually in human blood cells.

Mitosis and Meiosis

Mitosis produces two genetically identical daughter cells from one parent cell. It is used for growth, repair, and asexual reproduction. Before mitosis, the cell copies its DNA so that each new cell receives a full set of chromosomes.

Meiosis is the type of cell division that produces gametes -- sperm and egg cells in animals, pollen and egg cells in plants. Meiosis is distinctive because it involves two rounds of division, resulting in four genetically different cells, each with half the normal chromosome number. In humans, this means gametes contain 23 chromosomes rather than the full 46.

Meiosis generates genetic variation through two key mechanisms. First, the chromosomes are shuffled randomly when they separate, meaning each gamete receives a different combination of maternal and paternal chromosomes. Second, crossing over occurs during the first division, where sections of chromatids are exchanged between homologous chromosome pairs, creating entirely new combinations of alleles.

DNA Structure and the Genome

DNA (deoxyribonucleic acid) is the molecule that carries the genetic instructions for all living organisms. Its structure is a double helix -- two strands wound around each other, connected by pairs of complementary bases. Each strand is made of repeating units called nucleotides, and each nucleotide consists of a sugar, a phosphate group, and one of four bases: adenine (A), thymine (T), cytosine (C), and guanine (G).

The bases pair according to strict rules: A always pairs with T, and C always pairs with G. This complementary base pairing is essential for DNA replication and for the transmission of genetic information.

A gene is a short section of DNA on a chromosome that codes for a specific protein. Different forms of the same gene are called alleles. The entire set of genetic material in an organism is called its genome. Understanding the human genome has allowed scientists to search for genes linked to diseases, develop new medicines, and trace human migration and evolution.

Genetic Inheritance

Genetic inheritance describes how alleles are passed from parents to offspring. Some alleles are dominant -- they are expressed whenever present. Others are recessive -- they are only expressed when two copies are present (one from each parent).

Key terminology you must know:

• Homozygous -- both alleles for a gene are the same (e.g. BB or bb)
• Heterozygous -- the two alleles for a gene are different (e.g. Bb)
• Genotype -- the combination of alleles an organism has
• Phenotype -- the physical characteristic that results from the genotype

Punnett squares are the standard tool for predicting the outcomes of genetic crosses. You place one parent's alleles along the top and the other's down the side, then fill in the grid to show all possible combinations. From this, you can calculate the probability of each genotype and phenotype in the offspring. For example, if both parents are heterozygous (Bb), the expected ratio of offspring is 3 dominant phenotype to 1 recessive phenotype.

Family pedigree diagrams are another way to track the inheritance of a trait across generations. You need to be able to read these diagrams and deduce the genotypes of individuals based on the phenotypes shown.

Inherited Disorders

You need to know three key inherited disorders:

• Polydactyly (extra fingers or toes) is caused by a dominant allele. Only one copy of the allele is needed for the condition to appear. An affected person can be either homozygous dominant or heterozygous.
• Cystic fibrosis is caused by a recessive allele. A person must inherit two copies of the faulty allele (one from each parent) to be affected. People with one copy are carriers -- they do not show symptoms but can pass the allele to their children.
• Sickle cell disease is also caused by a recessive allele. It affects the shape of red blood cells, making them sickle-shaped and less efficient at carrying oxygen. Carriers of one sickle cell allele have some resistance to malaria, which explains why the allele is more common in populations from regions where malaria is prevalent.

Sex Determination

In humans, biological sex is determined by the 23rd pair of chromosomes -- the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). A Punnett square shows that there is always a 50% chance of the offspring being male and a 50% chance of being female, because the mother always contributes an X chromosome while the father contributes either an X or a Y.

Variation

Variation between organisms arises from three sources:

• Genetic causes -- differences in genes inherited from parents, and mutations in DNA
• Environmental causes -- factors such as diet, climate, lifestyle, and injury
• A combination of both -- most characteristics are influenced by both genes and environment. For example, a person may have a genetic potential for a certain height, but their actual height is also influenced by nutrition during childhood.

Evolution and Natural Selection

Charles Darwin's theory of evolution by natural selection is one of the most important ideas in biology. It can be summarised in a few key steps: individuals within a population show variation; some individuals have characteristics better suited to the environment; these individuals are more likely to survive and reproduce; they pass on the advantageous alleles to their offspring; over many generations, the frequency of the beneficial alleles increases in the population.

Alfred Russel Wallace independently developed a very similar theory at the same time as Darwin. Wallace's work on biogeography -- studying how species are distributed across different regions -- provided strong supporting evidence. Wallace also contributed to the theory of speciation through natural selection and is credited alongside Darwin for the initial presentation of the theory.

Evidence for evolution comes from several sources:

• Fossils show how organisms have changed over time, though the fossil record is incomplete because many organisms did not fossilise, soft tissue decays, and geological activity destroys some fossils.
• Antibiotic resistance in bacteria is a modern, observable example of natural selection. When a population of bacteria is exposed to an antibiotic, most die, but any individuals with a mutation that gives resistance survive and reproduce rapidly, passing on the resistance gene.
• Rapid changes in species -- such as changes in beak size in finch populations in response to food availability -- provide further direct evidence of natural selection in action.

Selective Breeding

Selective breeding (artificial selection) involves humans choosing organisms with desirable characteristics and breeding them together over many generations. This is used to produce cattle with higher milk yields, crops with greater disease resistance, dogs with particular temperaments, and plants with larger flowers.

The process involves selecting the individuals with the most desirable traits, breeding them together, selecting the best offspring from the next generation, and repeating over many generations. The main risk is that selective breeding reduces the gene pool, which can lead to health problems (such as hip dysplasia in certain dog breeds) and makes the population more vulnerable to new diseases.

Genetic Engineering

Genetic engineering is the process of modifying an organism's genome by introducing a gene from a different organism. The basic steps are:

1. The desired gene is identified and cut out of the donor organism's DNA using restriction enzymes.
2. The gene is inserted into a vector, usually a bacterial plasmid (a small ring of DNA).
3. DNA ligase is used to join the gene into the plasmid.
4. The vector is introduced into the target organism's cells, where the new gene is expressed.

Examples include genetically modified (GM) crops that are resistant to pests or herbicides, and the production of human insulin by genetically engineered bacteria -- a breakthrough for treating diabetes.

The benefits of genetic engineering include improved crop yields, the ability to produce medicines cheaply and in large quantities, and the potential to treat genetic disorders. The risks and concerns include unknown long-term effects on health and the environment, the possibility of genes transferring to wild populations, and ethical objections to modifying living organisms.

Cloning

Plant cloning can be done through simple cuttings -- taking a section of stem, placing it in soil, and allowing it to grow roots. A more advanced method is tissue culture, which involves taking a small group of cells from a plant, placing them in a growth medium with nutrients and hormones, and growing them into new identical plants. Tissue culture is used commercially to produce large numbers of genetically identical plants quickly.

Animal cloning is more complex. Adult cell cloning -- the method used to produce Dolly the Sheep -- involves taking the nucleus from an adult body cell, inserting it into an egg cell that has had its nucleus removed, stimulating the egg to divide with an electric shock, and then implanting the resulting embryo into a surrogate mother.

Advantages of cloning include the ability to mass-produce organisms with desirable characteristics and to preserve endangered species. Disadvantages include reduced genetic variation within cloned populations, a high failure rate in animal cloning, and concerns about the long-term health of cloned animals.

Classification

Living organisms are classified into groups based on their characteristics. Carl Linnaeus developed the traditional classification system, which organises organisms into a hierarchy: Kingdom, Phylum, Class, Order, Family, Genus, Species. This system is based on observable structural similarities.

More recently, Carl Woese proposed the three-domain system, which uses evidence from chemical analysis, particularly RNA sequences, to classify organisms into three broad domains:

• Archaea -- primitive bacteria often found in extreme environments
• Bacteria -- true bacteria
• Eukarya -- organisms whose cells contain a nucleus, including all animals, plants, fungi, and protists

The three-domain system is considered more scientifically accurate because it reflects evolutionary relationships revealed by genetic analysis rather than just physical appearance.

Extinction

A species becomes extinct when there are no remaining individuals. Causes of extinction include new predators, new diseases, increased competition, catastrophic events (such as volcanic eruptions or asteroid impacts), and environmental changes to which the species cannot adapt quickly enough. Human activity -- including habitat destruction, pollution, and hunting -- has dramatically accelerated extinction rates in recent centuries.

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Ecology

Ecosystems and Interdependence

An ecosystem is the interaction of a community of living organisms (biotic factors) with the non-living components (abiotic factors) of their environment. Key terms to understand:

• Population -- all the organisms of one species in a habitat
• Community -- all the populations of different species in a habitat
• Habitat -- the place where an organism lives
• Interdependence -- the reliance of every species on other species in the community for food, shelter, pollination, seed dispersal, and other needs

Competition occurs when organisms need the same limited resource. Interspecific competition is competition between different species, while intraspecific competition is competition between individuals of the same species. Intraspecific competition is often more intense because the organisms have identical resource needs.

Abiotic factors affecting ecosystems include light intensity, temperature, moisture levels, soil pH, wind speed and direction, carbon dioxide concentration, and oxygen levels in water. Biotic factors include food availability, new predators, new pathogens, and competition from other species.

Adaptations

Organisms have features that help them survive in their environment. These adaptations fall into three categories:

• Structural adaptations -- physical features such as thick fur in arctic animals, large surface-area-to-volume ratios in desert animals for heat loss, or thorns on plants
• Behavioural adaptations -- actions such as migration, hibernation, or courtship displays
• Functional adaptations -- biochemical processes such as producing venom, producing antifreeze proteins, or the ability of some bacteria to break down unusual substrates

Extremophiles are organisms adapted to survive in extreme conditions, such as high temperatures (near volcanic vents), high salt concentrations, or high pressure (deep ocean). Many extremophiles are archaea.

Levels of Organisation in an Ecosystem

Ecology can be studied at multiple levels: individual organisms, populations, communities, and entire ecosystems. Each level reveals different patterns and interactions. Understanding these levels helps explain how a change at one level (such as the loss of a predator) can cascade through the entire system.

Food Chains and Food Webs

Energy flows through ecosystems via feeding relationships. A food chain shows a single pathway of energy transfer:

Producer --> Primary consumer --> Secondary consumer --> Tertiary consumer --> Apex predator

• Producers (usually plants or algae) photosynthesise to convert light energy into chemical energy in biomass
• Primary consumers are herbivores that eat producers
• Secondary consumers eat primary consumers
• Tertiary consumers eat secondary consumers
• Apex predators sit at the top of the food chain with no natural predators
• Decomposers (bacteria and fungi) break down dead organisms and waste material, recycling nutrients back into the soil

A food web is a network of interconnected food chains within a community. Food webs provide a more realistic picture of feeding relationships and help explain why the removal of one species can affect many others.

Measuring Population Size -- Required Practical

You need to know how to estimate the population size of a species in a habitat using quadrats and transects:

• A quadrat is a square frame placed randomly in the study area. You count the organisms inside each quadrat, repeat in multiple random locations, and calculate the mean number per quadrat. You can then scale this up to estimate the total population in the whole area.
• A transect is a line laid across the habitat. Quadrats are placed at regular intervals along the transect to study how the distribution of organisms changes across the habitat -- for example, from a pond edge into grassland.

The capture-recapture method is used for mobile animals. A sample is caught, counted, marked, and released. After a period of time, a second sample is caught. The number of marked individuals in the second sample is used to estimate the total population with this formula:

Total population = (number in first sample x number in second sample) / number of marked individuals recaptured

This method assumes that the marked organisms mix evenly back into the population, that marking does not affect survival, and that there is no significant immigration, emigration, birth, or death between samples.

Cycling of Materials

Materials are constantly recycled through ecosystems:

The carbon cycle describes how carbon moves between the atmosphere, living organisms, and the Earth. Plants absorb carbon dioxide during photosynthesis and incorporate carbon into their biomass. Animals obtain carbon by eating plants or other animals. Both plants and animals release carbon dioxide back into the atmosphere through respiration. When organisms die, decomposers break down the organic material, releasing carbon dioxide through their own respiration. Carbon is also released by the combustion of fossil fuels, which were formed from organisms that died millions of years ago.

The water cycle involves evaporation of water from seas, lakes, and rivers; transpiration from plants; condensation into clouds; and precipitation back to the ground, where it may be absorbed by plants, flow into waterways, or percolate into underground reserves.

Decomposition is the breakdown of dead organic matter by microorganisms. The rate of decomposition is affected by temperature, moisture, oxygen availability, and the number of decomposers. Compost bins exploit these factors to speed up decomposition for garden use.

Biodiversity

Biodiversity is the variety of all the different species of organisms on Earth, or within a particular ecosystem. High biodiversity makes ecosystems more resilient to change and is essential for the stability of food webs, the cycling of nutrients, and the provision of resources that humans depend upon.

Human activities have significantly reduced biodiversity in many regions:

• Deforestation -- clearing large areas of forest destroys habitats, reduces the number of species, and releases stored carbon dioxide
• Pollution -- contamination of air, water, and land harms organisms and can make habitats uninhabitable
• Global warming -- rising temperatures caused by increased greenhouse gas emissions alter habitats and disrupt ecosystems
• Land use -- converting natural habitats into farmland, housing, or industrial areas reduces the space available for wildlife

Global Warming

The enhanced greenhouse effect is caused by increased concentrations of greenhouse gases -- primarily carbon dioxide and methane -- in the atmosphere. These gases trap heat that would otherwise escape into space, leading to a rise in global temperatures.

The effects of global warming on ecosystems include:

• Changes in the distribution of species as habitats shift towards the poles or to higher altitudes
• Loss of habitats such as coral reefs due to ocean warming and acidification
• Altered migration patterns and breeding seasons
• Increased frequency of extreme weather events
• Rising sea levels threatening coastal ecosystems and human communities

Maintaining Biodiversity

There are many strategies for maintaining biodiversity:

• Breeding programmes for endangered species in zoos and wildlife reserves, with the aim of reintroducing animals into the wild
• Seed banks that store seeds from a wide variety of plant species, protecting genetic diversity against extinction
• Hedgerow replanting and the creation of wildlife corridors to reconnect fragmented habitats
• Reduction of deforestation through international agreements, sustainable forestry, and the protection of rainforests
• Recycling and reducing waste to lower demand for raw materials and decrease habitat destruction

Conservation programmes require cooperation between governments, scientists, and local communities to be effective. Understanding the science behind these efforts is not only essential for your exam -- it is one of the most important applications of biology in the modern world.

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Prepare with LearningBro

Strengthen your revision for these Paper 2 topics with focused practice questions and instant feedback:

• AQA GCSE Biology: Inheritance, Variation and Evolution
• AQA GCSE Biology: Ecology

Active recall is the most effective way to move knowledge from short-term to long-term memory. Use these courses to test yourself on every key concept covered in this guide, identify your weak areas, and build the confidence you need for exam day.