Variation and Selection

The One Thing

Inherited variation means that some individuals are better suited to particular environmental conditions than others. Natural selection increases the frequency of advantageous inherited characteristics over successive generations, producing adaptations and causing populations to evolve.

Reactions, Processes and Experiments

Key Terms

• Variation: Differences in characteristics between individuals of the same species.
• Genetic variation: Variation caused by differences in genes or alleles, inherited from parents.
• Environmental variation: Variation caused by conditions such as diet, climate, exercise, disease, or lifestyle.
• Mutation: A random change in genetic material that can produce a new allele.
• Continuous variation: Variation with a range of intermediate values, such as height or body mass, usually influenced by several genes and the environment.
• Discontinuous variation: Variation that falls into distinct categories, often controlled by one or a few genes, such as blood group.
• Gene: A length of DNA that codes for a protein or influences a characteristic.
• Allele: An alternative form of a gene.
• Natural selection: The process in which individuals with advantageous inherited characteristics are more likely to survive and reproduce.
• Selection pressure: An environmental factor, such as predation, competition, disease, or climate, that affects survival and reproduction.
• Adaptation: An inherited feature that makes an organism better suited to its environment.
• Evolution: The change in the inherited characteristics of a population over successive generations.
• Population: All the organisms of one species living in the same area at the same time.
• Species: A group of organisms that can reproduce with one another to produce fertile offspring.
• Selective breeding: The deliberate breeding of organisms with desired characteristics to produce offspring with those characteristics.
• Antibiotic resistance: The ability of bacteria with resistance alleles to survive antibiotic treatment and reproduce.
• Fossil: The preserved remains or traces of organisms from the past, providing evidence for evolution.
• Extinction: The permanent disappearance of a species when no living individuals remain.

Easily Confused

• Genetic variation vs environmental variation: Genetic variation is inherited through differences in genes or alleles, whereas environmental variation results from conditions such as diet, exercise or climate and is generally not inherited.
• Mutation vs adaptation: A mutation is a random genetic change that may produce a new allele; an adaptation is an inherited feature that improves survival and reproduction in a particular environment.
• Continuous variation vs discontinuous variation: Continuous variation has a range of intermediate values, such as height or body mass; discontinuous variation falls into distinct categories, such as blood group.
• Natural selection vs selective breeding: Natural selection occurs through environmental pressures, whereas selective breeding is deliberately controlled by humans.
• Adaptation vs evolution: An adaptation is an inherited feature that improves suitability to an environment; evolution is the change in inherited characteristics of a population over successive generations.
• Individual organisms vs populations: Individuals do not evolve during their lifetimes; evolution affects populations over many generations.
• Antibiotic resistance vs antibiotics creating resistance: Antibiotics select resistant bacteria that already possess resistance alleles; they do not create resistance because bacteria need it.
• Species formation vs extinction: A new species forms when isolated populations become unable to produce fertile offspring together; extinction occurs when no living individuals of a species remain.
• Inherited characteristics vs acquired characteristics: Natural selection can increase the frequency of inherited characteristics, but acquired features such as increased muscle size caused by exercise are not passed genetically to offspring.

What Gets Asked

• Explain the causes of variation, distinguishing genetic variation produced by mutations and the mixing of alleles during meiosis and fertilisation from environmental variation such as exercise, diet, climate, disease and lifestyle. A common error is to treat increased muscle size caused by exercise as inherited.
• Describe the sequence of natural selection: variation exists, a selection pressure acts, better-adapted individuals survive, they reproduce more, and advantageous alleles become more common over generations. Marks are lost if organisms are described as consciously developing features because they need them.
• Explain antibiotic resistance using the specific mechanism that resistant bacteria survive antibiotic treatment and reproduce. Antibiotics do not create resistance.
• Identify and explain adaptations as structural, physiological or behavioural, using thick fur, producing concentrated urine and migration respectively. The characteristic must be inherited for natural selection to increase its frequency.
• Use the equations percentage change = (change in value ÷ original value) × 100 and frequency of an allele = number of copies of the allele ÷ total number of copies of the gene. The relevant change, original value, or total number of gene copies must not be confused.
• Explain evidence for evolution and the formation of new species, including fossils, similarities in body structures, similarities in DNA and proteins, observed changes such as antibiotic resistance, and isolation followed by different selection pressures and accumulation of genetic differences. Darwin and Wallace independently proposed natural selection as an explanation for evolution in the nineteenth century.