Source: Lecture notes / textbook readings
Difficulty: Introductory | Prerequisites: Characteristics of Life notes
Evolution by natural selection is the central organising principle of biology. This unit explains how populations change over time, what drives that change, and how we know it happens. You will study the mechanism of natural selection, the evidence for evolution from multiple scientific fields, and two classic case studies (peppered moths and lactase persistence in humans) that show natural selection in action. If you are comfortable with the characteristics of life, especially reproduction, genetic variation and adaptation, you are ready for this material.
Natural selection is the mechanism by which populations evolve: individuals with heritable traits that improve survival and reproduction leave more offspring, so those traits become more common. Multiple lines of evidence (fossils, molecular biology, comparative anatomy, biogeography, embryology, observed changes) support evolution. The peppered moth and human lactase persistence are two well-documented examples of natural selection responding to environmental change.
Evolution
The change in the genetic composition of a population over successive generations. In simple terms, evolution is how populations shift over time as certain traits become more or less common.
Natural selection
The process by which individuals with heritable traits better suited to their environment survive and reproduce at higher rates, causing those traits to spread through the population.
Directional selection
A mode of natural selection in which one extreme phenotype is favoured, shifting the population's trait distribution in one direction. Think of it as the whole bell curve sliding left or right.
Stabilising selection
A mode of natural selection that favours intermediate phenotypes and reduces variation. The bell curve gets narrower.
Disruptive selection
A mode of natural selection that favours both extremes over the intermediate phenotype. The bell curve splits into two peaks.
Homologous structures
Anatomical features in different species that share a common evolutionary origin but may serve different functions. The classic example: the human arm, whale flipper and bat wing all share the same underlying bone arrangement.
Biogeography
The study of the geographic distribution of species. Patterns of distribution (e.g. similar but distinct species on neighbouring islands) provide evidence for evolution.
Lactase persistence
A genetic trait that allows adults to continue producing the lactase enzyme and digest milk. In most mammals, lactase production declines after weaning.
Cortex gene
The gene in peppered moths responsible for colour variation between light (mottled) and dark (melanic) morphs.
Melanic morph
The dark-coloured form of the peppered moth (Biston betularia), which became more common during industrial pollution due to better camouflage on soot-darkened trees.
Natural selection requires three conditions:
Variation. Individuals in a population differ in their traits.
Heritability. Those differences are at least partly genetic and can be passed to offspring.
Differential reproduction. Some variants survive and reproduce more successfully than others in a given environment.
Over generations, the traits that confer an advantage become more common. This is evolution by natural selection.
Natural selection acts only on existing variation. It cannot anticipate future environmental changes, and it does not produce "perfect" organisms. It adapts populations to current conditions.
Directional selection shifts the population toward one phenotypic extreme. Example: larger body size being favoured in a cold climate.
Stabilising selection narrows variation around the mean. Example: human birth weight, where very large and very small babies have lower survival rates.
Disruptive selection favours both extremes over the middle. Example: beak size in a bird population where medium-sized seeds are scarce but small and large seeds are abundant.
Evolution is supported by converging evidence from six fields:
Fossil record. Shows gradual changes in organisms over geological time. Transitional fossils (e.g. Tiktaalik, a fish-tetrapod intermediate) document major evolutionary transitions.
Molecular biology. DNA and protein sequence comparisons reveal genetic similarities among species. More closely related species share more genetic sequence.
Embryology. Early embryos of vertebrates (fish, reptiles, birds, mammals) look strikingly similar, reflecting shared developmental genes.
Comparative anatomy. Homologous structures (same bones, different functions) indicate common ancestry. Vestigial structures (e.g. the human appendix, whale pelvis bones) are remnants of ancestral features.
Biogeography. Species distribution patterns match predictions of evolution. Island species resemble nearby mainland species but are distinct, reflecting divergence after colonisation.
Observed evolution. Real-time examples include antibiotic resistance in bacteria, pesticide resistance in insects, and the peppered moth case study.
The peppered moth (Biston betularia) is one of the best-documented examples of natural selection in the wild.
Before industrialisation: Light-coloured (mottled) moths were common. They blended in against lichen-covered tree bark. Dark (melanic) moths were rare and easily spotted by bird predators.
During industrialisation: Pollution killed lichen and coated trees in soot. Dark moths now had better camouflage. Birds preferentially ate the conspicuous light moths. The dark morph became dominant.
After pollution controls: Clean-air legislation allowed lichen to regrow. Light moths regained their camouflage advantage. Their frequency increased again, demonstrating that natural selection is reversible when the environment changes.
The genetics: Mutations in the cortex gene are responsible for the colour difference between morphs, confirming that the variation is heritable and genetic.
Why it matters for exams: This case neatly demonstrates all three conditions for natural selection (variation, heritability, differential reproduction) and shows that selection responds to environmental change.
Most mammals stop producing the lactase enzyme after weaning. In humans, lactase production typically declines after childhood, making undigested lactose cause discomfort (lactose intolerance).
Some human populations, particularly in Europe and parts of East Africa, carry genetic mutations that keep lactase production active into adulthood. This trait is called lactase persistence.
How it evolved: Populations that domesticated dairy animals gained a nutritional advantage from being able to digest milk as adults. Individuals with lactase persistence had better nutrition and higher survival rates, so the trait spread through positive selection.
Convergent evolution: Lactase persistence evolved independently in European and African populations within the last 20,000 years, through different mutations. This is a strong signal of positive selection: two separate populations arrived at the same solution to the same environmental pressure.
Why it matters: Lactase persistence is a clear example of ongoing human evolution driven by the interaction of genetic mutation, cultural practice (dairying) and environmental pressure.
Antibiotic resistance in bacteria is natural selection happening in real time. When a population of bacteria is exposed to an antibiotic, most die, but any individuals with a resistance mutation survive and reproduce. Within a few generations the resistant strain dominates. This is why doctors emphasise completing a full course of antibiotics and why overuse accelerates resistance.
Pesticide resistance in agricultural insects follows exactly the same logic, and it is a major driver of integrated pest management strategies.
Students often think evolution causes individuals to change during their lifetime. It does not. Natural selection acts on populations over generations; individuals do not evolve.
Students often assume evolution means progress, that organisms are always "getting better." Evolution has no direction or goal. It adapts populations to current conditions, which may change.
The giraffe's long neck is frequently cited as an individual stretching to reach leaves. In reality, neck length is a population-level trait: giraffes with slightly longer necks had better access to food, survived more, and passed the trait on.
Students sometimes say humans descended from monkeys. Humans and modern apes share a common ancestor. Neither descended from the other.
⚠️ Be able to state the three conditions required for natural selection (variation, heritability, differential reproduction) and apply them to a novel example.
⚠️ Know the six lines of evidence for evolution and be able to give a one-sentence example of each.
⚠️ Be ready to walk through the peppered moth case: what happened before, during and after industrialisation, and which condition of natural selection each element illustrates.
⚠️ Understand lactase persistence as an example of ongoing human evolution and gene-culture co-evolution.
⚠️ Distinguish directional, stabilising and disruptive selection. Exam questions often give a scenario and ask you to identify the type.
True or false: Natural selection can cause an individual organism to evolve during its lifetime. (False. Selection acts on populations over generations.)
Fill in the blank: The three conditions for natural selection are variation, heritability and __________. (Differential reproduction.)
True or false: Lactase persistence evolved only once in human history. (False. It evolved independently in European and African populations.)
Fill in the blank: The gene responsible for colour differences in peppered moths is the __________ gene. (Cortex.)
True or false: Humans descended directly from modern monkeys. (False. Humans and modern apes share a common ancestor.)
Q: State the three conditions necessary for natural selection to occur.
A: (1) There must be variation in traits within a population. (2) That variation must be heritable (genetic). (3) Some variants must have a reproductive advantage over others in the current environment (differential reproduction).
Q: Explain how the peppered moth case demonstrates reversible natural selection.
A: Before industrialisation, light moths were camouflaged on lichen-covered bark and had higher survival. Pollution darkened the trees, giving dark moths the camouflage advantage and increasing their frequency. When pollution controls restored lichen, the light morph regained its advantage and increased in frequency again. The direction of selection reversed with the environment.
Q: How does lactase persistence illustrate gene-culture co-evolution?
A: The cultural practice of dairy farming created an environment in which adults who could digest milk had a nutritional advantage. This cultural change (dairying) drove positive selection for mutations that kept lactase production active into adulthood. The gene and the culture reinforced each other.
Q: A population of beetles varies in colour from green to brown. Birds eat the beetles. The habitat is a green meadow. Over ten generations, the proportion of green beetles increases. What type of selection is this?
A: Directional selection. One extreme phenotype (green) is favoured because it provides better camouflage, shifting the population's colour distribution toward green.
Q: Why does natural selection not produce "perfect" organisms?
A: Natural selection can only act on existing genetic variation. It cannot anticipate future conditions, and trade-offs between traits mean that improving one function may compromise another. Organisms are adapted to current conditions, not optimised for all possible environments.
This material builds directly on the characteristics of life (reproduction with variation and adaptation are prerequisites for evolution). It also connects to the Earth's history and fossil record unit, where you will see how radiometric dating provides the timeline for evolutionary events. Understanding natural selection is essential for later topics in ecology (predator-prey dynamics, competition) and genetics (allele frequencies, Hardy-Weinberg equilibrium).
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