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CIE A-Level Biology Study Notes

17.2.1 Natural Selection Explained

Natural selection, a cornerstone of evolutionary biology, is the process by which species adapt and evolve through the differential survival and reproduction of individuals. This concept, profoundly articulated by Charles Darwin, illustrates how traits advantageous for survival and reproduction become more common in a population over time.

Understanding the Mechanism of Natural Selection

The Basis of Natural Selection

  • Variation in Populations: All natural populations exhibit genetic variation in traits. These traits, ranging from physical attributes like size and colour to behavioural tendencies, are often heritable. This variation is crucial for natural selection to occur.
  • Competition for Resources: Due to limited resources, individuals within a population face competition for survival essentials like food, shelter, and mates. This competition forms the basis of the survival of the fittest.
  • Differential Survival and Reproduction: Individuals possessing traits that offer a survival or reproductive advantage are more likely to survive and pass these advantageous traits to their offspring. Over time, this leads to changes in the population's genetic makeup.
  • Accumulation of Favourable Traits: Beneficial traits accumulate in the population over generations, leading to evolutionary changes. This gradual change can result in the emergence of new species.
A demonstration of natural selection.

Image courtesy of Ccaldwell19

Survival and Reproductive Advantages

  • Predator-Prey Dynamics: Traits that enhance an organism's ability to avoid predators, like camouflage in certain insects or speed in gazelles, directly influence survival rates.
  • Mating Success: Traits affecting mating success, such as elaborate plumage or courtship rituals in birds, play a significant role in reproductive success.
  • Resource Utilisation: Variations in traits that affect the efficiency of resource utilisation, like different beak shapes in birds for specific diets, directly impact survival and reproduction.

Exploring the Concept of Fitness

Definition of Fitness

  • Fitness in the context of natural selection refers to the relative ability of an organism to survive and reproduce in its environment. It is a measure of reproductive success rather than physical strength or health.
Evolutionary Fitness and  reproductive success using an example

Image courtesy of Understanding Evolution

Factors Influencing Fitness

  • Environmental Adaptations: Organisms well-adapted to their environments, such as polar bears in Arctic climates or cacti in deserts, have higher fitness in those specific settings.
  • Changing Environments: An organism's fitness can vary if the environment changes. A trait advantageous in one environment might become detrimental if environmental conditions change.

Fitness and Genetic Variation

  • Genetic diversity within a population affects overall fitness. Populations with a broader range of genetic variations are better equipped to adapt to changing environments, enhancing their survival prospects.

Examples of Natural Selection in Action

The Peppered Moth and Industrial Melanism

  • A classic example of natural selection is the evolution of the peppered moth in Britain during the Industrial Revolution. The once-common light-coloured variety became rare as pollution darkened tree barks, favouring the survival of a darker variety. This phenomenon, known as industrial melanism, illustrates how human activities can influence natural selection.
Industrial melanism-evolution of the peppered moth

Image courtesy of Alex Hyde

Darwin’s Finches in the Galápagos Islands

  • Charles Darwin observed finches on the Galápagos Islands with varied beak shapes, each adapted to different food sources. This observation was pivotal in developing his theory of natural selection, showing how species evolve to fit specific ecological niches.
A diagram of Finches from the Galapagos Archipelago.

Species of Finches from the Galapagos Archipelago that Darwin observed.

Image courtesy of Robert Taylor Pritchett

Antibiotic Resistance in Bacteria

  • The evolution of antibiotic resistance in bacteria is a contemporary example of natural selection. Bacteria that randomly mutate to resist antibiotics have a survival advantage in environments with these drugs, leading to a rapid increase in resistant strains.

Fitness in Various Environments

Adaptations to Diverse Ecosystems

  • In biodiverse ecosystems like tropical rainforests, a wide array of adaptations is observed, leading to high biodiversity. Species evolve to exploit different niches, resulting in varied forms, behaviours, and survival strategies.
  • In contrast, in more uniform environments like deserts, adaptations tend to focus on surviving extreme conditions. For example, desert plants and animals often evolve water retention capabilities and temperature regulation strategies.

Human Impact on Natural Selection

  • Human activities, including urban development, pollution, and deforestation, can significantly alter the course of natural selection by changing environmental conditions and selective pressures.

Climate Change and Evolution

  • Climate change is imposing new selective pressures on species globally. Traits that enable species to withstand higher temperatures, altered precipitation patterns, and extreme weather events are becoming increasingly advantageous.


Natural selection is a dynamic, continuous process that underpins the diversity of life on Earth. It operates through the differential survival and reproduction of individuals with certain heritable traits, leading to gradual evolutionary changes over generations. This process highlights the intricate relationship between organisms and their environments, emphasising the importance of genetic variation and adaptation. Understanding natural selection is essential for comprehending the complexities of biology and the ongoing evolution of life on our planet.


Mutations are random changes in an organism's DNA and are a primary source of genetic variation, which is crucial for natural selection. While most mutations are neutral or harmful, occasionally, a mutation can result in a new trait that is advantageous in a particular environment. If this advantageous trait is heritable, the individuals possessing it have a higher chance of surviving and reproducing than those without it. This differential reproductive success means that the advantageous trait, introduced by the mutation, will become more common in subsequent generations. Thus, mutations can introduce new traits that can be acted upon by natural selection, driving evolutionary change.

Genetic variation is essential for natural selection because it provides the raw material on which natural selection acts. Without genetic variation, all individuals in a population would be genetically identical, and there would be no differential survival or reproduction based on heritable traits. Genetic variation arises from mutations, gene flow, and sexual reproduction, creating a diversity of traits within a population. When environmental conditions change, it is this pool of genetic diversity that allows some individuals to survive and reproduce more effectively than others. Over time, the traits that confer a survival and reproductive advantage become more common, leading to evolutionary change.

The concept of fitness in natural selection is closely related to the environment, as it depends on an organism's ability to survive and reproduce in its specific environmental context. Fitness is not an absolute measure but is relative to the conditions of the environment. For example, a trait that confers high fitness in one environment (such as thick fur in arctic conditions) may be detrimental in another (like a tropical rainforest). Environmental changes can therefore alter the fitness landscape, making previously advantageous traits less beneficial or even harmful. This dynamic interplay between organisms and their environments is a key aspect of natural selection, with environmental conditions shaping the path of evolutionary change.

An individual organism cannot evolve during its lifetime. Evolution is a process that occurs at the population level over multiple generations through changes in the genetic composition of populations. The traits of an individual are determined by its genes, which remain constant throughout its life (excluding mutations, which are rare and usually not significant in the short term). Evolution involves the gradual change in allele frequencies within a gene pool over time, as a result of mechanisms like natural selection, genetic drift, and gene flow. While individuals can acclimate or adapt behaviourally to their environment, these changes are not heritable and thus do not constitute evolution.

Genetic drift is a mechanism of evolution that involves random changes in allele frequencies within a population, unlike natural selection which is non-random and based on differential survival and reproduction. In genetic drift, chance events cause certain alleles to become more or less common in a population, irrespective of their effect on fitness. For example, in a small population, if individuals carrying a particular allele happen to die accidentally or fail to reproduce for reasons unrelated to fitness, that allele's frequency might decrease or be lost entirely. This process can lead to significant genetic changes over time, especially in small populations, but it does not necessarily produce adaptations like natural selection does.

Practice Questions

Explain how the concept of 'fitness' in natural selection differs from its common usage and provide an example to illustrate this difference.

Fitness in the context of natural selection refers to an organism's ability to survive and reproduce in its environment, rather than physical strength or health. For example, in a population of beetles, a trait that allows better camouflage from predators can be considered a fitness advantage. This trait increases the likelihood of these beetles surviving and reproducing, thus passing on their genes to the next generation. Over time, this leads to an increase in the frequency of the camouflage trait in the population, demonstrating natural selection in action. Fitness here is about reproductive success and not about the physical robustness or vitality of the individual beetle.

Describe an example of natural selection in a specific species, highlighting the key aspects of variation, competition, survival, and reproduction.

The Darwin's finches of the Galápagos Islands provide a classic example of natural selection. These finches exhibit significant variation in beak size and shape, adapted to different food sources. For instance, finches with larger, stronger beaks can crack open hard seeds, while those with finer beaks are better suited for eating insects. This variation arises from genetic differences and is heritable. The finches compete for food resources, and those whose beak shapes are best suited to available food sources have a survival advantage. These finches are more likely to survive and reproduce, passing on their advantageous beak traits. Over generations, these adaptations become more pronounced in the population, showcasing natural selection through differential survival and reproductive success based on beak morphology.

Dr Shubhi Khandelwal avatar
Written by: Dr Shubhi Khandelwal
Qualified Dentist and Expert Science Educator

Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.

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