Natural Selection (AQA A-Level Biology): Revision Notes
Natural Selection
Natural selection is the process by which organisms become better adapted to their environment over time. Every living organism faces selection based on how well suited it is for survival under current environmental conditions.
Natural selection is one of the fundamental mechanisms driving evolution, working alongside other factors like genetic drift and gene flow to shape the characteristics of populations over time.
Gene pools and selection pressures
A gene pool represents all the alleles of every gene found in all individuals within a population at any given moment. This genetic diversity forms the raw material upon which natural selection can act.
Selection pressures are environmental factors that limit population size and determine which individuals survive and reproduce. These pressures include:
Major Selection Pressures:
- Predation by other organisms
- Disease and parasites
- Competition for resources (food, water, light, space, mates)
- Extreme environmental conditions (temperature, natural disasters)
Understanding these pressures is crucial because they directly determine which traits become advantageous or disadvantageous in a given environment.
Selection pressures change over time and vary between locations, causing allele frequencies within gene pools to shift as populations adapt to different challenges.
Over-production of offspring and competition
Charles Darwin recognised that all species have the biological potential for exponential population growth. However, in reality, population sizes rarely increase at such dramatic rates due to high death rates.
Most species produce far more offspring than the environment can support. This over-production creates intense competition between individuals for limited resources - known as intraspecific competition (competition within the same species).
When resources become scarce, not all individuals can survive. Those deaths are not random events. Instead, individuals with characteristics that make them better adapted to prevailing conditions have higher survival chances. These survivors are more likely to reproduce successfully, passing their advantageous genetic combinations to the next generation.
This process gradually changes allele frequencies in the population, as beneficial traits become more common while less advantageous traits become rarer.
The role of variation in natural selection
Genetic variation within populations provides the foundation for evolutionary change. Without variation, natural selection cannot occur effectively.
If all organisms in a population were genetically identical, they would respond similarly to environmental challenges. However, genetic variation means that some individuals possess combinations of alleles that provide advantages in specific circumstances.
When environmental conditions change, this variation becomes valuable. Populations with greater genetic diversity have higher chances of containing individuals capable of surviving new challenges. These adapted individuals can then reproduce and pass on their beneficial traits.
Advantages of Large, Genetically Diverse Populations:
- Greater probability of containing individuals with advantageous trait combinations
- Better capacity to adapt to environmental changes
- Reduced vulnerability to new diseases or climate shifts
- Enhanced potential for long-term survival
This is why conservation biologists emphasise maintaining large population sizes and genetic diversity in endangered species.
Case study: Copper tolerance in grasses
Worked Example: Evolution of Copper Tolerance in Agrostis Species
The evolution of copper tolerance in grass species demonstrates natural selection in action. Species like Agrostis tenuis and Agrostis capillaris show varying degrees of copper tolerance across different environments.
Initial Conditions: Near old copper mines, soils contain high copper concentrations that are toxic to most plants. Initially, most grass varieties in these areas had low copper tolerance and could not survive in contaminated soil.
Selection Process: However, some individuals possessed genetic variants that provided copper tolerance. In copper-contaminated environments, these tolerant plants had significant survival advantages over non-tolerant varieties. They could grow and reproduce successfully while non-tolerant plants died.
Outcome: Over many generations, the frequency of copper-tolerance alleles increased in populations growing on contaminated soils. Meanwhile, in uncontaminated areas, copper tolerance provided no particular advantage, so both tolerant and non-tolerant varieties continued to coexist.
Key Principles Demonstrated:
- Environmental pressure (copper toxicity) acted as a selective force
- Genetic variation existed within the population
- Advantageous traits (copper tolerance) increased in frequency where they provided benefits
- Selection was environment-specific - tolerance was only advantageous in copper-rich soils
Remember!
Key Points to Remember:
- Natural selection requires over-production of offspring, genetic variation, and environmental selection pressures
- Gene pools contain all alleles present in a population and change as selection alters allele frequencies
- Individuals with advantageous traits have higher survival and reproduction rates, passing these traits to offspring
- Genetic variation provides the raw material for adaptation to changing environmental conditions
- Selection pressures vary over time and space, leading to different evolutionary outcomes in different environments