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Natural Selection

Laura Armstrong & Joe Wolfensohn

Teachers

Laura Armstrong Joe Wolfensohn

Contents

Natural Selection

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The Process of Natural Selection

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Types of Natural Selection

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Recall Questions

This topic requires prior knowledge of natural selection from GCSE. You can test your knowledge on this below.

What is natural selection?

Natural selection is the process where organisms with advantageous traits are more likely to survive, reproduce, and pass on their beneficial alleles to the next generation.

What is the role of mutations in natural selection?

Mutations create genetic variation by introducing new alleles. If a mutation provides a survival advantage, it may increase in frequency in a population over generations.

How does antibiotic resistance in bacteria demonstrate natural selection?

Some bacteria develop mutations that make them resistant to antibiotics. These bacteria survive, reproduce, and pass on the resistance gene, leading to an increase in antibiotic-resistant bacteria in the population.

Topic Explainer Video

Check out this @JoeDoesBiology video that explains natural selection or read the full notes below. Once you've gone through the whole note, try out the practice questions!

Natural Selection

What is Natural Selection?

Natural Selection is a mechanism of evolution proposed by Charles Darwin. It explains how species change over time as individuals with advantageous traits survive and reproduce, passing their beneficial alleles to their offspring.

  • Natural selection occurs due to genetic variation in a population.
  • Selection pressures (e.g., predators, climate, disease) affect which individuals survive.
  • Beneficial alleles become more common over generations, leading to adaptation.

The Process of Natural Selection

  1. Variation: Individuals in a population show genetic variation due to mutations, crossing over, and independent assortment during meiosis.
  2. Mutations give rise to new alleles.
  3. Selection Pressure: Environmental factors (e.g., competition, predators, disease) act as pressures that affect survival chances in a population.
  4. Survival of the Fittest: Individuals with advantageous alleles survive and reproduce more successfully.
  5. Inheritance: Beneficial alleles are passed on to the next generation.
  6. Change in Allele Frequency: Over generations, the frequency of advantageous alleles increases, leading to evolution.

Example: Peppered Moths

  • There was genetic variation in the moth population due to mutations.
  • In pre-industrial England, light-coloured moths were more common as they blended into tree bark.
  • Industrial pollution darkened trees, making dark-coloured moths better camouflaged from predators.
  • Dark moths survived and reproduced.
  • Passing on the advantageous allele for the darker colour.
  • Over time the frequency of this allele increased in the population.

Types of Natural Selection

1. Directional Selection

  • Favours one extreme phenotype, shifting the population towards it.
  • Occurs when the environment changes.
  • Example: Bacteria evolving antibiotic resistance.

2. Stabilising Selection

  • Favours the average phenotype and selects against extremes.
  • Occurs in a stable environment where extremes reduce survival.
  • Example: Human birth weight – babies that are too small or too large have lower survival rates.

3. Disruptive Selection

  • Favours both extremes but selects against the average.
  • Can lead to speciation if populations become genetically distinct.
  • Example: Beak size in finches – small and large beaks may be advantageous, but medium-sized beaks are less efficient.

Key Terms

  • Natural Selection: The process where organisms with advantageous traits survive and pass on their alleles.
  • Selection Pressure: An environmental factor (e.g., predation, climate) that affects survival and reproduction.
  • Adaptation: A trait that increases an organism’s survival and reproductive success.
  • Allele Frequency: The proportion of a specific allele in a population’s gene pool.
  • Directional Selection: Selection that favours one extreme phenotype.
  • Stabilising Selection: Selection that favours intermediate phenotypes and reduces variation.
  • Disruptive Selection: Selection that favours extreme phenotypes at both ends of a spectrum.
No answer provided.

Exam Tips

When explaining natural selection, always structure your answer using:

  1. Genetic variation due to mutations.
  2. Selection Pressures.
  3. Survival and increased reproductive success.
  4. Inheritance of the advantageous allele.
  5. Change in Allele Frequency.
No answer provided.

Explain how natural selection has led to antibiotic resistance in bacteria. (5 marks)

  1. Mutations occur in bacterial populations, leading to genetic variation. (1)
  2. Some bacteria develop resistance genes through mutations. (1)
  3. When exposed to antibiotics, non-resistant bacteria die, while resistant bacteria survive and reproduce. (1)
  4. The resistance allele is passed on, increasing its frequency in future generations. (1)
  5. Over time, the bacterial population becomes more resistant to the antibiotic. (1)

Practice Question 1

Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!

Directional selection