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GCSEBiology: Combined ScienceOCR GatewayRevision Notes5. Genes, Inheritance & Selection5.2 Natural Selection & Evolution5.2.1 Genetic Variation Within a Population

Genetic Variation Within a Population (OCR Gateway GCSE Biology: Combined Science)

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Variation in Populations

  • Variation within populations is called intraspecific variation
  • Familiar examples of intraspecific variation might include
    • Breeds of dogs
    • Race in humans
    • Varieties of plants
  • It occurs due to different alleles found in the population
  • These alleles originated from mutations of an ancestral gene
  • Variation in a population is responsible for the differences in characteristics but also may determine how likely an individual is to survive
  • Periodically, a new mutation will occur and introduce a new allele into the population
  • Whether this allele is successful or whether it is quickly lost again from the population depends on whether it provides any kind of advantage or disadvantage to individuals that possess it

Mutations

  • Mutations occur continuously and are the cause of new alleles in a population
  • Mutations are rare, random changes that occur in the sequence of DNA bases in a gene or a chromosome
  • As the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to a change in the protein that the gene codes for
  • Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
  • There are different ways that a mutation in the DNA base sequence can occur

The Effects of Mutations

  • Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
  • However, a small number of mutations code for a significantly altered protein with a different shape
  • This may affect the ability of the protein to perform its function. For example:
    • If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site
    • A structural protein (like collagen) may lose its strength if its shape changes
  • On rare occasions mutations lead to the development of new alleles and so new phenotypes
  • Occasionally, the new allele (and its resulting phenotype) gives the individual a survival advantage over other members of the species
  • For example:
    • A bird develops a mutation leading to a change in feather colours
    • This makes it more attractive to birds of the opposite sex
    • Which causes the bird to breed more frequently and have more chances of passing on the mutated phenotype to the next generation
  • Mutations can also lead to harmful changes that can have dramatic effects on the body - for example, sickle cell anaemia in humans

The Causes of Mutations

  • Mutations happen spontaneously and continuously but their frequency can be increased by exposure to the following:
    • Gamma rays, x - rays and ultraviolet rays - all types of ionising radiation which can damage bonds and cause changes in base sequences
    • Certain types of chemicals - for example chemicals such as tar in tobacco
  • Increased rates of mutation can cause cells to become cancerous, which is why the above are linked to increased incidence of different types of cancer

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    Ruth

    Author: Ruth

    Expertise: Biology

    Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. She gained 10 years of teaching experience across the 3 key science disciplines and physical education. Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.