Natural Selection

• Natural selection is the main mechanism which drives evolution
• Variation already present due to mutations, these can produce advantageous alleles. Which can be selected for increasing survival, increasing reproductive success. So these alleles are passed on to increase the frequency of these alleles.
• Types of selection - selection causes some traits to survive and spread, while others are lost. A selection pressure determines which traits are successful.

  Predation, disease and competition are selection pressures which result in natural selection.
• Stabilising selection:

  • this occurs in an unchanging enviroment
  • this mean/median/modal value for a characteristic (its alleles) is selected for
• Example of stabilising selection:

  • very large babies' alleles are selected against during childbirth due to difficulties in actually being born i.e. movement through the cervix.
  • very small babies have a large sa:volume and so lose heat very quickly. This inability to maintain body temperature (enzymes) makes it difficult for them to survive.
• Directional Selection:

  • occurs in a changing environment.
• Example of directional selection:

  • antibiotic resistance in bacteria.
  • the allele for antibiotic resistance is selected for in an environment where antibiotics are present, e.g. a hospital.
  • in this, new environment the resistant allele confers advantage and is passed on, increasing its frequency in the population very quickly.