Natural selection

Natural selection

  • the mechanism for evolution

  • individuals that are better adapted to their environment survive and reproduce

    • survival of the fittest

  • eventually new species are formed

Process of natural selection

  1. there is natural variation in a population (new alleles can arise by mutation)

  2. Environmental selection occurs - those with mutations that give rise to variations which allow the organism to overcome the selective pressure have a survival advantage

  3. thus best adapted organisms in a population outcompete those that are less well adapted (survival of the fittest)

  4. these individuals survive and reproduce

  5. individuals in the next generation with these alleles are also more likely to survive

  6. over many generations the frequency of the advantageous alleles are also more likely to survive

  7. process continues and may eventually lead to a new species formation

EXAMPLE CASE: antibiotic resistance

  • bacteria can carry antibiotic resistance genes in their plasmids

  • they will be able to survive and reproduce in the presence of the antibiotic passing on the resistance gene to their offspring

How the resistance evolves

  • the original population of bacteria shows variation in its level of antibiotic resistance

  • the antibiotic (selective pressure) is added

  • only bacteria with the advantageous allele (selective advantage) of the gene for resistance survive and then reproduce

  • eventually all of the bacteria that survive will carry the advantageous allele for the antibiotic resistance gene

  • antibiotic resistance