natural selection

• first stage 


  variation exists within a species population, some due to random mutations which introduce new alleles= population possess different phenotypes
• second stage


  environmental factors or changes act as selection pressures on population eg disease, predation
• third stage 


  some individuals posses advantageous alleles which give them a selective advantage and allow them to survive and reproduce
• fourth stage 


  the advantageous alles then passed to offspring which increases frequency of advantageous alleles in gene pool

  frequency of alleles within the population changes leading to evolution