Evolution: Natural Selection

Nota:

• Natural Selection:  the process whereby organisms better adapted to their environment tend to survive and produce more offspring. 

1. Resistance to AZT is created because of natural selection: natural selection increases the amount of AZT resistant forms until all/majority of HIV cells have evolved with this trait
   1. Natural Selection is in favour of the virus not the human
2. Variation
   1. Variation is a result of meiosis and sex. Offspring are GENETICALLY DIFFERENT from their parents
      1. Variation in meiosis is a result of crossing over and independant assortment

         Nota:

         • Crossing over: when homologous chromosomes lay on top of one another and swap DNA    Independent Assortment:  occurs in meiosis I and II during metaphase; chromosomes are randomly lined up (spindle fibers randomly attach to kinetochores) causing a random mixture of chromosomes/chromatids to be separated into daughter cells  
   2. Variation in offspring can result in mutations/changes that can have no, good or bad effects on the survival and reproductive abilities for the offspring

      Nota:

      • Better mutations will have a higher success rate in both survival and reproduction  Bad mutations will reduce success rate and reproduction and eventually disappear 
   3. Variation is a result of genes spreading between populations
      1. Speciation: formation of new and distinct species

         Nota:

         • Most speciation happens in allopatry: physical or distance barrier to gene flow
         1. Once the species are isolated in some way (pre-zygotic or post-zygotic) each species will evolve separately

            Nota:

            • Natural selection will act differently, depending on the type of isolation different mutations will be more successful than others 
         2. Species sometimes have secondary contact where the isolation is removed, if the two groups can still mate then more variation is introduced to the gene pool

            Nota:

            • While the species were apart they evolved separately, different mutations will be present in each species 
3. Heritability

   Nota:

   • Heritability:  estimate of how much of the diversity regarding a certain trait is due to diversity in the population gene pool