Evolution: Natural Selection

Evolution: Natural Selection

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

   • Natural Selection is in favour of the virus not the human

2. Variation

   • Variation is a result of meiosis and sex. Offspring are GENETICALLY DIFFERENT from their parents
     • Variation in meiosis is a result of crossing over and independant assortment
       • 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  
   • 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
     • Better mutations will have a higher success rate in both survival and reproduction  Bad mutations will reduce success rate and reproduction and eventually disappear 
   • Variation is a result of genes spreading between populations
     • Speciation: formation of new and distinct species
       • Most speciation happens in allopatry: physical or distance barrier to gene flow
       • Once the species are isolated in some way (pre-zygotic or post-zygotic) each species will evolve separately
         • Natural selection will act differently, depending on the type of isolation different mutations will be more successful than others 
       • 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
         • While the species were apart they evolved separately, different mutations will be present in each species 

3. Heritability

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