(Unit 1.7) Evolution

• what is evolution?


  the gradual change in characteristics of a population of organisms over generations as a result of variation in the population's genomes.
• what is natural selection?

  Natural selection is the non-random increase in the frequency of DNA sequences that increase survival and the non-random reduction in the frequency of deleterious sequences.
• what is stabalising selection?


  selection pressure goes against extreme variants and favours the intermediate versions of that trait. This leads to reduction in genetic diversity.
• what is an example of stablising selection?


  Human birth mass remains within 3-4kg. Babies with lower mass are more exposed to disease, higher mass have difficulties during birth.
• what is directional selection?


  it is common during period of environmental change. selection favours a version in which was initially less common, causing a progressive shift in the mean value.
• what is an example of directional selection?


  European black bears increased in mass during each ice age. as larger bodies lose relatively less heat than smaller ones.
• what is disruptive selection?


  selection pressure selects extreme versions of a trait at the expense of the intermediate versions. can result in the population being split into 2 distinct groups. this is the driving force behind sympatric speciation.
• what is horizontal gene transfer?


  when genes are transferred across from an individual to others in the same generation.
• what is vertical gene transfer?


  when genes transferred down generations. ie. from parent to off-spring as a result of sexual or asexual reproduction.
• what are the 2 types of speciation?

  1. allopatric
  2. sympatric
• what is allopatric speciation?


  it occurs when gene flow between 2 or more populations is prevented by a geographical barrier. (ie. rivers, mountain ranges, desert, sea)
• what is sympatric speciation?


  2 or more populations living in the same area but still become genetically isolated. this happens due to a behavioral or ecological barrier.