genetic diversity and natural selection

• Stabilising selection is where individuals with alleles for characteristics towards the middle range are more likely to survive and reproduce, when an environment does not change and it reduces the range of possible characteristics. 
• Directional selection is where individuals with alleles of an extreme type are more likely to survive and reproduce, possibly a response to an environmental change. 
• Anatomical adaptations are structural features of an organisms body that increase chances of survival. 

  • Whales have a thick layer of blubber to retain heat
  • Cacti grow thorns to deter predators 
• Physiological adaptations are processes inside an organisms body that increase chances for survival. 

  • Brown bears hibernating 
  • Some bacteria produce antibiotics 
• Behavioural adaptations are ways an organisms acts in order to increase its chances for survival. 

  • Possums play dead
  • Birds do a mating dance 
• Some mutations are beneficial to organisms. If an allele increases the chance of an individuals survival then it may produce more offspring. This means that a greater proportion of the next generation have the beneficial allele and reproduce increasing the number of  individuals with the allele in future generations. 
• The founder effect describes what happens when only a few individuals start a new colony with only a small number of different alleles in the initial gene pool. The frequency of alleles in the new population may be different than that in the original. 
• A genetic bottleneck is an event that causes a large reduction in a population, reducing the number of alleles in the gene pool. The surviving population will reproduce and their offspring will contain the same alleles. 
• Genetic diversity may ne increased by mutations in the DNA and different alleles being introduced to a population. 
• Genetic diversity is the number of different alleles of genes in a species. 
• Different versions of a single gene are called alleles, which code for different versions of characteristics.  
• Each homologous pair is made from one chromosome from each parent. When homologous pairs are separated in meiosis I, its completely random which chromosome from each pair ends up in which daughter cell. The four daughter have completely different combinations of those parental chromosomes. This is what leads to genetic variation. 
• During meiosis I chromosomes pair up, the chromatids twist around each other and pieces swap over. The chromatids still contain the same genes but now have different combinations of alleles. 


  example