Evolutionary forces

Question 1

What is normal distribution?

Answer 1

• the traits or characteristics of all living organism show variation within populations
• the distribution of the different vairants will take the form of a bell-shaped curve if plotted on a graph

Question 2

Why does the bell shape form?

Answer 2

• average -
  
  • most common and form the peak
• extremes are less common
  
  • may lead to loww chance of survival
  • unlikely of occuring

Question 3

What is stabalising selection? How is normal distribution an example?

Answer 3

• type of natural selection in which the population mean stabalised on a particular non-extrenme trait value
• the norm or average is selcted for (positive selection)
• and the extremes are selected against (negative selection)

Question 4

What does stabalising selection result in?

Answer 4

• reduction in the freq of alleles at the extremes
• increase in the fre at ‘average’ alleles

Question 5

What is directional selection?

Answer 5

• directional selection occurs when there is change in the environment
• the normal phenotype is no longer the most advantageous
• organisms which are less comon and have more extreme phenotypes and are positively selection
• the allele frequency then shifts towards the extreme phenotypes and evolution occurs

Question 6

What is an example of directional selection

Answer 6

• peppered moths in the industrial revolution
• smoke was released from facotires, killed lichens growing on barks of tree
  
  • soot made bark black
• peppered moths were white - camoflauge by the lichen from predation by birds
• always a few darker moth present
• white moths quickly eaten quickly
  
  • allele freq decreased

Question 7

What is disruptive selection?

Answer 7

• in disruptive selection the extremes are selected for and the normal selected against
• e.g. finches observed by Darwin in the Galapagos Islands
• opposite to stabalising slection

Question 8

Draw the graphs for the types of selection

Answer 8