types of selection

how does a single bacterium become antibiotic resistant?

• a bacterium becomes resistant due to a random mutation in its DNA

• this means that the bacterium produces a protein that makes the antibiotic ineffective

• the protein in the cell wall is no longer weakened by antibiotics

• → enzyme breaks down antibiotic and carrier protein pumps antibiotics out of cell

describe the process of antibiotic resistance:

• a bacteria population has a gene pool w/ a wide variety of alleles

• random mutation may introduce antibiotic resistant alleles, increasing variation

• environmental pressure of antibiotic kills bacteria w/o antibiotic resistant mutation

• differential reproduction occurs - only the antibiotic resistant bacteria survive and reproduce, passing on their advantageous alleles to the next generation: heredity

• over many generations, advantageous alleles increase in frequency, forming a population of antibiotic resistant bacteria

what is directional selection?

• individuals w/ alleles for more extreme characteristics are more likely to survive and reproduce

• due to a selection pressure - environmental change

• on a graph, this shifts the mean frequency to the right

give an example of directional selection:

antibiotic resistance

what is stabilising selection?

• individuals w/ alleles for characteristics towards middle of range are more likely to survive and reproduce

• no selection pressure/environmental change

give an example of stabilising selection:

human birth weights

explain human birth weights in terms of stabilising selection:

• very small babies less likely to survive - difficulty maintaining body temp → these alleles less likely to be passed on

• very large babies less likely to survive - difficulty in childbirth → these alleles less likely to be passed on

• → conditions most favourable for medium sized babies ∴ human birth weights stabilise towards the mean w/ less variations at the extremes