Production of variation Selection for/against  Pop evolves:  cumulative change in heritable characteristics in population  Nat sel can act on pop w/o speciation occurring  Adapting to changes in local conditions

5.4.7 Explain how natural selection leads to evolution.

 STRONGEST, BIGGEST not always “best” or “most fit”  Changes in environment  May not have the advantage

Staphylococcus aureus  Bacteria associated w/skin & lung infections  Genetic mutations allow it to survive antibiotic treatment  population of S. aureus has diverged, 2 forms:  Methicillin-Resistant Staphylococcus aureus (MRSA)  Methicillin-Susceptible Staphylococcus aureus (MSSA)  still contained by use of antibiotic Methicillin  Genomes compared in 2004—signif diffs  ** resistance = generational; immunity = lifetime US Hospitals

 Antibiotics select against susceptible forms of the bacteria  Antibiotic = selective pressure on pop  Random mutation: resistance gene, low freq in pop  Gene transferred to other bacteria (plasmids)  Frequent use of antibiotic   MRSA has advantage, MSSA has selective disadvantage  MRSA survives, reproduces  Offspring have resistant gene, increases in freq in pop