Why do organisms ‘match’ their
environment?
fitness
Natural selection:
differential survival and reproduction of individuals
based on genetic differences in some trait(s)
adaptation
• The ‘match’ is genetic and induced by
environment.
Charles Darwin figured out how natural
selection could lead to adaptive evolution.
• offspring resemble parents
• variation in traits can be inherited
• life is full of challenges!
• variation in traits  variation in fitness
Fitness:
- ability to survive and reproduce
- depends on the environment
How natural selection works
Resistance to antibacterial soap
Generation 1: 100% not resistant
0.00% resistant
How natural selection works
Resistance to antibacterial soap
Generation 1: 100% not resistant
0.00% resistant
How natural selection works
Resistance to antibacterial soap
Generation 1: 100% not resistant
0.00% resistant
Generation 2: 96% not resistant
4% resistant
mutation!
How natural selection works
Resistance to antibacterial soap
Generation 1: 100% not resistant
0.00% resistant
Generation 2: 96% not resistant
4% resistant
Generation 3: 76% not resistant
24% resistant
How natural selection works
Resistance to antibacterial soap
Generation 1: 100% not resistant
0.00% resistant
Generation 2: 96% not resistant
4% resistant
Generation 3: 76% not resistant
24% resistant
Generation 4: 12% not resistant
88% resistant
Natural selection – key points
differential survival and reproduction of individuals
based on genetic differences in some trait(s)
For adaptations to evolve:
- differences in the trait must cause differences in fitness
- differences in the trait must be heritable
Fitness depends on the environment.
local adaptation
Natural Selection
• Directional Selection: natural selection
removes organisms at one extreme and in
the middle, favoring one extreme:
– The peppered moth
– Bacterial resistance to antibiotics
– Insect resistance to pesticides
Natural Selection
• Stabilizing Selection: natural selection
removes organisms at the extremes of a
population:
– Birth weight in humans (heaviest and lightest
babies have the highest mortality)
Natural Selection
• Disruptive selection: natural selection that
favors two extremes of a characteristic
– Grass plants in Welsh copper mines
– Sickle cell heterozygotes in Africa
Why do organisms ‘match’ their
environment?
fitness
Natural selection:
differential survival and reproduction of individuals
based on genetic differences in some trait(s)
adaptation
• The ‘match’ is genetic and induced by
environment.