Natural Selection
• Darwinian view: the history of life is like a tree
Hyracoidea
(Hyraxes)
Sirenia
(Manatees
and relatives)
Elephas
maximus
(Asia)
Figure 22.7
Loxodonta
africana
(Africa)
Loxodonta
cyclotis
(Africa)
Natural Selection and Adaptation
3 inferences based on 5 observations
• Observation #1: Population sizes increase
exponentially
Figure 22.8
• Observation #2: Nonetheless, populations tend
to be stable in size
• Observation #3: Resources are limited
• Inference #1: Production of more individuals
than the environment can support
struggle for existence, with only a fraction of the
offspring survive
• Observation #4: Population varies extensively
No two individuals exactly alike
Figure 22.9
• Observation #5: Much of this variation is
heritable
• Inference #2: Survival depends in part on
inherited traits
– Individuals whose inherited traits give them a high
probability of surviving and reproducing are likely to
leave more offspring than other individuals
• Inference #3: Unequal survival and reproduction
gradual change in a population, favorable
characteristics accumulating over generations
• Natural selection is differential success in
reproduction
– Results f/ interaction between individuals that vary
in heritable traits and their environment
• Natural selection  increase over time of adaptation
to environment
(a) A flower mantid
in Malaysia
(b) A stick mantid
in Africa
Figure 22.11
The Evolution of Drug-Resistant HIV
Evolution of antibiotic resistance