Natural Selection

Natural Selection
Differential success in reproduction
3 modes of selection
Original population
population population
(a) Directional selection shifts the overall
makeup of the population by favoring
variants at one extreme of the
distribution. In this case, darker mice are
favored because they live among dark
rocks and a darker fur color conceals them
from predators.
Fig 23.12 A–C
Phenotypes (fur color)
(b) Disruptive selection favors variants
at both ends of the distribution. These
mice have colonized a patchy habitat
made up of light and dark rocks, with the
result that mice of an intermediate color are
at a disadvantage.
Stabilizing selection removes
extreme variants from the population
and preserves intermediate types. If
the environment consists of rocks of
an intermediate color, both light and
dark mice will be selected against.
• Sexual selection
– Natural selection for mating success
sexual dimorphism, differences between the sexes
• Chance and natural selection interact
• Selection can only edit existing variations