Natural Selection and Fitness

Body Position Review: Humans
Body Position Review: Animals
Natural Selection and Fitness
What is Fitness
• Fitness = determined by
reproductive success
• Not necessarily bigger,
faster, stronger
• Fitness is not
determined by one trait
but by the totality of traits
in the organism
What is Natural Selection?
• Certain phenotypes are
more successful
• Particular environments
“select” certain phenotypes
• Adaptive Process
Natural Selection in a nut shell
Natural Selection in a nut shell
Natural Selection in a nut shell
Modes of Selection
Stabilizing selection – acts against extreme
phenotypes and favors intermediate
Modes of Selection
Directional Selection – shifts the overall
makeup of the population by favoring
variants at one extreme of the distribution
Modes of Selection
• Diversifying Selection – favors variants at
both ends of the distribution.
Mechanism : Natural Selection
• Fact 1: Over-reproduction occurs in
• Fact 2: Populations do not increase
• Fact 3: There are limited natural
resources (food, shelter)
Mechanism: Natural Selection
• Fact 4: Variation exists in populations
• Fact 5: Much of the variation is heritable
– Fact 4 was physically observed. Darwin’s
weakness was the 5th fact
• Inference 1:
– struggle for survival ensues
• Inference 2:
– Organisms with the best variations survive the
struggle for life
• Inference 3:
– Unequal survival of organisms with different
variations leads to favorable variations
accumulating over time
Examples of Natural Selection
• Darwin’s Finches
• Peppered Moths
• Antibiotic
resistance in
• Arms Race
Environmental Influence
• Environmental plays a
huge role on selective
• Organisms adapt to their
• Ex. Eye sockets in blind
salamanders (Vestigial
Vestigial structures
Eye sockets in blind salamanders
Key Concepts
• Natural Selection happens because of
heritable variations
• Overtime, natural selection results in
changes in the inherited characteristics of
a population.
• These changes increase a species fitness
(survival rate).
Natural Selection Does Not Design
Perfect Organisms
• Organisms are locked into historical
• Adaptations are often compromises as
with seals that need to swim and walk
• Not all evolution is adaptive
• Organisms can’t get a trait just because
they “need” it.