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Evolution of Populations
•What
are the three ways that natural
selection can change the distribution of a
trait?
•How does genetic drift lead to the
evolution of populations?
Video Explanation
The Making of the Fittest
Series HHMI
“Natural Selection in Humans”
Individuals in Populations with
Beneficial Genetic Traits Can Leave
More Offspring

1.
STEPS to EVOLUTION
Development of genetic variations
◦
2.
Occurs through mutations in reproductive cells
Natural selection
◦
◦
◦
Acts on individuals
Genetically based traits enhance your ability to
survive & reproduce
For natural selection to occur, a trait must be
heritable & lead to differential reproduction
Summary of Biological Evolution
Genes mutate
 Individuals are selected
 Populations evolve

◦ Are now better adapted to survive &
reproduce under existing environmental
conditions
Evolution by Natural Selection
3 Choices!

When environmental conditions change,
populations
◦ Adapt
◦ Migrate
◦ Become extinct
Natural Selection in Populations

Traits are normally distributed in a bellshaped curve
Microevolution is the change in the
frequencies of alleles over time
 The frequencies can change in one of
three ways

Natural Selection in Populations

Directional Selection occurs when one
extreme phenotype is favored
Natural Selection in Populations

Stabilizing Selection occurs when
intermediate phenotypes are favored
Natural Selection in Populations

Disruptive Selection occurs when both
extreme phenotypes are favored
This is an example of disruptive selection
because one phenotype is favored on the lava
flows and the other is favored on the sandstone
rocks.
Video Explanation
The Making of the Fittest
Series HHMI
“Natural Selection and Adaptation”
Other Mechanisms of Evolution

Gene flow – movement of alleles between
populations (migration)
◦ Increase in movement, increases genetic
variation
◦ Decrease in movement, decreases genetic
variation but increases evolution of new
species
Loss of Genetic Variation
The original population is on the left with three
possible founder populations on the right.
Other Mechanisms of Evolution
 Genetic
drift
◦ Changes in allele frequency due to chance
◦ Affects small populations more than large
populations
◦ Two causes:
 Bottleneck effect – event reduces size of
population
 Founder effect – small number of individuals
colonize a new area
Founder Effect and Ellis-van Creveld
Syndrome
Other Mechanisms of Evolution
 Sexual
Selection occurs when
traits increase mating success
◦ Intrasexual selection is between
males of same species
Intersexual selection occurs when
males display traits that attract the
female