Download Chapter 17: Evolution of Populations

Chapter 17:
Evolution of Populations
Section 17-2: Evolution as Genetic
Change in Populations
How Natural Selection Works
• Evolutionary fitness = success in passing on
genes
• Evolutionary adaptation = any genetically
controlled trait that increases an organism’s
ability to pass along its alleles
Natural Selection on Single-Gene
Traits
• Changes allele frequencies
• Ex: Body color in lizards
Natural Selection on Polygenic
Traits
• Range of phenotypes
• Fitness varies throughout the curve
• Natural selection can affect phenotype range,
changing shape of bell curve
Directional Selection
• Occurs when individuals at one end of the curve
have higher fitness than individuals in the
middle or at the other end
Stabilizing Selection
• Occurs when individuals near the center of the
curve have higher fitness than individuals at
either end
• Keeps center of curve, narrows overall graph
Disruptive Selection
• Occurs when individuals at the upper and lower
ends of the curve have higher fitness than
individuals near the middle
• Acts against the intermediate phenotype, can
create two distinct phenotypes
Genetic Drift
• Occurs in small populations
• Allele becomes more or less common by chance
• Random
Genetic Bottlenecks
• The bottleneck effect is a change in allele
frequency following a dramatic reduction in
population size
• Ex: Natural disaster
The Founder Effect
• Occurs when allele frequencies change as a
result of the migration of a small subgroup of a
population
• Can create new populations very different from
original group
The Founder Effect
Evolution vs Genetic Equilibrium
• If allele frequencies in a population remain the
same it is in genetic equilibrium
• No evolution
• The Hardy-Weinberg principle describes the
conditions under which evolution will not occur
• States that allele frequencies will remain
constant unless something causes them the
change
Hardy-Weinberg Principle
• 5 conditions can disturb genetic equilibrium and
cause evolution:
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Nonrandom mating – sexual selection
Small population size
Immigration or emigration
Mutations
Natural selection
Sexual Reproduction and Allele
Frequency
• Sexual reproduction alone does not
change relative allele frequency