Transcript Chapter 11: The Evolution of Populations

Section One: Genetic Variation
within Populations
 Genetic Variation
 Increases the chance of
survival
 Can come from mutations
or through recombination
 Gene pool: the combined
alleles of all the
individuals in a population

Allele frequency: the
measure of how common a
certain allele is in the
population
Section Two: Natural Selection in
Populations
 Distribution of Traits
 Normal distribution
 Directional selection
 Stabilizing selection
 Disruptive selection
 All can lead to microevoluion: the observable change in
allele frequencies of a population over time
Section Two: Natural Selection in
Populations
 Normal Distribution
 The allele frequency is highest near the mean value and
decreases toward each extreme end of the range
Section Two: Natural Selection in
Populations
 Directional Selection
 A type of selection that favors phenotypes at one
extreme of a trait’s range
Section Two: Natural Selection in
Populations
 Stabilizing selection
 The intermediate phenotype is favored and becomes
more common in the population
Section Two: Natural Selection in
Populations
 Disruptive selection
 Both extreme phenotypes are favored, while individual
with intermediate phenotypes are selected against by
something in nature
Section Three: Other Mechanisms
of Evolution
 Gene Flow
 The movement of alleles
from one population to
another



Occurs when animals
move into or out of a
population
Effects allele frequency
Increases genetic variation
Section Three: Other Mechanisms
of Evolution
 Genetic Drift
 A change in allele
frequencies due to
chance alone, occurring
most commonly in
small populations

Types


Bottleneck Effect
Founder Effect
Section Three: Other Mechanisms
of Evolution
 Genetic Drift
 Bottleneck Effect:
genetic drift that occurs
after an event greatly
reduces the size of the
population
Section Three: Other Mechanisms
of Evolution
 Genetic Drift
 Founder Effect: genetic drift that occurs after a small
number of individuals colonize a new area

New population is not representative of the original
Section Three: Other Mechanisms
of Evolution
 Sexual selection
 When certain traits increase mating success


Intrasexual: competition between males
Intersexual: competition where males display certain traits
that attract the female
Section Five: Speciation Through
Isolation
 Isolation can lead to
speciation
 Speciation: the rise of
two or more species
from one existing
species
 Reproductive isolation:
when members of
different populations
can no longer mate
successfully with one
another.
Section Five: Speciation Through
Isolation
 Population Isolation
 Behavioral Isolation

Isolation caused by differences in courtship or mating
behaviors
 Geographic Isolation

Physical barriers that divide a population into two or more
groups
 Temporal Isolation

Timing prevents reproduction between populations
Section Five: Speciation Through
Isolation
Section Six: Patterns in Evolution
 Evolution through Natural
Selection
 Convergent Evolution
 Evolution toward similar
characteristics in unrelated
species
 Such as analogous
structures
 Ex. Bat wings and bird
wings
 Divergent Evolution
 Closely related species
evolve in different
directions and become
increasingly different
Section Six: Patterns of Evolution
 Coevolution
 The process in which
two or more species
evolve in response to
changes in each other

Ex: flowers and pollinators
Section Six: Patterns of Evolution
 Extinction
 The elimination of a species from Earth
 Background extinctions



Usually only affect one or a few species
Occur continuously but at a very low rate
Part of the cycle of earth
 Mass extinction
 Much more rare than background
 Have a global effect
 Destroy many species
 Ex: Dinosaur extinction
Section Six: Patterns of Evolution
 Speciation through
Patterns
 Punctuated Equilibrium
 Episodes of speciation
occur suddenly in geologic
time and are followed by
long periods of little
evolutionary change
 Adaptive Radiation
 The diversification of one
ancestral species into
many descendant species