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Evolution
11.1 – Genetic Variation
Within Populations
 Key Concept:
 A population shares a common gene pool.
Genetic variation in a population increases the
chance that some individuals will survive.
 Genetic variation leads to phenotype variation (different coat
color, hair color)
 Necessary for natural selection
 Genetic variation is stored in a population’s gene pool
 Made up of all the alleles of all individuals in a population
 Allele frequency: a measure of how common a certain allele is in a
population. Can be impacted by natural selection.
Genetic variation comes from
several sources.
 Mutations
 Can form a new allele
 Passed to offspring
if in a gamete
 Recombination
 Usually occurs during meiosis
 Parents’ alleles rearranged during gamete formation
11.2 – Natural Selection
in Populations
 Key Concept:
 Populations, not individuals, evolve.
Microevolution
 Evolution within a population
 Observable change in allele frequencies
 Can result from natural selection
 Types:
 Directional selection
 Stabilizing selection
 Disruptive selection
Directional Selection
 Favors phenotypes at one extreme
Stabilizing Selection
 Favors the intermediate phenotype
Disruptive Selection
 Favors both extreme phenotypes
 http://www.youtube.com/watch?v=aTftyFboC_M
 (From 7.35 – Sexual Selection)
11.3 – Other mechanisms of
Evolution
 Key Concept:
 Natural selection is not the only mechanism through
which populations evolve.
Gene Flow
 Movement of alleles between populations
 Occurs when individuals
join new populations
and reproduce

Their alleles become part of
gene pool
 Keeps neighboring
populations similar
 Low gene flow
increases the chance
that two populations
will evolve into different
species
bald eagle migration
Genetic Drift
 Change in allele frequencies due to chance
 Causes a loss of genetic diversity in a population
 Common in small populations
 Bottleneck Effect is genetic
drift after a bottleneck event
 Occurs when an event
drastically reduces population size
 Founder Effect is genetic drift that occurs after the
start of a new population
 Occurs when a few individuals start a new population
Sexual selection occurs when certain
traits increase mating success.
 Sexual selection
 Occurs due to higher cost of reproduction for females
 Males produce sperm continuously
 Females are more limited in potential offspring each cycle
 Two types:
 Intrasexual selection: competition among males
 Intersexual selection: males display certain traits to females
 http://www.youtube.com/watch?v=aTftyFboC_M
 (From Sexual Selection to the end)
11.5: Speciation through Isolation
 Key Concept: New species can arise when populations
are isolated.
 Several kinds of barriers can prevent mating between
populations, leading to reproductive isolation. These are
all known as pre-zygotic isolation.
 Behavioral isolation: differences in courtship or mating
behaviors. (Different song)
 Geographic isolation: physical barriers that divide a
population into two or more groups. (See pic) also called
allopatric speciation.
 Temporal isolation: timing prevents reproduction between
populations. (Different pollination times in trees)
 https://www.youtube.com/watch?v=2oKlKmrbLoU
 (10 min Speciation Crash Course Video – Have to click on
speciation)
 https://www.youtube.com/watch?v=11iYk0Yrx3g
 (CP – start at 6:50)
 Quick Intro to speciation
 https://www.youtube.com/watch?v=Q2vsG77PZ80
11.6 – Patterns in Evolution
 Key Concept:
 Evolution occurs in patterns.
Species can become extinct.
 Extinction: elimination of a species from Earth
 Background extinction
 Mass extinction
Background Extinction
 Occur randomly, but at a low rate
 Usually affect only a few species
in a small area
 Can by caused by local changes
in the environment
Mass Extinction
 Rare, but very intense
 Can operate at a global level
 Caused by a catastrophic event such as an ice age
 At least 5 mass extinctions in the last 600 million years
Extinction
 Species go extinct because they lack the variation
needed to adapt