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15.3 Notes (pg. 92)
Section 3: Shaping Evolutionary Theory
• E.Q.- What are the four types of natural
selection? Give an example of two types.
Chapter 15
Evolution
Is it possible for no change in a population?
 Population genetics
 Hardy-Weinberg principle states that when
allelic frequencies remain constant, a population
is in genetic equilibrium.
15.3 Shaping Evolutionary Theory
Hardy-Weinberg principle is based on five
conditions
1. Population must be large.
2. No immigration or emmigration
3. Mating must be random
4. Mutations are not allowed.
5. Natural Selection is not allowed.
Chapter 15
Evolution
Genetic Drift
 A change in the allelic frequencies in a
population that is due to chance.
(natural disaster, flood, etc….)
Small populations are
affected easily/quickly
Large populations are
less likely to be affected
by genetic drift.
Chapter 15
Evolution
Founder Effect
 Occurs when a small sample of a population
settles in a location separated from the rest
of the population
 Alleles that were uncommon in the original
population might be common in the new
population.
Amish population (sixth finger)
Chapter 15
Evolution
Gene Flow
 Genes moving from one population to another.
• What is immigration
and emigration?
I = into
E = exiting
Nonrandom Mating
 Promotes inbreeding and could lead to a
change in allelic proportions favoring individuals
that are homozygous for particular traits
15.3 Did You Get it?
1. Identify two parts of the HW principle and
explain why it won’t happen in nature.
2. Write a question using your notes.
(include answer)
Chapter 15
Evolution
Natural Selection
 Acts to select the
individuals that
are best adapted
for survival and
reproduction
 4 types
Chapter 15
Evolution
First type of Natural Selection
 Stabilizing selection operates to eliminate
extreme expressions of a trait when the
average expression leads to higher fitness.
Chapter 15
Evolution
Second type of Natural Selection
 Directional selection
 Individuals with one extreme survive,
resulting in a change in the population.
Chapter 15
Evolution
Third type of Natural Selection
 Disruptive selection
 Individuals with either extreme survive,
resulting in a split of the population.
Chapter 15
Evolution
Fourth type of Natural Selection
 Sexual selection
• Males in a population have very distinct
traits to attract a mate.
 Males appearance
can sometimes
attract a mate but
at the same time
make it harder to
survive.
15.3 Did You Get it?
3. Identify the four types of Natural Selection
(draw graph for each)
4. Using your notes, write a question
(include answer)
Chapter 15
Evolution
Types of Reproductive Isolation
 Prezygotic isolation
prevents reproduction
by making fertilization
unlikely.
Different mating seasons,
or different mating calls.
 Prevents genotypes
from entering a
population’s gene pool
Eastern meadowlark and Western meadowlark
Chapter 15
Evolution
Types of Reproductive Isolation
 Postzygotic isolation occurs when fertilization
has occurred but
a hybrid offspring
cannot develop
or reproduce.
 Prevents offspring
survival or
reproduction
Liger
mule
Chapter 15
Evolution
Types of speciation
Allopatric Speciation (Geographic Isolation)
 A physical barrier divides one population
into two or more populations.
Abert squirrel
Kaibab squirrel
Chapter 15
Evolution
Types of speciation
Sympatric Speciation
 A species evolves into a new species without
a physical barrier.
 The ancestor species and the new species live
side by side during the speciation process.
Chapter 15
Evolution
Types of speciation
Adaptive Radiation
 Can occur in a relatively short time when one species
gives rise to
many different
species in
response to the
creation of new
habitat or some
other ecological
opportunity
 Follows large-scale extinction events
Chapter 15
Evolution
Types of speciation
Coevolution
 The relationship between two species might
be so close that the evolution of one species
affects the evolution of the other species.
 Mutualism
Chapter 15
Evolution
Types of speciation
Convergent Evolution
 Unrelated species
evolve similar traits
even though they
live in different parts
of the world.
Chapter 15
Evolution
Rates of Speciation
 Punctuated equilibrium
explains rapid spurts of
genetic change causing
species to diverge
quickly.
 Evolution proceeds in
small, gradual steps
according to a theory
called gradualism.
15.3 Did You Get it?
5.
Chapter 15
Evolution
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Chapter 15
Evolution
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