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Chapter 15
• How Organisms Evolve
How Are Populations, Genes, and
Evolution Related?
• 1. The determination of a trait in diploid
individuals depends on
• A. _________________________:
–
–
–
–
__________
__________
Incomplete dominance
Sex-linked
• B. __________
– The changes an individual experiences as it
grows and develops are not evolutionary
changes
Interaction between alleles
• For example, coat color in hamster is
determined by 2 alleles for pigment:
– B  black, __________
– b  brown, __________
How Are Populations, Genes, and
Evolution Related?
• 2. Evolution is a property of
__________ , not individuals.
–Evolutionary changes:
• Occur from generation to
generation
• Cause descendants to _____ from
their ______________
• Occur at the population level
How Are Populations, Genes, and
Evolution Related?
• 3. Can we measure evolution?
– _____, if we can measure allele change in the gene pool
of a population.
GENE ________:
– ________________________
– Allele frequency (__________ ) = specific allele number/ Total
number of possible alleles in a population
– Evolution Is the _________________________________
Within a Population. In order to measure change we
need a point of reference
The Gene Pool
• For example, coat
color in hamsters:
– A population of 25
hamsters contains 50
alleles of the coat color
gene (hamsters are
diploid)
– If 20 of those 50 alleles
code for black coats,
then the frequency of
the black allele is_____
_____________
The Equilibrium Population
•
•
4. What would be required for a population to
be in __________ (without evolving)?
________________ proposed that, given the
following conditions, the frequencies of alleles
and genotypes in a sexually reproducing
population remain constant from one
generation to the next
1.
2.
3.
4.
5.
,
.
.
.
.
5.
What Causes Evolution?
Answer: the opposite of equilibrium.
Violation of one or more of these five
conditions may allow changes in allele
frequencies
Equilibrium requires:
•a. No mutation
•b. No gene flow
between populations
•c. Large population
•d. Random mating
•e. No natural selection
Evolution requires:
Mutations
• Mutations are _______________
___________________ in a gene
– Usually have _____ gametes) little
effect on Hardy-Weinberg proportions
of common alleles
– Are the __________________ .
Sources of genetic variability.
– In gamete offspring.
– Can be ________________________
– _____________ ______ . Arise
spontaneously, not as a result of, or in
anticipation of, environmental
necessity
Gene Flow: migration
– ___________________
from one population to
another.
• New alleles can be
transferred
• Tend to ____________
alleles in separate
populations (reduce the
differences in the gene
pools).
• __________ of gene flow
may create __________
Allele Frequencies Drift
• ________ ______ is the random change in
allele frequencies over time, brought about
by chance alone
– Has minor impact in very large populations
– Occurs more rapidly and has bigger effect on
__________ __________
Genetic Drift. In small populations
(a) Population size = 10,000
(b) Population size = 4
1.0
1.0
0.9
0.9
frequency of allele A
• Frequencies of particular alleles may
change by chance alone.
0.8
0.7
0.6
0.8
0.7
0.6
• Important in small populations.
0.5
0.5
– Bottleneck Effect - Drastic reduction
in population, and
0.4
gene pool size.
0.3
– Founder Effect - Few individuals
0.2 found new population
(Small allelic pool).
0.1
0.4
0.3
0.2
0.1
0.0
0.0
0
1
2
3
generation
4
5
6
0
1
2
3
generation
The effect of population size on genetic drift
4
5
6
Causes of Genetic Drift
• There are two causes of
genetic drift:
• A. Population
__________ . Drastic
reduction in population
size brought about by a
natural __________ or
__________
– It can change allele
frequencies and reduce
genetic variation
Population Bottleneck
• Northern elephant seal
–
–
–
–
Hunted almost to extinction in the 1800s
By 1890s, only 20 individuals remained
Hunting ban allowed population to increase to 30,000
Biochemical analysis shows that present-day northern
elephant seals are almost __________ __________
Causes of Genetic Drift
• B. _______ _____ . It occurs when a small
number of individuals leave a large
population and establish a __________
population
Isolated Founding Populations
• By chance, allele frequencies of founders
may differ from those of original population
• Over time, new population may exhibit
allele frequencies that _____ from ______
population
Non-random mating
• Within species:
• Sometimes choosing the
__________ (snow geese)
• Sometimes choosing the
__________
• Sometimes choosing the
most __________
Natural and Sexual Selection
• __________ - Nature exerts
selection.
• __________ must exist among
individuals.
• Variation must result in differences
in __________ of viable offspring
produced.
• Variation must be genetically
__________ .
– Natural Selection is a __________ ,
and Evolution is an __________ .
How Natural and Sexual
Selection Work
• Natural selection is often associated with
the phrase “survival of the fittest”
• The fittest individuals are those that not
only survive, but are able to __________
____________________
Success of Phenotypes
• Successful phenotypes are those that have
the best adaptations to their present
environment
– __________ are characteristics that help an
individual __________ and reproduce
• Adaptations arise from the __________ of
organisms with both the _______ and ____
parts of their environments
The Environment
• Nonliving (abiotic) components
include:
–
–
–
–
–
__________
Availability of __________
Concentration of _______ in the ___
Living (biotic) components include:
Interactions with other organisms:
• __________
• __________
• Sexual __________
Agents of Selection
• ____________ is an interaction among
individuals who attempt to utilize a _______
__________
– May be between individuals of ______ species
or __________ species
– Most intense among members of the _______
species
Agents of Selection
• __________ is the
evolution of __________ in
two species due to their
extensive interaction
– e.g. __________
relationships, akin to a
“biological arms race”
• Wolf predation selects against
slow, careless deer
• Alert, swift deer select against
slow, clumsy wolves
Sexual Selection
• Sexual selection is a type of natural
selection that favors traits that help an
organism acquire a __________
– __________ features (bright colors, long
feathers or fins, elaborate antlers)
– Bizarre courtship __________
– Loud, complex courting __________
• Traits derived by sexual selection make
males more __________ to predators
Sexual Selection
• Male-male competition for access to
females
– ________ evolution of features that provide an
__________ in fights or ritual __________ of
aggression
Sexual Selection
• Female mate choice
– Male structures, _______
, and displays that do not
enhance survival might
provide an outward sign
of a male’s _______ and
vigor
Selection Influences Populations
• Natural selection and sexual selection can
affect populations in three ways:
– __________ selection
– __________ selection
– __________ selection
BEFORE
SELECTION
– Selection __________ ___
______ from a phenotypic
array.
• e.g. pesticide resistance,
antibiotic resistance
time
• Directional Selection
percent of population
Forms of Natural
Selection
AFTER
SELECTION
range of a particular characteristic (siz
STABILIZING SELECTION
Forms of Natural
Selection
• Stabilizing Selection
– __________ ________ is favored
– Selection acts to eliminate both
extremes.
– Phenotypic __________ _______
– Environmental conditions are
relatively constant
– e.g. body size in Aristelliger lizards
• Smallest lizards have a difficulty
defending territory
• Largest lizards more likely to be
eaten by owls
• __________
Selection
Forms of Natural
Selection
– Selection acts to _____________
_______
and eliminate the
average.
– Environment has more than one
type of useful resource:
Population divides into ____
__________ _________ over time.
• e.g. beak size in black-bellied
seedcrackers
– Birds with _____ beaks eat ____
seeds
– Birds with _____ beaks eat _____
seeds
DIRECTIONAL SELECTION
STABILIZING SELECTION
percent of population
time
BEFORE
SELECTION
AFTER
SELECTION
range of a particular characteristic (size, color, etc.)
DISRUPTIVE SELECTION
Notes on selection:
A. Evolution is a
compromise between
_______________ …
• Not all genotypes
changes are
beneficial.
– i.e. giraffe’s neck:
• Larger can help on a
fight
• Can make it
vulnerable
Notes on selection:
B. Antibiotic _________
Evolves by Natural
Selection.
• Penicillin kills most
Bacteria. The ones
with penicillinresistant mutation
survive and
reproduce.
Notes on selection:
• C. ______selection: __________ and Selective
__________ . Some individuals may enhance the
survival of individuals from their own species.
Notes on Selection
D. __________ Selection
• __________ exert selection.
Notes on Selection
E. __________ __________ is the prolonged
maintenance of two or more alleles in a
population
• Balanced polymorphism often occurs when
environmental conditions favor
__________
– e.g. normal and sickle-cell hemoglobin alleles
in malaria-prone regions of Africa
HH
Hh
hh
Balanced Polymorphism
dies of malaria
lives and
reproduces
dies of
sickle-cell anemia
HH
Hh
hh
dies of malaria
lives and
reproduces
dies of
sickle-cell anemia
HH
Hh
hh
Heterozygote advantage
• Heterozygote
advantage (balanced
polymorphism)