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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. Interaction between alleles:
–
–
–
–
Dominant-recessive
Codominant
Incomplete dominance
Sex-linked
• B. Environment
– 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, dominant
– b  brown, recessive
How Are Populations, Genes, and
Evolution Related?
• 2. Evolution is a property of
populations, not individuals.
–Evolutionary changes:
• Occur from generation to
generation
• Cause descendants to differ from
their ancestors
• Occur at the population level
How Are Populations, Genes, and
Evolution Related?
• 3. Can we measure evolution?
– Yes, if we can measure allele change in the gene pool of
a population.
GENE POOL:
– Sum of all genes in a population
– Allele frequency (proportion) = specific allele number/ Total
number of possible alleles in a population
– Evolution Is the Change Over Time of Allele Frequencies
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 0.40
or 40% [20/50 = 0.40]
The Equilibrium Population
•
•
4. What would be required for a population to
be in Equilibrium (without evolving)?
Hardy-Weinberg 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.
No mutation
No gene flow between populations
Population must be very large
Mating must be random
No natural selection
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:
Evolution requires:
•a. No mutation
•b. No gene flow
between populations
•c. Large population
•d. Random mating
•e. No natural selection
•a. mutation
•b. gene flow between
populations
•c. Small population
•d. Selective mating (nonrandom)
•e. Natural Selection of
individuals (the fittest)
Mutations
• Mutations are rare changes in the
base sequence of DNA in a gene
– Usually have little or no immediate
effect (1 in 100,000-1,000,000
gametes) little effect on HardyWeinberg proportions of common
alleles
– Are the source of new alleles.
Sources of genetic variability.
– In gamete offspring.
– Can be beneficial, harmful, or neutral
– Not goal-directed. Arise
spontaneously, not as a result of, or in
anticipation of, environmental
necessity
Gene Flow: migration
– Movement of alleles from
one population to
another.
• New alleles can be
transferred
• Tend to homogenize
alleles in separate
populations (reduce the
differences in the gene
pools).
• Blockage of gene flow
may create new species
Allele Frequencies Drift
• Genetic 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
small populations
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
bottleneck. Drastic
reduction in population
size brought about by a
natural catastrophe or
over-hunting
– 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 genetically identical
Causes of Genetic Drift
• B. Founder effect. It occurs when a small
number of individuals leave a large
population and establish a new isolated
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 differ from original
population
Non-random mating
• Within species:
• Sometimes choosing the
similar (snow geese)
• Sometimes choosing the
strongest
• Sometimes choosing the
most attractive
Natural and Sexual Selection
• Natural - Nature exerts
selection.
• Variation must exist among
individuals.
• Variation must result in differences
in numbers of viable offspring
produced.
• Variation must be genetically
inherited.
– Natural Selection is a process, and
Evolution is an outcome.
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 leave the most
offspring behind
Success of Phenotypes
• Successful phenotypes are those that have
the best adaptations to their present
environment
– Adaptations are characteristics that help an
individual survive and reproduce
• Adaptations arise from the interactions of
organisms with both the nonliving and living
parts of their environments
The Environment
• Nonliving (abiotic) components
include:
– Climate
– Availability of water
– Concentration of minerals in the soil
• Living (biotic) components
include:
– Interactions with other organisms:
• Competition
• Coevolution
• Sexual selection
Agents of Selection
• Competition is an interaction among
individuals who attempt to utilize a limited
resource
– May be between individuals of same species
or different species
– Most intense among members of the same
species
Agents of Selection
• Coevolution is the
evolution of adaptations in
two species due to their
extensive interaction
– e.g. predator-prey
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 mate
– Conspicuous features (bright colors, long
feathers or fins, elaborate antlers)
– Bizarre courtship behaviors
– Loud, complex courting songs
• Traits derived by sexual selection make
males more vulnerable to predators
Sexual Selection
• Male-male competition for access to
females
– Favors evolution of features that provide an
advantage in fights or ritual displays of
aggression
Sexual Selection
• Female mate choice
– Male structures, colors,
and displays that do not
enhance survival might
provide an outward sign
of a male’s health and
vigor
Selection Influences Populations
• Natural selection and sexual selection can
affect populations in three ways:
– Directional selection
– Stabilizing selection
– Disruptive selection
BEFORE
SELECTION
– Selection eliminates one
extreme 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
– Intermediate phenotype is favored
– Selection acts to eliminate both
extremes.
– Phenotypic variability declines
– 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
• Disruptive
Selection
Forms of Natural
Selection
– Selection acts to promote both
extremes and eliminate the
average.
– Environment has more than one
type of useful resource:
Population divides into two
phenotypic groups over time.
• e.g. beak size in black-bellied
seedcrackers
– Birds with large beaks eat hard
seeds
– Birds with small beaks eat soft
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
opposing pressures…
• 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 Resistance
Evolves by Natural
Selection.
• Penicillin kills most
Bacteria. The ones
with penicillinresistant mutation
survive and
reproduce.
Notes on selection:
• C. Kin selection: Altruism and Selective
cannibalism. Some individuals may enhance the
survival of individuals from their own species.
Notes on Selection
D. Artificial Selection
• Breeders exert selection.
Notes on Selection
E. Balanced polymorphism is the
prolonged maintenance of two or more
alleles in a population
• Balanced polymorphism often occurs when
environmental conditions favor
heterozygotes
– 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)