Download Variation Within a Population

Population Evolution
ch. 23
•
One misconception is that organisms evolve, in the Darwinian sense, during
their lifetimes
•
populations evolve
Natural selection acts on individuals, but only _____________________
•
Genetic variations
______________________
in populations contribute to evolution
•
Microevolution
_____________________
is a change in allele frequencies in a
population over generations
•
mutation and sexual reproduction produce the
Two processes, ____________________________,
variation in gene pools that contributes to differences among individuals
•
Variation
__________________
in individual genotype leads to variation in
individual phenotype
•
phenotypic
Not all _______________
variation is heritable
•
variation
Natural selection can only act on _____________with
a genetic
component
Fig. 23-1
Fig. 23-2a
(a)
Fig. 23-2b
(b)
Variation Within a Population
•
Both discrete and quantitative characters contribute to variation within a
population
•
Discrete characters
___________________
can be classified on an either-or basis
•
Quantitative characters vary along a continuum within a population
___________________
•
Population geneticists measure polymorphisms in a population by
amount of heterozygosity
determining the ____________________________
at the gene and
molecular levels
Variation Between Populations
•
geographic variation differences between gene
Most species exhibit __________________,
pools of separate populations or population subgroups
•
a graded change in some trait along a geographic axis
Cline - __________________________________________________
Fig. 23-3
Geographic variation in isolated mouse populations on Madeira
1
2.4
8.11
9.12
3.14
5.18
10.16 13.17
6
7.15
19
XX
1
2.19
3.8
4.16 5.14
9.10 11.12 13.17 15.18
6.7
XX
Figure 23.8 Clinal variation in a plant
variations within
a population
A cline determined by temperature
Fig. 23-4
1.0
0.8
0.6
0.4
0.2
0
46
44
Maine
Cold (6°C)
42
40
38
36
Latitude (°N)
34
32
30
Georgia
Warm (21°C)
Mutation
•
Mutations
_______________are
changes in the nucleotide sequence of DNA
•
genes and alleles to arise
Mutations cause new ___________________
•
gametes
Only mutations in cells that produce ______________
can be passed to
offspring
•
point mutation
A ____________________
is a change in one base in a gene
•
The effects of point mutations can vary:
–
harmless
Mutations in noncoding regions of DNA are often ______________
•
Chromosomal mutations that delete, disrupt, or rearrange many loci are
harmful
typically ________________
•
harmful
Duplication of large chromosome segments is usually ______________
•
animals and plants
Mutation rates are low in _______________________________
•
The average is about one mutation in every 100,000 genes per generation
•
lower
Mutations rates are often ______________
in prokaryotes and
higher
____________
in viruses
Sexual Reproduction
•
Sexual reproduction can shuffle existing alleles into new
_____________________
combinations
•
sexually
recombination
In organisms that reproduce _____________,
____________________
mutation in producing
of alleles
_________________
is more important than ___________
the genetic differences that make adaptation possible
The Hardy-Weinberg equation can be
used to test whether a population is evolving
•
population
A _________________
is a localized group of individuals capable of
fertile
interbreeding and producing _____________
offspring
•
gene pool
A _______________
consists of all the alleles for all loci in a population
•
A locus is fixed if all individuals in a population are homozygous for the
same allele
Fig. 23-5
One species, two
populations
Porcupine herd
MAP
AREA
Beaufort Sea
Porcupine
herd range
Fortymile
herd range
Fortymile herd
Hardy-Weinberg Principle
•
The frequency of an allele in a population can be calculated
–
diploid
For ___________
organisms, the total number of alleles at a locus is
x 2
the total number of individuals _______
•
p and q
By convention, if there are 2 alleles at a locus, _______________ are
frequencies
used to represent their _________________
•
The frequency of all alleles in a population will add up to 1
–
For example
p + q = 1
•
NOT evolving
The Hardy-Weinberg principle describes a population that is ____
•
not meet
If a population does ____________
the criteria of the Hardy-Weinberg
evolving
principle, it can be concluded that the population is ____________
Alleles
Hardy Weinbeg Equation: p + q = 1 p=A, q=a
p2 + 2pq + q2 = 1
AA
Aa
aa
Requirements for Hardy Weinberg Equilibrium:
1. no migration
2. no natural selection
3. no net mutations 4. large population
5. random mating
Hardy-Weinberg Equilibrium
•
frequencies
The Hardy-Weinberg principle states that _______________
of
alleles and genotypes
____________________________
in a population remain constant from
_____________________________
generation to generation
•
In a given population where gametes contribute to the next generation
NOT
randomly, allele frequencies will ________
change
•
preserves
Mendelian inheritance ______________
genetic variation in a population
•
Hardy-Weinberg equilibrium describes the constant frequency of alleles in
such a gene pool
•
If p and q represent the relative frequencies of the only two possible alleles
in a population at a particular locus, then
– p2 + 2pq + q2 = 1
homozygous
– where p2 and q2 represent the frequencies of the _____________
genotypes
heterozygous
– 2pq represents the frequency of the _________________
genotype
Conditions for Hardy-Weinberg Equilibrium
•
hypothetical
The Hardy-Weinberg theorem describes a _______________
population
•
DO
In real populations, allele and genotype frequencies ____change
over time
•
The five conditions for nonevolving populations are rarely met in nature:
•
–
No mutations
__________________________
–
Random mating
__________________________
–
No natural selection
__________________________
–
Extremely large population size
__________________________
–
No gene flow
__________________________
Natural populations can evolve at some loci, while being in Hardy-Weinberg
equilibrium at other loci
Natural selection, genetic drift, and gene flow can alter allele
frequencies in a population
•
Three major factors alter allele frequencies and bring about most
evolutionary change:
–
Natural selection
_____________________
–
Genetic drift
(small populations)
_____________________
–
Gene flow
(migration)
_____________________
Natural Selection
•
Differential success in reproduction results in certain alleles being passed
greater proportions
to the next generation in ______________________
Genetic Drift
• The smaller a sample, the greater the chance of deviation from a
predicted result
•
Genetic drift
__________________
describes how allele frequencies fluctuate
unpredictably from one generation to the next
•
reduce
loss
Genetic drift tends to __________
genetic variation through _______
of alleles
•
founder effect
The ________________
occurs when a few individuals become isolated
from a larger population
•
Allele frequencies in the small founder population can be different from
those in the larger parent population
•
bottleneck effect
The ________________
is a sudden reduction in population size due to a
change in the environment
•
The resulting gene pool may no longer be reflective of the original
population’s gene pool
•
If the population remains small, it may be further affected by genetic
drift
Genetic Drift
not in Hardy
Weinberg
equilibrium
p and q are not staying
constant throughout the
generations
Bottleneck Effect: the population undergoes a
dramatic decrease in size
- floods
- volcanoes
- ice ages
Case Study: Impact of Genetic Drift on the Greater Prairie Chicken
Loss of prairie habitat caused a severe reduction in the population of greater prairie
chickens in Illinois. The surviving birds had low levels of genetic variation,
and only 50% of their eggs hatched
Fig. 23-10a
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Range
of greater
prairie
chicken
•
Researchers used DNA from museum specimens to compare genetic variation in the
population before and after the bottleneck
•
The results showed a loss of alleles at several loci
•
Researchers introduced greater prairie chickens from population in other states
and were successful in introducing new alleles and increasing the egg hatch rate to
90%
Effects of Genetic Drift: A Summary
1.
small populations
Genetic drift is significant in _________________________
2.
random
Genetic drift causes allele frequencies to change at ____________
3.
genetic variation within populations
Genetic drift can lead to a loss of _______________
4.
fixed
Genetic drift can cause harmful alleles to become ___________
Gene Flow
•
movement
Gene flow consists of the ______________
of alleles among populations
•
Alleles can be transferred through the movement of fertile individuals or
gametes (for example, pollen)
•
reduce differences
Gene flow tends to __________________
between populations over time
•
Gene flow is more likely than mutation to alter allele frequencies directly
Fig. 23-11
Gene Flow
•
Gene flow can decrease the
fitness of a population
70
60
MINE
SOIL
NONMINE
SOIL
NONMINE
SOIL
50
Prevailing wind direction
40
30
20
10
0
20
0
20
0
20
40
60
80
100
120
140
160
Distance from mine edge (meters)
•
In bent grass, alleles for copper tolerance are beneficial in populations near copper
mines, but harmful to populations in other soils
•
Windblown pollen moves these alleles between populations
•
The movement of unfavorable alleles into a population results in a decrease in fit
between organism and environment
Natural selection is the only mechanism that
consistently causes adaptive evolution
•
adaptive evolution
Only natural selection consistently results in ______________________
•
Natural selection brings about adaptive evolution by acting on an
phenotype
organism’s ______________
Relative Fitness
•
The phrases “struggle for existence” and “survival of the fittest” are
misleading as they imply direct competition among individuals
•
Reproductive success is generally more subtle and depends on many
factors
•
Relative fitness
__________________
is the contribution an individual makes to the
__________________
of the next generation, relative to the contributions
gene pool
of other individuals
•
genotypes
phenotypes
Selection favors certain ______________
by acting on the ____________
of certain organisms
Directional, Disruptive, and Stabilizing Selection
•
Three modes of selection:
–
Directional selection
_____________________
favors individuals at one end of the
phenotypic range
–
Disruptive selection
_____________________
favors individuals at both extremes of
the phenotypic range
–
Stabilizing selection
_____________________
favors intermediate variants and acts
against extreme phenotypes
Original population
Original
Evolved
population population
(a) Directional selection
Phenotypes (fur color)
(b) Disruptive selection
(c) Stabilizing selection
The Key Role of Natural Selection in Adaptive Evolution
•
Natural selection increases the frequencies of alleles that enhance
survival and reproduction
_______________________________
•
Adaptive evolution occurs as the match between an organism and its
increases
environment _________________
•
Because the environment can change, adaptive evolution is a
continuous
_______________
process
•
to adaptive
Genetic drift and gene flow do not consistently lead ______________
evolution as they can increase or decrease the match between an organism
and its environment
Fig. 23-14b
Movable bones
Examples of adaptations
(b) Movable jaw
bones in
snakes
Fig. 23-14a
(a) Color-changing ability in cuttlefish
Sexual Selection
•
mating success
Sexual selection is natural selection for _____________________
•
sexual dimorphism
It can result in ___________________,
marked differences between
the sexes in secondary sexual characteristics
•
competition
Intrasexual selection is _______________
___________________
among individuals of one
sex (often males) for mates of the opposite sex
Intersexual selection
_______________________,
often called mate choice, occurs when
individuals of one sex (usually females) are choosy in selecting their mates
•
•
Male showiness due to mate choice can increase a male’s chances of
attracting a female, while decreasing his chances of survival
Fig. 23-15
The distinctive feature of this monkey is their protruding
nose. It’s not defined about the purpose of the large nose,
but it has been determined as the result of sexual
selection. The female Proboscis Monkey prefers big-nosed
male, thus propagating the trait.
The Preservation of Genetic Variation
•
Various mechanisms help to preserve genetic variation in a population
► Balancing Selection
•
stable
Balancing selection occurs when natural selection maintains _________
phenotypic forms
frequencies of two or more ________________________
in a population
► Diploidy
•
hidden recessive alleles
Diploidy maintains genetic variation in the form of ___________________
► Heterozygote Advantage
•
higher
Heterozygote advantage occurs when heterozygotes have a ___________
fitness
than do both homozygotes
•
Natural selection will tend to maintain two or more alleles at that locus
•
The sickle-cell allele causes mutations in hemoglobin but also confers malaria
resistance
► Frequency-Dependent Selection
•
declines
In frequency-dependent selection, the fitness of a phenotype _____________
if
too common
it becomes ________________
in the population
•
less common
Selection can favor whichever phenotype is _________________
in a population
Fig. 23-17
Frequencies of the
sickle-cell allele
0–2.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Fig. 23-18
Frequency-dependent
selection in scale-eating
fish (Perissodus microlepis)
“Right-mouthed”
1.0
“Left-mouthed”
0.5
0
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year