Download Chapter 23: The Evolution of a Population

Chapter 23: The Evolution of a
Population
Population
Group of individuals in a geographic location, capable of
interbreeding and producing fertile offspring
Natural selection acts on individuals,
but only populations evolve
Galapagos Islands
• Population of medium ground
finches on Daphne Major
Island
– During a drought, largebeaked birds were more
likely to crack large seeds
and survive
– Evolution by natural
selection
Types of Evolution
• Microevolution= change in the gene pool of a
population over many generations
• 4 Methods of Microevolution
– Mutations
– Natural Selection
– Genetic Drift= chance events cause genetic changes
from one population to the next
– Gene Flow= individuals or gametes move to a
different population
Genetic Drift
• Genetic drift describes how allele frequencies
fluctuate unpredictably from one generation to
the next
– Bottleneck Effect= event kills a large number of
individuals and only a small subset of population is left
– Founder Effect= Small number of individuals colonize
new location
Case Study: Impact of Genetic Drift on
the Greater Prairie Chicken
• Loss of prairie habitat= severe reduction in the population
of greater prairie chickens in Illinois
– Low levels of genetic variation, only 50% of their eggs hatched
Pre-bottleneck
(Illinois, 1820)
Greater prairie chicken
Range
of greater
prairie
chicken
Post-bottleneck
(Illinois, 1993)
Case Study: Impact of Genetic Drift on
the Greater Prairie Chicken
• Researchers used DNA from museum specimens
to compare genetic variation in the population
before and after the bottleneck
• Results showed a loss of alleles at several loci
Figure 23.11b
Location
Illinois
1930–1960s
1993
Population
size
Number Percentage
of alleles of eggs
per locus hatched
1,000–25,000
<50
5.2
3.7
93
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
Case Study: Impact of Genetic Drift on
the Greater Prairie Chicken
• Researchers introduced greater prairie chickens
from population in other states
• Introduced new alleles into population
• Increased the egg hatch rate to 90%
Genetic Drift: Summary
1. Genetic drift is significant in small
populations
2. Genetic drift causes allele frequencies to
change at random
3. Genetic drift can lead to a loss of genetic
variation within populations
4. Genetic drift can cause harmful alleles to
become fixed
Gene Flow
• Gene flow = movement of alleles among
populations
• Alleles can be transferred through:
– Movement of fertile individuals
– Gametes
• Gene flow tends to reduce variation among
populations over time
• Barriers to dispersal can limit gene flow
between populations
Geographic Variation
• Most species exhibit geographic variation
– Differences between gene pools of separate populations
• Mice in Madeira
– Island in Atlantic Ocean
– Several isolated populations of non-native mice
• Mountain range prevents gene flow
– Fusion of chromosomes
Geographic Variation
Variation among
populations is due to
drift, not natural
selection
Geographic Variation
Cline= graded
change in a trait
along a geographic
axis
Effect of natural
selection
Mummichog Fish and Cold-Adapted Allele
Natural Selection
• Evolution by natural selection involves both change
and “sorting”
– New genetic variations arise by chance
– Beneficial alleles are “sorted” and favored by natural
selection
• Only natural selection consistently results in adaptive
evolution
Evolution
• Evolution by natural selection is possible because
of genetic variation in population
– Gene pool= all the alleles for all loci in a population
• Genetic Variation = differences in DNA sequences
– Gene variability
• Average heterozygosity= average percent of loci that are
heterozygous in a population
• Fixed loci= all individuals in a population have same allele
– Nucleotide variability
• Measured by comparing the DNA sequences of pairs of
individuals
Evolution
• Phenotype= product of inherited genotype +
environmental influences
– Discrete characters= classified on an either-or basis
• Flower color: red or white
– Quantitative characters= vary along a continuum
within a population
• Skin color in humans
• Natural selection can only act on variation with a
genetic component
Evolution
• Genetic variation primarily comes from 2
sources:
1. Mutation and Gene Duplication
•
•
Original source of new alleles or genes
May be neutral before it becomes an advantage
• “raw material” of evolution
2. Sexual Reproduction= unique combination of
genes following crossing over, independent
assortment of chromosomes, and random
fertilization
Evolution
• Genetic drift and gene flow do not consistently lead
to adaptive evolution
– Can increase or decrease the match between an
organism and its environment
• Natural selection increases the frequencies of
alleles that enhance survival and reproduction
– Adaptive evolution occurs as the match between an
organism and its environment increases
• Because the environment can change, adaptive
evolution is a continuous process
Evolution of a Population
• Types of Natural Selection
– Directional Selection
• Highest reproduction in one
extreme phenotype
– Stabilizing Selection
• Highest reproduction of
intermediate phenotypes
– Disruptive Selection
• Highest reproduction of two
extreme phenotypes
1
Generations
2
3
Frequency of
individuals
Figure 23.13
Original
population
Evolved
population
(a) Directional selection
Original population
Phenotypes (fur color)
(b) Disruptive selection
(c) Stabilizing selection
Sexual Selection
• Form of natural selection
• Individuals with certain traits are more likely to mate
• Sexual Dimorphism= differences in appearance of
males and females
• Vertebrates= males usually “showier” of sexes or
engage in competition for
females
• Characteristics may be
disadvantage
– Male birds with bright feathers
more obvious to predators
Sexual Selection
• Intrasexual selection=
competition among
individuals of one sex
(often males) for mates of
the opposite sex
• Intersexual selection (mate
choice)= individuals of one
sex (usually females) are
choosy in selecting their
mates
Sexual Selection
• How do female preferences evolve?
• Good genes hypothesis= if a trait is related to
male health, both the male trait and female
preference for that trait should increase in
frequency
Example of Sexual Selection
• Females select males based on traits indicating
defenses against parasites and pathogens
– Bird, Mammal, and Fish Species
• Female stickleback fish and Major Histocompatibility
Complex (MHC)
• Higher reproductive
success increases
frequency of defense
trait in next generation
Genetic Variation in Populations
• Neutral variation= genetic variation that does
not confer a selective advantage or
disadvantage
• Various mechanisms help to preserve genetic
variation in a population
• Diploidy= maintains genetic variation in the
form of hidden recessive alleles
– Heterozygotes can carry recessive alleles that are
hidden from the effects of selection
Genetic Variation in Populations
• Balancing selection= natural selection maintains
stable frequencies of two or more phenotypic forms
in a population
• Balancing selection includes
– Heterozygote advantage
– Frequency-dependent selection
Heterozygote Advantage
• Malaria caused by protist,
Plasmodium
• 1-2 million people die/yr
from disease
• Modifies red blood
cells to obtain
nutrients, escape
destruction by
spleen
Heterozygote Advantage
• Sickle-Cell Allele: recessive allele
– Produces abnormal hemoglobin
proteins
Normal Blood Cell
Sickle-Cell
Heterozygote Advantage
• Homozygous Dominant
– No sickle cell allele
– Susceptible to malaria
• Homozygous Recessive
– Sickle-cell disease
– Resistant to malaria
• Heterozygotes= co-dominant
alleles, both types of blood cells
– Heterozygotes have decreased
symptoms of malaria and
decreased symptoms of sickle-cell
disease
– Higher survival
Frequency-Dependent Selection
• Fitness of a phenotype declines if it becomes
too common in the population
• Selection can favor whichever phenotype is
less common in a population
• Example: Predators can form a “search image”
of their prey
– Most common phenotype
– Rare phenotypes may avoid detection by
predators, increasing survival and reproduction
Limits of Natural Selection
• Selection can only act on existing variation in a
population.
– New alleles do not appear when needed
• Evolution is limited by historical constraints.
– Ancestral structures are adapted to new situations
• Adaptations are usually compromises.
– One characteristic may be an adaptation in one situation, a
disadvantage in another
• Natural selection interacts with chance/random
events and the environment.
– Chance events can alter allele frequencies in population
– Environment can change
Evolution of Populations
• Measured by calculating changes in gene pool
over time
– Frequency of an allele in a population
• Diploid organisms: total number of alleles at a locus
is the total number of individuals times 2
– Homozygous Dominant= 2 dominant alleles
– Heterozygous= 1 dominant, 1 recessive allele
– Homozygous Recessive= 2 recessive alleles
Frequency of Alleles in Population
• For a characteristic with 2 alleles, we can use
p and q to represent their frequencies
• Frequency of Alleles:
– p= frequency of “A” allele (dominant)
• Total number of “A” alleles/total number of alleles
– q= frequency of “a” allele (recessive)
• Total number of “a” alleles/total number of alleles
– Frequency of Alleles= p+ q =1
Frequency of Alleles in Population
• Population of wildflowers that is incompletely dominant for
color
– 320 red flowers (CRCR)
– 160 pink flowers (CRCW)
– 20 white flowers (CWCW)
• Calculate the number of copies of each allele:
– CR  (number of homozygotes for CR X 2) + number of
heterozygotes
• (320  2)  160  800
– CW  (number of homozygotes for CW x 2) + number of
heterozygotes
• (20  2)  160  200
Frequency of Alleles in Population
• To calculate the frequency of each allele:
• p  freq CR  number of CR alleles/total
number of alleles
– p = 800 / (800  200)  0.8
• q  freq CW  number of CW alleles/total
number of alleles
– q= 200 / (800  200)  0.2
• The sum of alleles is always 1
– 0.8  0.2  1
Hardy-Weinberg Principle
• The Hardy-Weinberg principle describes a
population that is not evolving
– frequency of alleles will not change from
generation to generation
– A “null hypothesis” to check for evidence of
evolution
• If a population does not meet the criteria of
the Hardy-Weinberg principle, it can be
concluded that the population is evolving
Figure 23.7
Alleles in the population
Gametes produced
Frequencies of alleles
p = frequency of
CR allele
= 0.8
Each egg:
Each sperm:
q = frequency of
CW allele
= 0.2
20%
80%
chance chance
20%
80%
chance chance
In a given population where gametes contribute to the
next generation randomly, allele frequencies will not
change
 Mendelian inheritance preserves genetic variation in a
population

Hardy-Weinberg Principle
• When allele frequencies remain constant from
generation to generation, the population is in
Hardy-Weinberg equilibrium
• Assumptions:
– No natural selection
– No mutation
– No gene flow
– Random mating
– Large population
Hardy-Weinberg Equilibrium
• Hardy-Weinberg Equilibrium Equations
• Frequency of Alleles: p + q = 1
• Frequency of Genotypes: p2 + 2pq + q2 = 1
Hardy-Weinberg Equilibrium
• The variables “p” and “q” come from Punnett
Square for populations
p= frequency of “A”
allele (dominant)
q= frequency of “a”
allele (recessive)
Hardy-Weinberg Equilibrium
• Hardy-Weinberg Equilibrium Equations
• Frequency of Alleles: p + q = 1
– p= 0.7
– q=0.3
• Frequency of Genotypes: p2 + 2pq + q2 = 1
– p2 = frequency of “AA” genotype
• Ex: 0.72 = 0.49
– 2pq = frequency of “Aa” genotype
• Ex: 2(0.7)(0.3)= 0.42
– q2 = frequency of “aa” genotype
• Ex: 0.32 = 0.09
– Frequency of Genotypes= 0.49 + 0.42 + 0.09 = 1
Population Variables: Hardy-Weinberg
Equilibrium
• Once scientists know what p and q are in the
population, they can track the population
through time and see if population is at
equilibrium or changing
• If p and q change through time, one of the HardyWeinberg Equilibrium assumptions are not being
met
– Evolution is occurring
• Populations can be at equilibrium at some loci,
but not at other loci
Real-World Example: Hardy-Weinberg
Equilibrium and Fisheries Management
• Kelp Grouper (Epinephelus bruneus)
• Commercial fish species in Korea
• Recent declines in landings
– 2007: IUCN Red List- Vulnerable
• Study by An et al. 2012
– Genotyped 12 gene loci from 30 fish
– 3 of 12 loci showed deviations from
Hardy-Weinberg Equilibrium