Transcript q 0.2 - Industrial ISD

Objective 5:
TSWBAT recognize that
genetic variation makes
evolution possible.
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Overview: The Smallest Unit of Evolution
 One common misconception is that organisms
evolve during their lifetimes
 Natural selection acts on individuals, but only
populations evolve
 Consider, for example, a population of medium
ground finches on Daphne Major Island
 During a drought, large-beaked birds were more likely
to crack large seeds and survive
 The finch population evolved by natural selection
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 Microevolution is a change in allele frequencies in
a population over generations
 Three mechanisms cause allele frequency change
 Natural selection
 Genetic drift
 Gene flow
 Only natural selection causes adaptive evolution
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Concept 21.1: Genetic variation makes evolution
possible
 Variation in heritable traits is a prerequisite for
evolution
 Mendel’s work on pea plants provided evidence of
discrete heritable units (genes)
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Genetic Variation
 Phenotypic variation often reflects genetic variation
 Genetic variation among individuals is caused by
differences in genes or other DNA sequences
 Some phenotypic differences are due to differences
in a single gene and can be classified on an “eitheror” basis
 Other phenotypic differences are due to the
influence of many genes and vary in gradations
along a continuum
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Figure 21.3
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 Genetic variation can be measured at the whole
gene level as gene variability
 Gene variability can be quantified as the average
percent of loci that are heterozygous
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 Genetic variation can be measured at the molecular
level of DNA as nucleotide variability
 Nucleotide variation rarely results in phenotypic
variation
 Most differences occur in noncoding regions (introns)
 Variations that occur in coding regions (exons) rarely
change the amino acid sequence of the encoded
protein
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Figure 21.4
Base-pair
substitutions
Insertion sites
1
500
1,000
Intron
Exon
Substitution resulting
in translation of
different amino acid
1,500
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Deletion
2,000
2,500
 Phenotype is the product of inherited genotype and
environmental influences
 Natural selection can only act on phenotypic variation
that has a genetic component
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Figure 21.5a
(a) Caterpillars raised on a diet of oak flowers
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Figure 21.5b
(b) Caterpillars raised on a diet of oak leaves
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Sources of Genetic Variation
 New genes and alleles can arise by mutation or
gene duplication
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Formation of New Alleles
 A mutation is a change in the nucleotide sequence
of DNA
 Only mutations in cells that produce gametes can be
passed to offspring
 A “point mutation” is a change in one base in a
gene
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 The effects of point mutations can vary
 Mutations in noncoding regions of DNA are often
harmless
 Mutations to genes can be neutral because of
redundancy in the genetic code
 Mutations that alter the phenotype are often harmful
 Mutations that result in a change in protein production
can sometimes be beneficial
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Altering Gene Number or Position
 Chromosomal mutations that delete, disrupt, or
rearrange many loci are typically harmful
 Duplication of small pieces of DNA increases
genome size and is usually less harmful
 Duplicated genes can take on new functions by
further mutation
 An ancestral odor-detecting gene has been
duplicated many times: Humans have 350 functional
copies of the gene; mice have 1,000
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Rapid Reproduction
 Mutation rates are low in animals and plants
 The average is about one mutation in every 100,000
genes per generation
 Mutation rates are often lower in prokaryotes and
higher in viruses
 Short generation times allow mutations to
accumulate rapidly in prokaryotes and viruses
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Sexual Reproduction
 In organisms that reproduce sexually, most genetic
variation results from recombination of alleles
 Sexual reproduction can shuffle existing alleles into
new combinations through three mechanisms:
crossing over, independent assortment, and
fertilization
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Objective 6
TSWBAT describe the basic
principles of population
genetics including us of the
Hardy-Weinberg theorem and
equation.
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Concept 21.2: The Hardy-Weinberg equation can
be used to test whether a population is evolving
 The first step in testing whether evolution is
occurring in a population is to clarify what we mean
by a population
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Gene Pools and Allele Frequencies
 A population is a localized group of individuals
capable of interbreeding and producing fertile
offspring
 A gene pool consists of all the alleles for all loci in a
population
 An allele for a particular locus is fixed if all
individuals in a population are homozygous for the
same allele
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MAP
AREA
Beaufort Sea
Porcupine
herd range
Fortymile
herd range
Fortymile herd
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CANADA
Porcupine herd
ALASKA
Figure 21.6
Figure 21.6a
Porcupine herd
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Figure 21.6b
Fortymile herd
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 The frequency of an allele in a population can be
calculated
 For diploid organisms, the total number of alleles at a
locus is the total number of individuals times 2
 The total number of dominant alleles at a locus is 2
alleles for each homozygous dominant individual
plus 1 allele for each heterozygous individual; the
same logic applies for recessive alleles
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 By convention, if there are 2 alleles at a locus, p
and q are used to represent their frequencies
 The frequency of all alleles in a population will add
up to 1
 For example, p  q  1
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Figure 21.UN01
CRCR
CWCW
CRCW
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 For example, consider a 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  (320  2)  160  800
 CW  (20  2)  160  200
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 To calculate the frequency of each allele
 p  freq CR  800 / (800  200)  0.8 (80%)
 q  1  p  0.2 (20%)
 The sum of alleles is always 1
 0.8  0.2  1
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The Hardy-Weinberg Principle
 The Hardy-Weinberg principle describes a
population that is not evolving
 If a population does not meet the criteria of the
Hardy-Weinberg principle, it can be concluded that
the population is evolving
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Hardy-Weinberg Equilibrium
 The Hardy-Weinberg principle states that
frequencies of alleles and genotypes in a population
remain constant from generation to generation
 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
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 Hardy-Weinberg equilibrium describes the constant
frequency of alleles in such a gene pool
 Consider, for example, the same population of 500
wildflowers and 1,000 alleles where
 p  freq CR  0.8
 q  freq CW  0.2
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Figure 21.7
Frequencies of alleles
p  frequency of CR allele
 0.8
q  frequency of CW allele
 0.2
Alleles in the population
Gametes produced
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Each egg:
Each sperm:
80%
20%
chance chance
20%
80%
chance chance
 The frequency of genotypes can be calculated
 CRCR  p2  (0.8)2  0.64
 CRCW  2pq  2(0.8)(0.2)  0.32
 CWCW  q2  (0.2)2  0.04
 The frequency of genotypes can be confirmed using
a Punnett square
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Figure 21.8
20% CW (q  0.2)
80% CR (p  0.8)
Sperm
CR p  0.8
CW q  0.2
CR
p  0.8
0.64 (p2)
CRCR
Eggs
CW
0.16 (pq)
CRCW
0.04 (q2)
CWCW
0.16 (qp)
CRCW
q  0.2
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants)
R
 16% C R
4% CW

(from CWCW plants)
(from C CW plants)
 80% CR  0.8  p
16% CW
 20% CW  0.2  q
(from CRCW plants)
With random mating, these gametes will result in the same
mix of genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
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Figure 21.8a
80% CR (p  0.8)
20% CW (q  0.2)
R
C
Sperm
CW q  0.2
p  0.8
CR
p  0.8
Eggs
CW
q  0.2
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0.64 (p2)
CRCR
0.16 (qp)
CRCW
0.16 (pq)
CRCW
0.04 (q2)
CWCW
Figure 21.8b
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants)
R
16%
C

 80% CR  0.8  p
R W
(from C C plants)
W
4% CW
16%
C

 20% CW  0.2  q
W W
R W
(from C C plants)
(from C C plants)
With random mating, these gametes will result in the same
mix of genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
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 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
where p2 and q2 represent the frequencies of the
homozygous genotypes and 2pq represents the
frequency of the heterozygous genotype
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Figure 21.UN02
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Conditions for Hardy-Weinberg Equilibrium
 The Hardy-Weinberg theorem describes a
hypothetical population that is not evolving
 In real populations, allele and genotype frequencies
do change over time
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 The five conditions for nonevolving populations are
rarely met in nature
1. No mutations
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow
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 Natural populations can evolve at some loci while
being in Hardy-Weinberg equilibrium at other loci
 Some populations evolve slowly enough that
evolution cannot be detected
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Applying the Hardy-Weinberg Principle
 We can assume the locus that causes
phenylketonuria (PKU) is in Hardy-Weinberg
equilibrium given that
1. The PKU gene mutation rate is low
2. Mate selection is random with respect to whether
or not an individual is a carrier for the PKU allele
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3. Natural selection can only act on rare homozygous
individuals who do not follow dietary restrictions
4. The population is large
5. Migration has no effect, as many other populations
have similar allele frequencies
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 The occurrence of PKU is 1 per 10,000 births
 q2  0.0001
 q  0.01
 The frequency of normal alleles is
 p  1 – q  1 – 0.01  0.99
 The frequency of carriers is
 2pq  2  0.99  0.01  0.0198
 or approximately 2% of the U.S. population
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