© 2014 pearson education, inc. objective 5: tswbat recognize that genetic variation makes evolution...
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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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Variation in heritable traits is a prerequisite for evolution
Mendel’s work on pea plants provided evidence of discrete heritable units (genes)
Concept 21.1: Genetic variation makes evolution possible
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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 “either-or” 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
1,000
Substitution resultingin translation ofdifferent amino acid
Base-pairsubstitutions Insertion sites
Deletion
Exon Intron
1 500
2,5002,0001,500
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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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Figure 21.6Porcupine herd
Beaufort Sea
Fortymile herd
Porcupine herd range
Fortymile herd range
MAPAREA
AL
AS
KA
CA
NA
DA
NO
RT
HW
ES
T
TE
RR
ITO
RIE
S
YU
KO
NA
LA
SK
A
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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
CRCW
CWCW
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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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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
Hardy-Weinberg Equilibrium
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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 21.7Frequencies of alleles
Gametes produced
p frequency of CR allele
q frequency of CW allele
Alleles in the population
Each egg: Each sperm:
0.8
0.2
80%chance
80%chance
20%chance
20%chance
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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
Sperm
Eggs
80% CR (p 0.8) 20% CW (q 0.2)
p 0.8 q 0.2 CR
CR
CW
CW
p 0.8
q 0.2
0.64 (p2)CRCR
0.16 (pq)CRCW
0.16 (qp)CRCW
0.04 (q2)CWCW
Gametes of this generation:
64% CRCR, 32% CRCW, and 4% CWCW
64% CR
(from CRCR plants)16% CR
(from CRCW plants)
4% CW
(from CWCW plants)16% CW
(from CRCW plants)
80% CR 0.8 p
20% CW 0.2 q
64% CRCR, 32% CRCW, and 4% CWCW plants
With random mating, these gametes will result in the samemix of genotypes in the next generation:
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Figure 21.8a
Sperm
Eggs
80% CR (p 0.8) 20% CW (q 0.2)
p 0.8 q 0.2 CR
CR
CW
CW
p 0.8
q 0.2
0.64 (p2)CRCR
0.16 (pq)CRCW
0.16 (qp)CRCW
0.04 (q2)CWCW
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Figure 21.8b
Gametes of this generation:
64% CRCR, 32% CRCW, and 4% CWCW
64% CR
(from CRCR plants)16% CR
(from CRCW plants)
4% CW
(from CWCW plants)16% CW
(from CRCW plants)
80% CR 0.8 p
20% CW 0.2 q
64% CRCR, 32% CRCW, and 4% CWCW plants
With random mating, these gametes will result in the samemix of genotypes in the next generation:
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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