Download Chapter 23 PowerPoint 2016 - Spring

Chapter 23
The Evolution of Populations
Original
population
Evolved
population
Directional
selection
Disruptive
selection
Stabilizing
selection
Overview: The Smallest Unit of Evolution
• Student misconception = organisms evolve
during their lifetimes
– Reminder- natural selection acts on
individuals, but only populations evolve
• Genetic variations in populations contribute to
evolution
• Microevolution is a change in allele
frequencies in a population over generations
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Concept 23.1: Mutation and sexual reproduction
produce the genetic variation that makes evolution
possible
• 2 processes produce variation in gene pools that
contributes to differences among individuals =
mutation and sexual reproduction
– Variation in individual genotype leads to
variation in individual phenotype
– Not all phenotypic variation is heritable
• Diet/environmentally-induced
– Natural selection can only act on variation with
a genetic component
Mutation
• Mutations are changes in the nucleotide
sequence of DNA
• Mutations cause new genes and alleles to arise
• Only mutations in cells that produce gametes
can be passed to offspring
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Point Mutations
• A point mutation is a change in one base in a gene
• Effects can vary:
– Mutations in noncoding regions  often harmless
– Mutations in gene might not affect protein
production b/c of redundancy in genetic code
– Mutations that result in change in protein
production are often harmful, but can sometimes
increase the fit between organism and
environment
Mutations That Alter Gene Number or Sequence
• Chromosomal mutations that delete, disrupt, or
rearrange many loci are typically harmful
• Duplication of large chromosome segments is
usually harmful
• Duplication of small pieces of DNA is sometimes
less harmful and increases the genome size
• Duplicated genes can take on new functions by
further mutation
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Mutation Rates
• Mutation rates are low in animals and plants
• The average is about one mutation in every
100,000 genes per generation
– lower in prokaryotes (but short generations
so mutations accumulate quickly)
– higher in viruses (and short generations so
mutations really accumulate quickly)
• RNA
Sexual Reproduction
• Sexual reproduction can shuffle existing alleles
into new combinations
– Crossing-over
– Independent assortment
– Random fertilization
• In organisms that reproduce sexually,
recombination of alleles is more important than
mutation in producing the genetic differences
that make adaptation possible
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Population genetics: study of how
populations change genetically over time
Population: group of individuals that live
in the same area and interbreed,
producing fertile offspring
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• Gene pool: all of the alleles for all genes in all the
members of the population
– Diploid species: 2 alleles for a gene
(homozygous/heterozygous)
• Fixed allele: all members of a population only
have 1 allele for a particular trait
– The more fixed alleles a population has, the
LOWER the species’ diversity
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Hardy-Weinberg Theorem
Hardy-Weinberg Theorem: The allele and
genotype frequencies of a population will remain
constant from generation to generation
…UNLESS they are acted upon by forces other
than Mendelian segregation and recombination of
alleles
Equilibrium = allele and genotype frequencies
remain constant from generation or generation
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Conditions for Hardy-Weinberg equilibrium
1. No mutations.
2. Random mating.
3. No natural selection.
4. Extremely large population size.
5. No gene flow.
If at least one of these conditions is NOT met, then
the population is EVOLVING!
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• The frequency of an allele in a population can be
calculated:
– For diploid organisms,
• total # of alleles at a locus = total # of
individuals x 2
• total # of dominant alleles at a locus = 2 alleles
for each homozygous dominant individual plus
1 allele for each heterozygous individual
• 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
– p = dominant
– q = recessive
• The frequency of all alleles in a population will
add up to 1
– For example, p + q = 1
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Hardy-Weinberg Principle
Allele Frequencies:
• Gene with 2 alleles : p, q
p = frequency of dominant allele (A)
q = frequency of recessive allele (a)
p+q=1
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Note:
1–p=q
1–q=p
Hardy-Weinberg Equation
Genotypic Frequencies:
• 3 genotypes (AA, Aa, aa)
2
p
+ 2pq +
2
q
=1
p2 = AA (homozygous dominant)
2pq = Aa (heterozygous)
q2 = aa (homozygous recessive)
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Allele frequencies
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Genotypic
frequencies
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Strategies for solving H-W Problems:
1. If you are given the genotypes (AA, Aa, aa),
calculate p and q by adding up the total # of A and a
alleles.
2. If you know phenotypes, then use “aa” to find q2,
and then q. (p = 1-q)
3. To find out if population is evolving, calculate p2 +
2pq + q2.
– If in equilibrium, it should = 1.
– If it DOES NOT = 1, then the population is
evolving!
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Hardy-Weinberg practice problem #1
The scarlet tiger moth has the following genotypes.
Calculate the allele and genotype frequencies (%) for a
population of 1612 moths.
AA = 1469
Aa = 138
Allele Frequencies:
A=
a=
Genotypic Frequencies:
AA =
Aa =
aa =
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aa = 5
Hardy-weinberg practice problem #2:
PTC Tasters
• Taster = AA or Aa
Nontaster = aa
• Tasters = ____
Nontasters = ___
q2 =
q=
p+q=1
p=1–q=
p2 + 2pq + q2 = 1
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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:
– The PKU gene mutation rate is low
– Mate selection is random with respect to whether
or not an individual is a carrier for the PKU allele
– Natural selection can only act on rare homozygous
individuals who do not follow dietary restrictions
– The population is large
– 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 x 0.99 x 0.01 = 0.0198
– or approximately 2% of the U.S. population
• Homework- p.475 #2 & 3, p.486 #1; check Appendix
CAUSES OF EVOLUTION
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Conditions for Hardy-Weinberg equilibrium
1. No mutations.
2. Random mating.
3. No natural selection.
4. Extremely large population size.
5. No gene flow.
If at least one of these conditions is NOT met, then
the population is EVOLVING!
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Minor Causes of Evolution:
#1 - Mutations
• Rare, very small changes in allele
frequencies
#2 - Nonrandom mating
• Affect genotypes, but not allele frequencies
Major Causes of Evolution:
• Natural selection, genetic drift, gene flow (#3-5)
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Major Causes of Evolution
#3 – Natural Selection
• Individuals with variations better suited to
environment pass more alleles to next
generation
• See Chapter 22
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Major Causes of Evolution
#4 – Genetic Drift
• Small populations have greater chance of
fluctuations in allele frequencies from one
generation to another
• Tends to reduce genetic variation through losses
of alleles
• Examples:
• Founder Effect
• Bottleneck Effect
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Genetic Drift
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Founder Effect
• A few individuals become isolated from
larger population
• Certain alleles under/over represented
Polydactyly in Amish population
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Bottleneck Effect
• Sudden change in environment drastically reduces
population size
• Resulting gene pool no longer reflective of the original
population’s gene pool
Northern elephant seals hunted
nearly to extinction in California
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Effects of Genetic Drift: A 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
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Major Causes of Evolution
#5 – Gene Flow
• Movement of fertile individuals
or gametes between
populations
• Gain/lose alleles
• Reduce genetic differences
between populations
• Can increase or decrease the
fitness of a population
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• Gene flow can decrease the fitness of a
population
• 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
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• Gene flow can increase the fitness of a
population
• Insecticides have been used to target
mosquitoes that carry West Nile virus and
malaria
• Alleles have evolved in some populations that
confer insecticide resistance to these
mosquitoes
• The flow of insecticide resistance alleles into a
population can cause an increase in fitness
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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 gene pool of the next generation,
relative to the contributions of other individuals
• Selection favors certain genotypes by acting on the
phenotypes 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
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Directional Selection:
eg. larger black bears
survive extreme cold
better than small ones
Disruptive Selection:
eg. small beaks for
small seeds; large
beaks for large seeds
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Stabilizing Selection:
eg. narrow range of
human birth weight
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 environment increases
– Because the environment can change, adaptive
evolution is a continuous process
• Genetic drift and gene flow do not consistently lead
to adaptive evolution as they can increase or
decrease the match between an organism and its
environment
Sexual Selection
• Sexual selection is natural selection for
mating success
• It can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
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• Intrasexual selection is competition among
individuals of one sex (often males) for mates
of the opposite sex http://abcnews.go.com/GMA/video/violentgiraffe-fight-video-viral-giraffes-slam-necks-18099718
• Intersexual selection, often called mate
choice, occurs when individuals of one sex
(usually females) are choosy in selecting their
mates http://www.youtube.com/watch?v=L54bxmZy_NE
• Male showiness due to mate choice can
increase a male’s chances of attracting a
female, while decreasing his chances of
survival (remember the guppies from Ch 22)
Preserving genetic variation
• Diploidy: hide recessive alleles that are less
favorable
• Heterozygote advantage: greater fitness than
homozygotes
• eg. Sickle cell disease
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Why Natural Selection Cannot Fashion Perfect
Organisms
1. Selection can act only on existing variations
2. Evolution is limited by historical constraints
-descent with modification
3. Adaptations are often compromises
4. Chance, natural selection, and the
environment interact
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You should now be able to:
1. Explain why the majority of point mutations are harmless
2. How does sexual recombination generate genetic variability?
3. Define: population, species, gene pool, relative fitness, neutral
variation, gene flow
4. List the five conditions of Hardy-Weinberg equilibrium
5. Apply the H-W equation to population genetics problems
6. Explain why natural selection is the only mechanism that
consistently produces adaptive change
7. Explain the role of population size in genetic drift; 2 types?
8. Distinguish among the following sets of selection terms:
directional/disruptive/stabilizing; intrasexual/intersexual
9. Why can’t natural selection produce perfect organisms?