chapter 23: the evolution of populations
DESCRIPTION
Chapter 23: The Evolution of Populations. Essential Knowledge. 1.a.1 – Natural selection is a major mechanism of evolution (23.2). 1.a.2 – Natural selection acts on phenotypic variations in populations (23.1 & 23.4). 1.a.3 – Evolutionary change is also driven by random processes (23.3). - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 23: The Evolution of
Populations
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Essential Knowledge 1.a.1 – Natural selection is a major mechanism of
evolution (23.2). 1.a.2 – Natural selection acts on phenotypic variations in
populations (23.1 & 23.4). 1.a.3 – Evolutionary change is also driven by random
processes (23.3). 2.c.1 – Changes in genotype can result in changes in
phenotype (23.4). 4.c.3 – The level of variation in a population affects
population dynamics (23.1 – 23.3). 4.c.4 – The diversity of species within an ecosystem may
influence the stability of the ecosystem (23.2).
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Question? Is the unit of evolution the
individual or the population? Answer – while evolution
affects individuals, it can only be tracked through time by looking at populations.
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So what do we study? We need to study populations,
not individuals. We need a method to track the
changes in populations over time. This is the area of Biology called
population genetics.
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Population Genetics The study of genetic variation
in populations. How do populations change,
genetically, over time? Represents the reconciliation
of Mendelism and Darwinism.
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Population A localized group of individuals
of the same species. Must produce viable offspring
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Species A group of similar organisms. A group of populations that
could interbreed (successfully) Populations are animals of the
same species that are isolated due to geography
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Gene Pool The total aggregate of genes in
a population. All alleles at all gene loci in all
individuals If evolution is occurring, then
changes must occur in the gene pool of the population over time.
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Microevolution Changes in the relative
frequencies of alleles in the gene pool.
Micro = small Microevolution is how we
study evolution at the genetics level
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Hardy-Weinberg Theorem
Developed in 1908. Use as a benchmark to study
evolutionary change in a population
Mathematical model of gene pool changes over time.
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H-W Theorem States:
The frequencies of alleles and genotypes in a population’s gene pool remain constant (in a population that is NOT evolving)
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Basic Equation p + q = 1 p = %/frequency of dominant
allele q = %/frequency of recessive
allele
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Expanded Equation p + q = 1 (p + q)2 = (1)2
p2 + 2pq + q2 = 1 We expand the equation to “fit”
all three types of genotypes (Ex: AA, Aa, aa)
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Genotypes p2 = Homozygous Dominant
frequency2pq = Heterozygous frequencyq2 = Homozygous Recessive frequency
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Example Calculation Let’s look at a population
where: A = red flowers a = white flowers
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Starting Population N = 500 Red = 480 (320 AA+ 160 Aa) White = 20 Total Genes/Alleles
= 2* x 500 = 1000*2 alleles per genotype
(hence the “2” in the equation)
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Dominant Allele
A = (320 x 2) + (160 x 1) = 800 = 800/1000 = 0.8 = 80%
320 = AA pop # (2 = # of dominant alleles in that AA genotype);
160 = Aa pop # (1 = # of dominant alleles in Aa genotype);
1000 = total genes
2 = # of times the dom allele is present in homozy dom genotype
1 = # of times the dom allele is present in heterozy genotype
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Recessive Allele
a = (160 x 1) + (20 x 2) = 200 = 200/1000 = .20 = 20%
20 = aa pop # (2 = # of recessive alleles in that aa/white genotype);
160 = Aa pop # (1 = # of recessive alleles in Aa genotype);
1000 = total genes
1 = # of times the rec allele is present in heterozy genotype
2 = # of times the rec allele is present in homozy rec genotype
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Importance of Hardy-Weinberg
Yardstick to measure rates of evolution.
Predicts that gene frequencies should NOT change over time as long as the H-W assumptions hold.
Way to calculate gene frequencies through time.
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Example What is the frequency of the
PKU allele? PKU is expressed only if the
individual is homozygous recessive (aa).
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PKU Frequency PKU is found at the rate of
1/10,000 births. PKU = aa = q2
q2 = .0001 q = .01 (frequency of
recessive alleles)
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Dominant Allele p + q = 1 p = 1- q p = 1- .01 p = .99
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Expanded Equation p2 + 2pq + q2 = 1(.99)2 + 2(.99x.01) + (.01)2 = 1.9801 + .0198 + .0001 = 1
Freq of Homozy Dom
genotype
Freq of Heterozy genotype
Freq of Homozy Rec
genotype
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Final Results All we did is convert the
frequencies (decimals) to % (by multiplying frequencies by 100%)
Normals (AA) = 98.01% Carriers (Aa) = 1.98% PKU (aa) = .01%
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AP Problems Using Hardy-Weinberg
Solve for q2 (% of total) Solve for q (equation) Solve for p (1- q) H-W is always on the national
AP Bio exam
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Hardy-Weinberg Assumptions
1. Large Population2. Isolation3. No Net Mutations4. Random Mating5. No Natural Selection
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If H-W assumptions hold true:
The gene frequencies will not change over time.
Evolution will not occur. How likely will natural
populations hold to the H-W assumptions?
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Microevolution Caused by violations of the
5 H-W assumptions.
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Causes of Microevolution
1. Genetic Drift2. Gene Flow3. Mutations4. Nonrandom Mating5. Natural Selection
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Genetic Drift Changes in the gene pool of a
small population by chance. Types:
1. Bottleneck Effect 2. Founder's Effect
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By Chance
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Bottleneck Effect Loss of most of the population
by disasters. Surviving population may have a
different gene pool than the original population.
Results: Some alleles lost, others are over-represented, genetic variety is decreased
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Importance Reduction of population size
may reduce gene pool for evolution to work with.
Ex: Cheetahs
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Founder's Effect Genetic drift in a new colony that
separates from a parent population.
Ex: Old-Order Amish Results: Genetic variety
reduced, some alleles increase while other lost
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Importance Very common in islands and
other groups that don't interbreed.
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Gene Flow Movement of genes in/out of
a population. Ex:
Immigration Emigration
Result: change in gene frequency
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Mutations Inherited changes in a gene.
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Result May change gene frequencies
(small population). Source of new alleles for
selection. Often lost by genetic drift.
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Nonrandom Mating Failure to choose mates at
random from the population.
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Causes Inbreeding within the same
“neighborhood”. Assortative mating
(like with like).
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Result Increases the number of
homozygous loci. Does not in itself alter the
overall gene frequencies in the population.
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Natural Selection Differential success in
survival and reproduction. Result - Shifts in gene
frequencies.
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Comment As the environment changes,
so does natural selection and gene frequencies.
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Result If the environment is
"patchy", the population may have many different local populations.
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Genetic Basis of Variation
1. Discrete Characters – Mendelian traits with clear phenotypes.
2. Quantitative Characters – Multigene traits with overlapping phenotypes.
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Polymorphism The existence of several
contrasting forms of the species in a population.
Usually inherited as Discrete Characteristics.
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Examples
Garter SnakesGaillardia
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Human Example ABO Blood Groups Morphs = A, B, AB, O
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Quantitative Characters Allow continuous variation in
the population. Result –
Geographical Variation Clines: a change along a
geographical axis
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Yarrow and Altitude
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Sources of Genetic Variation
Mutations. Meiosis - recombination
though sexual reproduction. Crossing-over Random fertilization
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Comment Population geneticists believe
that ALL genes that persist in a population must have had a selective advantage at one time.
Ex – Sickle Cell and Malaria, Tay-Sachs and Tuberculosis
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Fitness - Darwinian The relative contribution an
individual makes to the gene pool of the next generation. How likely is it that an organism
will survive and reproduce in a given environment?
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Relative Fitness Contribution of one genotype
to the next generation (when compared to other genotypes)
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Rate of Selection Differs between dominant and
recessive alleles. Selection pressure by the
environment/nature.
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Modes of Natural Selection
1. Stabilizing2. Directional3. Diversifying4. Sexual
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Stabilizing Selection toward the average
and against the extremes. Ex: birth weight in humans
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Directional Selection Selection toward one extreme. Ex: running speeds in race
animals Ex. Galapagos Finch beak size
and food source
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Diversifying(Disruptive) Selection toward both
extremes and against the norm.
Ex: bill size in birds
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Comment Diversifying Selection - can
split a species into several new species if it continues for a long enough period of time and the populations don’t interbreed.
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Sexual Mate selection May not be adaptive to the
environment, but increases reproduction success of the individual.
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Result Sexual dimorphism. Secondary sexual features
for attracting mates.
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Comments Females may drive sexual
selection and dimorphism since they often "choose" the mate.
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Question Does evolution result in
perfect organisms? No!?
Compromises Chance occurrences
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Summary Recognize the modern synthesis Theory of Evolution. Identify and use the Hardy-Weinberg Theorem for
population genetics. Identify the Hardy-Weinberg assumptions and how
they affect evolution of populations. Recognize causes and examples of microevolution. Identify modes of natural selection. Recognize why evolution does not produce "perfect"
organisms.