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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 Presentation
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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). 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).
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.
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.
Population Genetics The study of genetic variation
in populations. How do populations change,
genetically, over time? Represents the reconciliation
of Mendelism and Darwinism.
Population A localized group of individuals
of the same species. Must produce viable offspring
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
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.
Microevolution Changes in the relative
frequencies of alleles in the gene pool.
Micro = small Microevolution is how we
study evolution at the genetics level
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.
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)
Basic Equation p + q = 1 p = %/frequency of dominant
allele q = %/frequency of recessive
allele
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)
Genotypes p2 = Homozygous Dominant
frequency2pq = Heterozygous frequencyq2 = Homozygous Recessive frequency
Example Calculation Let’s look at a population
where: A = red flowers a = white flowers
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)
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
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
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.
Example What is the frequency of the
PKU allele? PKU is expressed only if the
individual is homozygous recessive (aa).
PKU Frequency PKU is found at the rate of
1/10,000 births. PKU = aa = q2
q2 = .0001 q = .01 (frequency of
recessive alleles)
Dominant Allele p + q = 1 p = 1- q p = 1- .01 p = .99
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
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%
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
Hardy-Weinberg Assumptions
1. Large Population2. Isolation3. No Net Mutations4. Random Mating5. No Natural Selection
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?
Microevolution Caused by violations of the
5 H-W assumptions.
Causes of Microevolution
1. Genetic Drift2. Gene Flow3. Mutations4. Nonrandom Mating5. Natural Selection
Genetic Drift Changes in the gene pool of a
small population by chance. Types:
1. Bottleneck Effect 2. Founder's Effect
By Chance
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
Importance Reduction of population size
may reduce gene pool for evolution to work with.
Ex: Cheetahs
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
Importance Very common in islands and
other groups that don't interbreed.
Gene Flow Movement of genes in/out of
a population. Ex:
Immigration Emigration
Result: change in gene frequency
Mutations Inherited changes in a gene.
Result May change gene frequencies
(small population). Source of new alleles for
selection. Often lost by genetic drift.
Nonrandom Mating Failure to choose mates at
random from the population.
Causes Inbreeding within the same
“neighborhood”. Assortative mating
(like with like).
Result Increases the number of
homozygous loci. Does not in itself alter the
overall gene frequencies in the population.
Natural Selection Differential success in
survival and reproduction. Result - Shifts in gene
frequencies.
Comment As the environment changes,
so does natural selection and gene frequencies.
Result If the environment is
"patchy", the population may have many different local populations.
Genetic Basis of Variation
1. Discrete Characters – Mendelian traits with clear phenotypes.
2. Quantitative Characters – Multigene traits with overlapping phenotypes.
Polymorphism The existence of several
contrasting forms of the species in a population.
Usually inherited as Discrete Characteristics.
Examples
Garter SnakesGaillardia
Human Example ABO Blood Groups Morphs = A, B, AB, O
Quantitative Characters Allow continuous variation in
the population. Result –
Geographical Variation Clines: a change along a
geographical axis
Yarrow and Altitude
Sources of Genetic Variation
Mutations. Meiosis - recombination
though sexual reproduction. Crossing-over Random fertilization
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
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?
Relative Fitness Contribution of one genotype
to the next generation (when compared to other genotypes)
Rate of Selection Differs between dominant and
recessive alleles. Selection pressure by the
environment/nature.
Modes of Natural Selection
1. Stabilizing2. Directional3. Diversifying4. Sexual
Stabilizing Selection toward the average
and against the extremes. Ex: birth weight in humans
Directional Selection Selection toward one extreme. Ex: running speeds in race
animals Ex. Galapagos Finch beak size
and food source
Diversifying(Disruptive) Selection toward both
extremes and against the norm.
Ex: bill size in birds
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.
Sexual Mate selection May not be adaptive to the
environment, but increases reproduction success of the individual.
Result Sexual dimorphism. Secondary sexual features
for attracting mates.
Comments Females may drive sexual
selection and dimorphism since they often "choose" the mate.
Question Does evolution result in
perfect organisms? No!?
Compromises Chance occurrences
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.