chapter 23: the evolution of populations
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Chapter 23: The Evolution of Populations. Question?. Is the unit of evolution the individual or the population ? Answer – while evolution effects individuals, it can only be tracked through time by looking at populations. So what do we study?. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 23:The Evolution of Populations
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Question? Is the unit of evolution the individual or the
population? Answer – while evolution effects 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. Represents the reconciliation of Mendelism
and Darwinism.
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Modern Synthesis
Uses population genetics as the means to track and study evolution.
Looks at the genetic basis of variation and natural selection.
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Population A localized group of individuals of the same
species.
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Species A group of similar organisms. A group of populations that could interbreed.
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Gene Pool The total aggregate of genes in a population. 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.
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Hardy-Weinberg Theorem Developed in 1908. Mathematical model of gene pool changes
over time.
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Basic Equation p + q = 1 p = % dominant allele q = % recessive allele
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Expanded Equation p + q = 1 (p + q)2 = (1)2
p2 + 2pq + q2 = 1
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Genotypes
p2 = Homozygous Dominants2pq = Heterozygousq2 = Homozygous Recessives
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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 = 2 x 500 = 1000
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Dominant Allele A = (320 x 2) + (160 x 1)
= 800
= 800/1000
A = 80%
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Recessive Allele a = (160 x 1) + (20 x 2)
= 200/1000
= .20
a = 20%
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A and a in HW equation Cross: Aa X Aa Result = AA + 2Aa + aa Remember: A = p, a = q
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Substitute the values for A and a p2 + 2pq + q2 = 1
(.8)2 + 2(.8)(.2) + (.2)2 = 1
.64 + .32 + .04 = 1
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Dominant Allele A = p2 + pq
= .64 + .16
= .80
= 80%
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Recessive Allele a = pq + q2
= .16 + .04
= .20
= 20%
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Result Gene pool is in a state of equilibrium and has
not changed because of sexual reproduction. No Evolution has occurred.
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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 HW 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
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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
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Final Results 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
(but no calculators are allowed).
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Hardy-Weinberg Assumptions1. Large Population
2. Isolation
3. No Net Mutations
4. Random Mating
5. 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. But, 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 Microevolution1. Genetic Drift
2. Gene Flow
3. Mutations
4. Nonrandom Mating
5. 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.
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Result Some alleles lost. Other alleles are over-represented. Genetic variation usually lost.
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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
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Result Genetic variation reduced. Some alleles increase in frequency while
others are lost (as compared to the parent population).
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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
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Result Changes in gene frequencies.
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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 Variation1. 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 Snakes Gaillardia
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Human Example ABO Blood Groups Morphs = A, B, AB, O
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Other examples
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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 Plants and Altitude
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Sources of Genetic Variation Mutations. Recombination though sexual reproduction.
Crossing-over Random fertilization
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Preserving Genetic Variation1. Diploidy - preserves recessives as
heterozygotes.
2. Balanced Polymorphisms - preservation of diversity by natural selection.
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Example Heterozygote Advantage - When the
heterozygote or hybrid survives better than the homozygotes. Also called Hybrid vigor.
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Result Can't bred "true“ and the diversity of the
population is maintained. Ex – Sickle Cell Anemia
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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.
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Relative Fitness
Contribution of one genotype to the next generation compared to other genotypes.
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Rate of Selection Differs between dominant and recessive
alleles. Selection pressure by the environment.
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Modes of Natural Selection1. Stabilizing
2. Directional
3. Diversifying
4. 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 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?
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Answer - No1. Historical Constraints
2. Compromises
3. Non-adaptive Evolution (chance)
4. Available variations
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Summary Know the difference between a species and a
population. Know that the unit of evolution is the
population and not the individual.
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Summary Know the H-W equations and how to use
them in calculations. Know the H-W assumptions and what
happens if each is violated.
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Summary Identify various means to introduce genetic
variation into populations. Know the various types of natural selection.