Download Ch. 23 The Evolution of Populations

Ch. 23
The Evolution of Populations
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Essential question:
Do populations evolve?
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Mutation and Sexual reproduction produce genetic variation that makes evolution possible
What is the smallest unit of evolution?
genetic variations come from microevolution = change in allele frequencies in a population over generations
Three main mechanisms that cause allele frequency:
1. natural selection*
2. genetic drift (chance events that alter allele frequencies)
3. gene flow (transfer of alleles between populations)
* natural selection is the only mechanism that improves the match
between organisms and their environment
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When looking at genetic variation, need to look at:
­discrete characters­ classified as either/or Ex. black or white
­ many determined by a single gene locus with different alleles at that locus (think dominant/recessive)
­Quantitative characters ­ characteristics that vary along a continuum within a population; 2 or more genes involved Examples?? Calico cats
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Scientists measure variation by looking at:
1. whole gene level (gene variability) 2. molecular level of DNA (nucleotide variation)
Gene variability can be quantified by Average heterozygosity = the average % of loci that are heterozygous
Heterozygous = two different alleles for a given locus
Ex. Fruit flies ­ 13,700 genes, heterozygous for 1,920 loci (14%)
Nucleotide variability ­ measured by comparing DNA sequences of 2 individuals in a population and then average data of many comparisons.
What is the difference between you and me?
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Variation between populations
geographic variation­ differences between gene pools of separate populations or population subgroups
Ex. mice in Madeira ­patterns of fused
chromosomes differ from one population to the other.
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clines­ a graded change in a trait along a geographic axis
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What factors do you think affect clines?
clines suggest natural selection because otherwise "there would be no reason to expect a close relationship between the environmental variable and the frequency of the allele"
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Ultimate source of new alleles is Mutations
mutation = any change in the nucleotide sequence of an organism's DNA
Types of mutations:
1. point mutation ­ change of one base in a gene
­as long as the amino acid that a section codes for is not changed, then it will have no effect.
­ but can be detrimental
2. translocation ­ movement of one part of chromosome to another chromosome or within itself.
­beneficial if it links DNA segments to have a beneficial effect
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Gene duplication ­ when a gene segment is duplicated due to errors in meiosis, slippage during DNA replication or pieces moving
if are not harmful, can lead to mutations and natural selection
Sexual reproduction leads to genetic variation and makes evolution possible
Mechanisms that make this possible:
1. crossing over during meiosis
2. independent assortment of chromosomes
3. fertilization
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Darwin couldn't explain how inherited variations were maintained in populations over time
Gregor Mendel had a model, but his manuscripts weren't really looked at until early 20th century
geneticists put Darwin's views and Mendel's view together = population genetics­ the study of how populations change over time
mid 20th century ­ modern synthesis­ a comprehensive theory of evolution that integrated ideas from many fields
Fisher (1890­1962) ­ demonstrated rules of Mendelian inheritance
Haldane (1892­1964) ­ studied rules of natural selection
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Population = a localized group of individuals that are capable of breeding and producing fertile offspring
­some populations of species are isolated so rarely exchange genetic material
Ex. Blue people of Kentucky
Gene pool = all of the alleles at all gene loci in all individuals of the population
fixed allele= one allele that exists at a particular locus in a population (all individuals are homozygous for that allele)
Each allele has a frequency in the population
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R
Ex. 500 wildflower plants with 2 alleles for color C = W
red, C = white ­absence of red pigment (total = 1000 alleles)
R R
W W
C C x C C red flowers
white flowers
R
W
C C
pink flowers ­ incomplete dominance
In the population of 500 plants:
320 plants = red 160 plants = pink 20 plants = white
R
R
R
C allele = 800 of the genes (320 x2 = 640 for C C plants, + R W
160 x1 = 160 for C C plants)
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R
W
W
C allele = 200 of the genes (160 x 1 = 160 for C C plants , +,
W W
20 x 2 for C C plants
if have two alleles for a particular locus use p to represent frequency of one allele and q to represent frequency of other allele
p = dominant allele frequency
q = recessive allele frequency
p + q = 1
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R
p = frequency of C allele in the gene pool of this population
R
C accounts for 800 of the genes (640 + 160)
R
800C alleles/ 1000 total alleles of population = 0.8 or 80% frequency
W
C accounts for 200 of the genes (160 + 40)
W
200 C alleles/ 1000 total alleles of population = 0.2 or 20 % frequency
W
q = frequency of the C allele in the gene pool of this population
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the sum of all frequencies must equal 1
therefore p + q = 1
Even if you have more than two alleles, the sum of the frequencies must still = 1
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Hardy­Weinberg Principle (Theorem)
1908 by Hardy and Weinberg ­ worked independently
= the frequencies of alleles and genotypes in a population's gene pool remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.
­works if gene pools not evolving
Hardy Weinberg Equilibrium = The condition describing a nonevolving population (one that is in genetic equilibrium)
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Conditions for Hardy­Weinberg equilibrium*
1. works for extremely large population size
­smaller the population, more chance for fluctuation in allele frequencies
2. No gene flow ­ no transfer of alleles between populations
3. no mutations ­ mutations could modify the gene pool
4. Random mating ­ allele frequencies would be different if choose mates with specific traits
5. No natural selection ­ allele frequencies could be changed if have different survival and reproductive success
*changes in these conditions result in evolution
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Hardy­
Weinberg Theorem
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Hardy­Weinberg equilibrium
­using previous example and rule of multiplication
­can calculate the frequencies of the three possible genotypes
assuming random union of sperm and egg
R
probability of two C alleles will come together
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0.8 x 0.8 = p x p = p = 0.64
R
64% of the next generation will have genotype C C
R
W W
The frequency of C C individuals 0.2 x 0.2 = q 2 =0.04 or 4%
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R
W
R
The frequency of heterozygotes C C can be from a sperm with C R
W
W
and egg with C or a sperm with C and an egg with C 0.8 x 0.2 = 0.16 or 16%
Can summarize unions of gametes using algebraic equation
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2
p + 2pq + q = 1
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Hardy­Weinberg can be used to estimate percentage of population carrying an allele for an inherited disease
Ex. PKU ­ phenylketonuria = metabolic disorder
1 in 10,000 people have this in U.S.
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because allele is recessive corresponds to q
frequency of the recessive allele:
q = 0.0001 = 0.01
frequency of dominant allele:
p = 1 ­ q = 1­ 0.01 = 0.99
frequency of carriers (heterozygous)
2pq = 2 x 0.99 x 0.01 = 0.0198 (approximately 2% of U.S. population
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Other ways to alter a population's genetic comparison
any change in the conditions that Hardy­Weinberg is based on 1. Natural Selection
individuals with variations in their heritable traits that are better suited to environmental conditions can produce more offspring
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Genetic Drift­ unpredictable fluctuations in allele frequencies from one generation to the next because of a population's small size
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Two situations of genetic drift
a. bottleneck effect­ happens when a sudden change in environment (like a fire or flood) drastically reduces the population size, so only survivors can pass genes on ­certain alleles may be more present than others, some may be eliminated
­humans ­ can cause other species to go through this
­endangered species
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b. The Founder Effect­ when a few individuals become isolated from main population and establish a new gene pool not reflective of original population.
accounts for some human genetic disorders
Ex.
1814 colonist brought recessive alleles for retinitis pigmentosa (form of blindness) to Tristan da Cunha
islands
1960's 4 people had disease (10x higher than normal)
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Gene Flow ­ genetic additions or subtractions from a population resulting from the movement of fertile individuals or gametes
­tends to reduce differences between populations
­could eventually form a single large gene pool
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Natural Selection is the primary mechanism of adaptive evolution
­accumulates and maintains favorable genotypes in a population
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Natural Selection
Evolutionary fitness
fitness = adaptive advantage, the contribution that an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
relative fitness = contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus
relative fitness = "0" if an animal or plant is sterile
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(Disruptive)
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Directional selection­ favors individuals that deviate from the average
­caused by environmental changes or migration of members to new environment with different environmental conditions than original habitat
Disruptive selection­ both extremes of phenotypic range are favored
Stabilizing selection ­ against extreme phenotypes and favors intermediate variants
selection favors heritable traits
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Directional Selection for beak size in Galapagos population of medium ground finch
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Sexual Selection ­ a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates.
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Sexual selection­ natural selection for mating success
sexual dimorphism =marked differences between sexes in secondary sexual characteristics, not associated with reproduction
differences can be size, color, ornamentation
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intrasexual selection ­selection "within same sex" like a direct competition among individuals of same sex
­usually in male birds
Bower bird ­ builds a colorful nest to attract female
peacocks ­ fanning their feathers
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intersexual selection = mate choice
mates are choosy about mates of other sex
female's choice depends on male's appearance or behavior
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Preservation of genetic variation in a population ­
1. Diploidy­ most eukaryotic organisms are diploid (2 of each chromosome ­ one from each parent) ­organisms have recessive alleles
­only shown when inherit both recessive genes
­ frequency is very low
­ the rarer the recessive allele, more protection from natural selection
2. Balancing selection ­ when two or more forms exist in a population. 38
a. heterozygote advantage heterozygote individuals have a greater fitness than homozygotes.
Ex. sickle ­cell allele ­ most common in Africa; also provides protection against malaria
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Why natural selection cannot make perfect organisms
1. Selection can edit only existing variations
favors only fittest phenotypes among those currently in the population
2. evolution is limited by historical constraints
Ex. ancestral anatomy can't be changed
3. Adaptations are often compromises
Ex. humans have prehensile hands and flexible limbs, but prone to injury
4. Chance, natural selection and the environment interact
­storms blowing insects or plant seeds to new areas
­ they might not be the most fit organisms
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