Chap 19
Biology 102 with Darville/martin/transue at Rutgers University - New Brunswick/Piscataway
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Evolutionary Change in Populations
Chapter 19
Population genetics is the study of genetic variability within a population and the evolutionary forces that act on it
What is a population?s gene pool
Population?s Gene Pool
- A population is a localized group of individuals belonging to the same species
- Potential to interbreed and produce offspring
- In any population, the gene pool includes all alleles for all loci present in population
- Genetic variation evident among individuals in any given population suggests that each individual has a different subset of alleles in the gene pool
Genotype, Phenotype and Allele Frequencies
- The proportion of a tested genotype in the population is the Genotype Frequency
- g. frequency of homozygous dominant = 490/1000 = 0.49
490
49
420
42
490+420
91
90
09
1000
- 0
- The proportion of an allele of a given genetic locus in the observed population is called the Allele Frequency
The proportion of an allele of a given genetic locus in the observed population is called the Allele Frequency
g. Frequency of the A allele is 0.7 (adds upto 1)
2pq
frequency of heterozygous genotype
Hardy?Weinberg Principle
- A population whose allele and genotype frequencies do not change from generation to generation is in genetic equilibrium
- ?.the relative proportions of alleles and genotypes in successive generations will therefore always be the same
A population whose allele and genotype frequencies do not change from generation to generation is in genetic equilibrium
e. no evolution is occurring
Can only apply if
- mating is random (not selected) in the population
- net mutations changing allele frequencies are absent
- The population is large to reduce the chance for random allele fluctuations
- no migration between populations
- No exchange of alleles
- natural selection does not occur
- No favoritism of one phenotype over another
Of 1000 individuals
Fertilization segregation by random assortment will
Learning Objective 2
- Application of the principle (while not ideal) helps us understand evolutionary change in sexually reproductive...
- In most cases, only the phenotype frequency is known
- When alleles are dominant or recessive, it is impossible to visibly distinguish the heterozygous from the homozygous...
- Therefore the Hardy-Weinberg principle allows us to use phenotypic frequencies to calculate expected genotype and...
null
q 2 = frequency of the homozygous recessive genotype
- We can use this equation to determine expected...
- We begin with knowing that 90 individuals are...
- Expected frequency of the aa genotype (q x q) =...
- Expected frequency of the a allele (q) = square...
- If p + q = 1, then the frequency of the dominant...
- We can now calculate the frequency of AA as (p x...
- Using the equation p 2 + 2pq + q 2 = 1
- Expected frequency of heterozygous (Aa)...
- Of 1000 individuals : Fertilization segregation by...
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Expected versus Actual
- This only represents the situation at genetic equilibrium
- Therefore
- By comparing expected numbers with the actual results observed, and reviewing any deviations, it?s possible to see how closely a population is to genetic equilibrium, and whether that population is indeed evolving
Microevolution
- Change in allele or genotype frequencies within a population over successive generations
- A departure from the Hardy-Weinberg principle because it involves small changes over a few generations
Learning Objective 3
- Each of the following microevolutionary forces alter allele frequencies in populations
- nonrandom mating
- mutation
- genetic drift
- gene flow
- natural selection
Nonrandom Mating (1)
- Individuals select their mates based on phenotype
- indirectly selecting corresponding genotypes
- Inbreeding
- mating of genetically similar individuals e.g. cousins breeding, self fertilization in certain plants
- Frequency of homozygous genotypes therefore increases with each successive generation of inbreeding
Mutations-
Are unpredictable changes in DNA
Nonrandom Mating (2)
- Inbred individuals have lower fitness than non-inbred individuals
- Which leads to inbreeding depression
- Average number of surviving offspring of inbred mice is lower, compared to the average number of surviving offspring of competing or non-inbred mice (harmful recessive genes)
null
Nonrandom Mating (3)
- If the process of selecting mates involves selection of the same phenotype, this is known as...
- Fruit flies with bristle types preferentially mating with the same bristle type
- Negative assortative mating is the selection of different phenotypes and is less common
- The fruit flies preferentially seek out the opposite bristle type
- Assortative mating will lead to a change in the genotype frequencies at a loci of choice
Mutations- Are unpredictable changes in DNA
- That provide sources of new alleles as a result of
- Change in nucleotide base pairs of a gene
- Rearrangement of genes within chromosomes so that...
- Change in chromosome structure
- Mutations
- Increase genetic variability occurring by natural...
- Affect both somatic (non-inherited) and germ...
- Can change the function of a gene
- Negligible as an evolutionary force, especially...
Genetic Drift (1)
- Is the random change in allele frequencies in a small population; that is passed from one generation to another purely by chance
- Low frequency alleles may be lost regardless of whether they are beneficial or harmful
- Genetic Drift in a small population with only a few individuals would therefore have a more dramatic effect upon rare alleles than would be expected in larger population
Genetic Drift (2)
- Two situations that can shrink larger populations down to a smaller size ? small enough for genetic drift to have a definitive effect
- are known as the bottle neck effect and the founder effect
Genetic Drift (3)
- The bottleneck effect occurs when a sudden decrease in population size is caused by adverse environmental factors (e.g. disease outbreak, natural disasters)
- The smaller surviving population may over represent certain alleles, and some may be eliminated altogether
- Which reduces the overall genetic variability in that population
Genetic Drift (4)
- The Founder effect occurs when a small population colonizes a new area and the individuals bring with them a small fraction of the genetic variation present in the original population
- Quite different from the original
- A rare allele causing homozygous recessive dwarfism [Ellis-Van Creveld syndrome] uncommon in the general population, is far more common in the Amish
Gene Flow
- The genetic exchange due to the migration of fertile individuals or gametes between populations
- As alleles flow from one population to another, gene variability in recipient population increases
- Counters the effects of natural selection and genetic drift
- A windstorm blows pollen from a pure white (rr) wildflower population to a close-by pure red...
- Increased gene flow from migrations of humans across the world once quite isolated
Natural Selection
- Causes change in allele frequencies that leads to adaptation (Darwin)
- Over successive generations, proportion of favorable alleles increases in the population
- changes the genetic composition of population to favor a particular environment
- reduces the frequency of alleles that result in the expression of unfavorable traits; providing offspring with selective advantage
KEY CONCEPTS
Microevolutionary forces include nonrandom mating, mutation, genetic drift, gene flow, and natural selection
Learning Objective 4
- Let?s discuss the difference between various types of natural selection
- stabilizing selection
- directional selection
- disruptive selection???
Modes of Selection
- Stabilizing selection
- Directional selection
- favors one phenotypic extreme over another, causing shift in phenotypic mean
- Disruptive selection
- favors two or more phenotypic extremes
- Selects against the average or intermediate phenotype
Stabilizing selection
- favors the mean over phenotypic extremes
- Individuals with average, or intermediate phenotypes are favored
- g. babies with average birth weights more likely to survive
- Decreases variation
favors one phenotypic extreme over another, causing shift in phenotypic mean
g. rams with large horns killed by hunters drives down the number, and birth of these types
Stabilizing Selection (Fig 19-5)
Directional Selection Table 19-1
null
null
Learning Objective 5
- The nature and extent of genetic variation can be evaluated several ways
- genetic polymorphism
- balanced polymorphism
- neutral variation
- geographic variation
null
Genetic polymorphism
- presence in a population of 2 or more alleles for any given locus
- extensive although many do not get expressed
Balanced polymorphism
- a special type of genetic polymorphism
- 2 or more alleles persist in a population over many generations due to natural selection
extensive although many do not get expressed
- different forms of a particular protein (different alleles, different amino acid sequences)- Do not get expressed and hence they are insignificant.
- Greater in plants than animals
Genetic Variation (2)
- Heterozygote advantage
- A form of balanced polymorphism that occurs when the heterozygote exhibits greater fitness than either homozygote
2 or more alleles persist in a population over many generations due to natural selection
Heterozygotic advantage, frequency dependent selection
Ex: RBC of sickle cell anemia heterozygote (carriers) are more resistant to malarial parasite because of a modified Hb
Advantageous over homozygous for sickle cell (death) or carrying normal gene (get malaria)
Ex
RBC of sickle cell anemia heterozygote (carriers) are more resistant to malarial parasite because of a modified Hb
Frequency-Dependent Selection
- A form of balanced polymorphism where fitness is dependent on the frequency of the phenotype in the population.
- The less common phenotype produces more offspring and therefore makes a greater relative contribution to the next generation
Neutral variation
- genetic variation that provides no detectable selective advantage
- random changes in DNA that do not alter protein structure usually do not affect the phenotype
- Difficult to determine because by definition not easy to demonstrate effectiveness
null
Geographic variation
- genetic variation among different populations within the same species
- Cline
- gradual change in species? phenotype and genotype frequencies
- through a series of geographically separate populations
null
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