This document provides an overview of population genetics and principles of evolution. It discusses how genetic variation is maintained in populations through mechanisms such as sexual reproduction, genetic drift, mutation and natural selection. A key concept is that evolution occurs through changes in allele frequencies in populations over generations. The document also covers Mendelian inheritance, Darwinian evolution, the Hardy-Weinberg principle of genetic equilibrium, and factors that can lead to deviations from equilibrium, driving microevolutionary changes within populations.
Population Genetics 2015 03-20 (AGB 32012)Suvanthinis
The document discusses Hardy-Weinberg equilibrium, which states that allele and genotype frequencies in a population will remain constant from one generation to the next if the population is large, randomly mating, and not experiencing mutation, immigration, emigration or natural selection. It defines key population genetics terms like population, gene pool, allele frequency. It outlines the five conditions for Hardy-Weinberg equilibrium and provides an example calculation of genotype frequencies given allele frequencies in a population of cats. Factors that can disrupt Hardy-Weinberg equilibrium include small population size, non-random mating, mutation, migration, and natural selection.
This document summarizes key concepts in population genetics and Hardy-Weinberg equilibrium. It defines population genetics as the study of gene and genotype frequencies in populations. The Hardy-Weinberg law states that allele and genotype frequencies remain constant from generation to generation in random mating populations of infinite size with no evolutionary influences. Factors like selection, mutation, migration, and genetic drift can disrupt Hardy-Weinberg equilibrium over time.
Population genetics reconciled Darwin and Mendel's ideas by showing how natural selection could act on variation present in populations. The Hardy-Weinberg theorem describes genetic equilibrium in a population where allele frequencies remain constant between generations unless disrupted by factors like genetic drift, migration, non-random mating, mutation, or natural selection. These disruptions to equilibrium allow for microevolution and populations to change over time through natural selection acting on genetic variation.
1) Quantitative genetics focuses on inheritance of quantitative traits controlled by multiple genes and influenced by the environment.
2) A basic single-gene model is used to explain quantitative genetic theory, including calculations of population mean, genetic effects, and variance components.
3) More complex multi-gene models and analyses like ANOVA and heritability are then introduced to better capture quantitative traits controlled by numerous genes and environmental influences.
Population genetics is the study of genetic variation within populations. A population's gene pool contains all the alleles of all individuals. Under Hardy-Weinberg equilibrium, allele frequencies remain constant between generations if there is no mutation, migration, genetic drift, or natural selection. Five agents cause evolution: mutation, gene flow, genetic drift, nonrandom mating, and natural selection, which is the only mechanism that leads to adaptation. Natural selection maintains genetic variation and can preserve polymorphisms through mechanisms like heterozygote advantage.
Population genetics is the study of genetic variation within populations over time. It examines changes in allele frequencies, genotype frequencies, and phenotype frequencies. The field originated from the synthesis of Darwin's theory of evolution by natural selection and Mendel's laws of inheritance. According to the Hardy-Weinberg principle, allele and genotype frequencies remain constant across generations in large, randomly mating populations without other evolutionary influences. Factors like mutation, migration, genetic drift, non-random mating, and natural selection can cause frequencies to change and drive evolution.
This document discusses quantitative traits, which are traits influenced by multiple genes and the environment, resulting in continuous variation in phenotypes rather than discrete categories. It provides examples of quantitative traits like height and explains how Mendelian genetics can still underlie such traits. For example, wheat kernel color is influenced by three genes, with partial dominance of alleles resulting in a quantitative distribution of colors in offspring. The document also discusses statistics used to describe and analyze quantitative traits, like mean, variance, and heritability.
Population Genetics 2015 03-20 (AGB 32012)Suvanthinis
The document discusses Hardy-Weinberg equilibrium, which states that allele and genotype frequencies in a population will remain constant from one generation to the next if the population is large, randomly mating, and not experiencing mutation, immigration, emigration or natural selection. It defines key population genetics terms like population, gene pool, allele frequency. It outlines the five conditions for Hardy-Weinberg equilibrium and provides an example calculation of genotype frequencies given allele frequencies in a population of cats. Factors that can disrupt Hardy-Weinberg equilibrium include small population size, non-random mating, mutation, migration, and natural selection.
This document summarizes key concepts in population genetics and Hardy-Weinberg equilibrium. It defines population genetics as the study of gene and genotype frequencies in populations. The Hardy-Weinberg law states that allele and genotype frequencies remain constant from generation to generation in random mating populations of infinite size with no evolutionary influences. Factors like selection, mutation, migration, and genetic drift can disrupt Hardy-Weinberg equilibrium over time.
Population genetics reconciled Darwin and Mendel's ideas by showing how natural selection could act on variation present in populations. The Hardy-Weinberg theorem describes genetic equilibrium in a population where allele frequencies remain constant between generations unless disrupted by factors like genetic drift, migration, non-random mating, mutation, or natural selection. These disruptions to equilibrium allow for microevolution and populations to change over time through natural selection acting on genetic variation.
1) Quantitative genetics focuses on inheritance of quantitative traits controlled by multiple genes and influenced by the environment.
2) A basic single-gene model is used to explain quantitative genetic theory, including calculations of population mean, genetic effects, and variance components.
3) More complex multi-gene models and analyses like ANOVA and heritability are then introduced to better capture quantitative traits controlled by numerous genes and environmental influences.
Population genetics is the study of genetic variation within populations. A population's gene pool contains all the alleles of all individuals. Under Hardy-Weinberg equilibrium, allele frequencies remain constant between generations if there is no mutation, migration, genetic drift, or natural selection. Five agents cause evolution: mutation, gene flow, genetic drift, nonrandom mating, and natural selection, which is the only mechanism that leads to adaptation. Natural selection maintains genetic variation and can preserve polymorphisms through mechanisms like heterozygote advantage.
Population genetics is the study of genetic variation within populations over time. It examines changes in allele frequencies, genotype frequencies, and phenotype frequencies. The field originated from the synthesis of Darwin's theory of evolution by natural selection and Mendel's laws of inheritance. According to the Hardy-Weinberg principle, allele and genotype frequencies remain constant across generations in large, randomly mating populations without other evolutionary influences. Factors like mutation, migration, genetic drift, non-random mating, and natural selection can cause frequencies to change and drive evolution.
This document discusses quantitative traits, which are traits influenced by multiple genes and the environment, resulting in continuous variation in phenotypes rather than discrete categories. It provides examples of quantitative traits like height and explains how Mendelian genetics can still underlie such traits. For example, wheat kernel color is influenced by three genes, with partial dominance of alleles resulting in a quantitative distribution of colors in offspring. The document also discusses statistics used to describe and analyze quantitative traits, like mean, variance, and heritability.
This document provides information about population genetics and the Hardy-Weinberg principle of genetic equilibrium. It defines key population genetics concepts such as gene pool, allele frequencies, and genotypes. It describes the five conditions required for Hardy-Weinberg equilibrium: large population size, random mating, no mutations, no migration, and no natural selection. Examples are provided to demonstrate how to calculate allele and genotype frequencies using the Hardy-Weinberg equation.
This document discusses population variation and selection. It provides an overview of key concepts:
1. Natural selection acts on individuals but only populations evolve over generations as allele frequencies change. This was shown using a population of finches where large-beaked birds survived a drought better.
2. Three mechanisms can cause changes in allele frequencies in a population: natural selection, genetic drift, and gene flow. Natural selection is the only mechanism that causes adaptive evolution.
3. Genetic variation within populations is required for evolution. Variation comes from new mutations and recombination during sexual reproduction. The Hardy-Weinberg principle describes populations where allele frequencies remain constant without evolutionary influences.
Srishti Agrawal presented on the Hardy-Weinberg law to Dr. Ajay Kumar. The Hardy-Weinberg law states that allele and genotype frequencies remain constant between generations in a population if it is large, mates randomly, has no migration, mutation, or selection. The law assumes organisms are diploid, reproduce sexually, have non-overlapping generations, random mating, an infinitely large population, equal allele frequencies between sexes. Violations of random mating can change frequencies from Hardy-Weinberg proportions. Selection, genetic drift, migration, and mutation can also influence allele and genotype frequencies across generations.
Population Genetics & Hardy - Weinberg Principle.pdfSuraj Singh
This document discusses population genetics and the Hardy-Weinberg principle of genetic equilibrium. It defines key terms like allele frequencies, genotype frequencies, and gene pools. It also describes how to calculate allele frequencies and genotype frequencies in a population. The document concludes by explaining that under certain conditions like a large panmictic population with random mating, the genetic structure will remain in equilibrium with allele and genotype frequencies remaining constant from generation to generation based on the Hardy-Weinberg principle.
This document discusses the Hardy-Weinberg law, which states that gene and genotype frequencies in a random mating population will remain constant from generation to generation if there is no selection, migration, mutation, or genetic drift. It provides an example of how to calculate genotype frequencies based on allele frequencies. Several factors that can disrupt Hardy-Weinberg equilibrium are described, including migration, mutation, genetic drift, inbreeding, and selection. The document was prepared by Kiran Dasanal to explain the basic principles of the Hardy-Weinberg law and factors affecting allele frequencies in populations.
This powerpoint gives a clear picture on inbreeding and also about outbreeding of higher organisms. This also explains the advantages and disadvantages of the above said topics. the methods of inbreeding and reasons for inbreeding also given in this powerpoint.
BIO 106
Lecture 10
Quantitative Inheritance
A. Inheritance of Quantitative Characters
1. Multiple Genes
2. Number of Genes in polygene Systems
3. Regression to the Mean
4. Effects of Dominance and Gene Interactions
5. Effects of Genes in Multiplying Effects
B. Analysis of Quantitative Characteristics
C. Components of Phenotypic Variance
D. Heredity
1. Heritability in the Narrow Sense
2. Heritability in the Broad Sense
A population is a group of the same species living in a geographical area at a given time. Variation exists between members of a population in structural, biochemical, physiological, developmental, and behavioral traits. This variation is caused by genetic and environmental factors and is maintained in a population at equilibrium by a balance of evolutionary forces. Over time, agents of change like natural selection, genetic drift, migration, and artificial selection can act on populations to change allele frequencies and potentially lead to the formation of new species.
Hardy-Weinberg equilibrium allows prediction of allele and genotype frequencies in a population over generations if the population is large, mates randomly, and is unaffected by mutations, migration or selection. It states that the allele frequencies will remain constant and can be used to determine the expected proportions of genotypes such as AA, Aa, and aa based on the allele frequencies p and q where p + q = 1.
Probability, Mendel, and Genetics PowerpointMrs. Henley
The document summarizes key concepts from Gregor Mendel's experiments with pea plants including:
- Mendel studied traits like plant height, seed shape and color in pea plants which existed in distinct forms (tall vs short, round vs wrinkled seeds)
- He performed controlled crosses between purebred (homozygous) pea plants and found that some traits were dominant over others in the offspring
- Mendel developed the concepts of dominant and recessive alleles and used Punnett squares to predict the probabilities of traits being expressed in offspring
This document discusses several evolutionary mechanisms: mutation, genetic drift, and natural selection. Mutation introduces heritable changes in DNA and is the ultimate source of genetic variation. Genetic drift is changes in allele frequencies that occur by chance, especially in small populations. It can reduce genetic variation over time. Natural selection leads to populations becoming locally adapted as individuals better suited to their environment leave more offspring.
The document discusses probability and genetics. It explains that Mendel's laws of segregation and independent assortment reflect the same laws of probability that apply to events like coin tosses or dice rolls. It then provides examples of how to calculate genetic probabilities, such as there being a 50% chance of a dominant trait being passed on, in the same way there is a 50% chance of tossing heads. The rule of multiplication is discussed, stating that the probability of two independent events occurring together is the probabilities multiplied. Examples of applying this to dihybrid crosses and calculating probabilities are also given.
This power point presentation is designed to explain deviation of Mendelian dihybrid ratio due to interaction of genes which may be of following types
1.Two gene pairs affecting same character – 9:3:3:1
2.Epistasis, one gene hides effect of other
a) Recessive Epistasis - 9:3:4
b) Dominant epistasis - 12:3:1
3.Complementary genes - 9:7 ( 2 genes responsible for production of a particular phenotype )
4. Duplicate genes – 15:1 ( same effect given by either of two genes )
5. Polymeric gene action - 9:6:1
6. Inhibitory gene action - 13 : 3
Each interaction is typical in itself and ratios obtained are different
Morgan conducted experiments in Drosophila melanogaster that discovered genetic linkage. He crossed a white-eyed, miniature-winged female to a wild-type male. In the F2 generation, most flies showed the parental phenotypes of white eyes and miniature wings, or wild-type eyes and wings. However, some flies showed non-parental phenotypes of white eyes and normal wings, or wild-type eyes and miniature wings, demonstrating genetic recombination between the two linked genes. This provided evidence that genes on the same chromosome may assort together during meiosis but can also undergo crossing over, resulting in new combinations of alleles. Morgan's discovery established the chromosomal theory of inheritance and allowed the field of genetics to construct genetic maps
Bio 106
Lecture 11 Genes in Populations
A. Population Genetics
B. Gene Frequencies and Equilibrium
1. Gene Frequencies
2. Gene Pool
3. Model System for Population Stability (Hardy – Weinberg Law)
2
cces2015
C. Changes in Gene Frequencies
1. Mutation
2. Selection
2.1 Relative Fitness
2.2 Selections and Variability
2.3 Selection and Mating
3. Systems
4. Migration
5. Genetic Drift
3
cces2015
D. Race and Species Formation
1. The Concept of Races
2. The Concept of Species
2.1 Reproductive Isolating Mechanisms
2.2 Rapid Speciation
This document discusses factors that affect genetic variation and change in populations, including evolution, natural selection, mutations, migration, and genetic drift. It provides details on each factor and how they influence allele frequencies in a gene pool over multiple generations, leading to evolution and potentially new species. Examples are given to illustrate concepts like founder effects and bottleneck effects on small populations.
This document provides an overview of the Hardy-Weinberg law in population genetics. It defines the Hardy-Weinberg principle as stating that allele and genotype frequencies will remain constant from one generation to the next in a population where no evolutionary influences are present. The history of the law developed from the work of Hardy in 1908 and Weinberg in 1908 is discussed. The key assumptions required for Hardy-Weinberg equilibrium - including random mating, large population size, no natural selection or gene flow - are outlined. Equations for calculating expected genotype frequencies based on allele frequencies are presented along with an example calculation.
It is the fundamental law of population genetics and provides the basis for studying Mendelian populations ( Mendelian population: A group of sexually inbreeding organisms living within a circumscribed area). It describes populations that are not evolving.
Population genetics is the study of genetic variation within species. The key concepts are:
1) The Hardy-Weinberg principle states that allele and genotype frequencies in a population will remain constant from generation to generation if mating is random and other evolutionary forces are absent.
2) Founder effects occur when new populations are established by a small number of individuals, resulting in a loss of genetic variation compared to the original population.
3) Factors like non-random mating, genetic drift, migration, mutation, and natural selection can cause changes in allele frequencies over time, known as microevolution.
This document discusses population genetics and key concepts in evolutionary biology. It covers Mendelian genetics, the Hardy-Weinberg principle of equilibrium, mechanisms of evolution like genetic variation, natural selection and genetic drift. It also discusses applications of population genetics like estimating allele frequencies, measuring genetic variation, and using DNA polymorphisms in forensics.
The document discusses several key concepts related to microevolution and population genetics, including:
- Gene pools and allelic frequencies changing over time through mutations, gene flow, genetic drift, and non-random mating can cause microevolution.
- Hardy-Weinberg principle states that allele frequencies remain stable in a population not experiencing these evolutionary forces.
- Types of natural selection include directional, stabilizing, and disruptive selection.
- Speciation occurs when reproductive isolation leads to the splitting of one species into two or more distinct species over time. Mechanisms of isolation can be prezygotic or postzygotic.
This document provides information about population genetics and the Hardy-Weinberg principle of genetic equilibrium. It defines key population genetics concepts such as gene pool, allele frequencies, and genotypes. It describes the five conditions required for Hardy-Weinberg equilibrium: large population size, random mating, no mutations, no migration, and no natural selection. Examples are provided to demonstrate how to calculate allele and genotype frequencies using the Hardy-Weinberg equation.
This document discusses population variation and selection. It provides an overview of key concepts:
1. Natural selection acts on individuals but only populations evolve over generations as allele frequencies change. This was shown using a population of finches where large-beaked birds survived a drought better.
2. Three mechanisms can cause changes in allele frequencies in a population: natural selection, genetic drift, and gene flow. Natural selection is the only mechanism that causes adaptive evolution.
3. Genetic variation within populations is required for evolution. Variation comes from new mutations and recombination during sexual reproduction. The Hardy-Weinberg principle describes populations where allele frequencies remain constant without evolutionary influences.
Srishti Agrawal presented on the Hardy-Weinberg law to Dr. Ajay Kumar. The Hardy-Weinberg law states that allele and genotype frequencies remain constant between generations in a population if it is large, mates randomly, has no migration, mutation, or selection. The law assumes organisms are diploid, reproduce sexually, have non-overlapping generations, random mating, an infinitely large population, equal allele frequencies between sexes. Violations of random mating can change frequencies from Hardy-Weinberg proportions. Selection, genetic drift, migration, and mutation can also influence allele and genotype frequencies across generations.
Population Genetics & Hardy - Weinberg Principle.pdfSuraj Singh
This document discusses population genetics and the Hardy-Weinberg principle of genetic equilibrium. It defines key terms like allele frequencies, genotype frequencies, and gene pools. It also describes how to calculate allele frequencies and genotype frequencies in a population. The document concludes by explaining that under certain conditions like a large panmictic population with random mating, the genetic structure will remain in equilibrium with allele and genotype frequencies remaining constant from generation to generation based on the Hardy-Weinberg principle.
This document discusses the Hardy-Weinberg law, which states that gene and genotype frequencies in a random mating population will remain constant from generation to generation if there is no selection, migration, mutation, or genetic drift. It provides an example of how to calculate genotype frequencies based on allele frequencies. Several factors that can disrupt Hardy-Weinberg equilibrium are described, including migration, mutation, genetic drift, inbreeding, and selection. The document was prepared by Kiran Dasanal to explain the basic principles of the Hardy-Weinberg law and factors affecting allele frequencies in populations.
This powerpoint gives a clear picture on inbreeding and also about outbreeding of higher organisms. This also explains the advantages and disadvantages of the above said topics. the methods of inbreeding and reasons for inbreeding also given in this powerpoint.
BIO 106
Lecture 10
Quantitative Inheritance
A. Inheritance of Quantitative Characters
1. Multiple Genes
2. Number of Genes in polygene Systems
3. Regression to the Mean
4. Effects of Dominance and Gene Interactions
5. Effects of Genes in Multiplying Effects
B. Analysis of Quantitative Characteristics
C. Components of Phenotypic Variance
D. Heredity
1. Heritability in the Narrow Sense
2. Heritability in the Broad Sense
A population is a group of the same species living in a geographical area at a given time. Variation exists between members of a population in structural, biochemical, physiological, developmental, and behavioral traits. This variation is caused by genetic and environmental factors and is maintained in a population at equilibrium by a balance of evolutionary forces. Over time, agents of change like natural selection, genetic drift, migration, and artificial selection can act on populations to change allele frequencies and potentially lead to the formation of new species.
Hardy-Weinberg equilibrium allows prediction of allele and genotype frequencies in a population over generations if the population is large, mates randomly, and is unaffected by mutations, migration or selection. It states that the allele frequencies will remain constant and can be used to determine the expected proportions of genotypes such as AA, Aa, and aa based on the allele frequencies p and q where p + q = 1.
Probability, Mendel, and Genetics PowerpointMrs. Henley
The document summarizes key concepts from Gregor Mendel's experiments with pea plants including:
- Mendel studied traits like plant height, seed shape and color in pea plants which existed in distinct forms (tall vs short, round vs wrinkled seeds)
- He performed controlled crosses between purebred (homozygous) pea plants and found that some traits were dominant over others in the offspring
- Mendel developed the concepts of dominant and recessive alleles and used Punnett squares to predict the probabilities of traits being expressed in offspring
This document discusses several evolutionary mechanisms: mutation, genetic drift, and natural selection. Mutation introduces heritable changes in DNA and is the ultimate source of genetic variation. Genetic drift is changes in allele frequencies that occur by chance, especially in small populations. It can reduce genetic variation over time. Natural selection leads to populations becoming locally adapted as individuals better suited to their environment leave more offspring.
The document discusses probability and genetics. It explains that Mendel's laws of segregation and independent assortment reflect the same laws of probability that apply to events like coin tosses or dice rolls. It then provides examples of how to calculate genetic probabilities, such as there being a 50% chance of a dominant trait being passed on, in the same way there is a 50% chance of tossing heads. The rule of multiplication is discussed, stating that the probability of two independent events occurring together is the probabilities multiplied. Examples of applying this to dihybrid crosses and calculating probabilities are also given.
This power point presentation is designed to explain deviation of Mendelian dihybrid ratio due to interaction of genes which may be of following types
1.Two gene pairs affecting same character – 9:3:3:1
2.Epistasis, one gene hides effect of other
a) Recessive Epistasis - 9:3:4
b) Dominant epistasis - 12:3:1
3.Complementary genes - 9:7 ( 2 genes responsible for production of a particular phenotype )
4. Duplicate genes – 15:1 ( same effect given by either of two genes )
5. Polymeric gene action - 9:6:1
6. Inhibitory gene action - 13 : 3
Each interaction is typical in itself and ratios obtained are different
Morgan conducted experiments in Drosophila melanogaster that discovered genetic linkage. He crossed a white-eyed, miniature-winged female to a wild-type male. In the F2 generation, most flies showed the parental phenotypes of white eyes and miniature wings, or wild-type eyes and wings. However, some flies showed non-parental phenotypes of white eyes and normal wings, or wild-type eyes and miniature wings, demonstrating genetic recombination between the two linked genes. This provided evidence that genes on the same chromosome may assort together during meiosis but can also undergo crossing over, resulting in new combinations of alleles. Morgan's discovery established the chromosomal theory of inheritance and allowed the field of genetics to construct genetic maps
Bio 106
Lecture 11 Genes in Populations
A. Population Genetics
B. Gene Frequencies and Equilibrium
1. Gene Frequencies
2. Gene Pool
3. Model System for Population Stability (Hardy – Weinberg Law)
2
cces2015
C. Changes in Gene Frequencies
1. Mutation
2. Selection
2.1 Relative Fitness
2.2 Selections and Variability
2.3 Selection and Mating
3. Systems
4. Migration
5. Genetic Drift
3
cces2015
D. Race and Species Formation
1. The Concept of Races
2. The Concept of Species
2.1 Reproductive Isolating Mechanisms
2.2 Rapid Speciation
This document discusses factors that affect genetic variation and change in populations, including evolution, natural selection, mutations, migration, and genetic drift. It provides details on each factor and how they influence allele frequencies in a gene pool over multiple generations, leading to evolution and potentially new species. Examples are given to illustrate concepts like founder effects and bottleneck effects on small populations.
This document provides an overview of the Hardy-Weinberg law in population genetics. It defines the Hardy-Weinberg principle as stating that allele and genotype frequencies will remain constant from one generation to the next in a population where no evolutionary influences are present. The history of the law developed from the work of Hardy in 1908 and Weinberg in 1908 is discussed. The key assumptions required for Hardy-Weinberg equilibrium - including random mating, large population size, no natural selection or gene flow - are outlined. Equations for calculating expected genotype frequencies based on allele frequencies are presented along with an example calculation.
It is the fundamental law of population genetics and provides the basis for studying Mendelian populations ( Mendelian population: A group of sexually inbreeding organisms living within a circumscribed area). It describes populations that are not evolving.
Population genetics is the study of genetic variation within species. The key concepts are:
1) The Hardy-Weinberg principle states that allele and genotype frequencies in a population will remain constant from generation to generation if mating is random and other evolutionary forces are absent.
2) Founder effects occur when new populations are established by a small number of individuals, resulting in a loss of genetic variation compared to the original population.
3) Factors like non-random mating, genetic drift, migration, mutation, and natural selection can cause changes in allele frequencies over time, known as microevolution.
This document discusses population genetics and key concepts in evolutionary biology. It covers Mendelian genetics, the Hardy-Weinberg principle of equilibrium, mechanisms of evolution like genetic variation, natural selection and genetic drift. It also discusses applications of population genetics like estimating allele frequencies, measuring genetic variation, and using DNA polymorphisms in forensics.
The document discusses several key concepts related to microevolution and population genetics, including:
- Gene pools and allelic frequencies changing over time through mutations, gene flow, genetic drift, and non-random mating can cause microevolution.
- Hardy-Weinberg principle states that allele frequencies remain stable in a population not experiencing these evolutionary forces.
- Types of natural selection include directional, stabilizing, and disruptive selection.
- Speciation occurs when reproductive isolation leads to the splitting of one species into two or more distinct species over time. Mechanisms of isolation can be prezygotic or postzygotic.
Hardy-Weinberg-Castle Principle of EquilibriumHenry Sergio Jr
The Hardy-Weinberg-Castle Principle states that under conditions of large population size, no mutation, no immigration or emigration, random mating, and random reproductive success, allelic and phenotypic frequencies will remain constant across generations. Natural populations rarely meet all these conditions, experiencing factors like non-random mating and selective pressures that drive evolution by changing frequencies over time. Studying this principle proves that natural selection is necessary for evolution and allows measurement of selective effects by comparing actual and expected allelic or phenotypic frequencies.
The document discusses the Hardy-Weinberg law, which states that allele and genotype frequencies in a population remain constant from generation to generation if the population meets certain conditions. These conditions include large population size, random mating, no mutations, no migration, and no natural selection. The Hardy-Weinberg equations can be used to calculate expected genotype frequencies based on allele frequencies in populations that are in Hardy-Weinberg equilibrium. Two examples are provided to demonstrate calculating allele and genotype frequencies.
The document discusses the Hardy-Weinberg law of genetic equilibrium. It states that the law describes how allele and genotype frequencies in a population remain constant from generation to generation if the population meets five assumptions: large population size, random mating, no mutations, no migration, and no natural selection. It provides examples of using the Hardy-Weinberg equations to calculate allele and genotype frequencies for populations of rats and cattle.
The document discusses Hardy-Weinberg equilibrium, which states that allele and genotype frequencies in a population will remain constant from generation to generation if the population meets certain conditions. These conditions are large population size, random mating, no mutations, no migration, and no natural selection. The Hardy-Weinberg principle can be used to calculate expected genotype frequencies and serves as a baseline for detecting deviations caused by non-random mating, genetic drift, natural selection or other evolutionary forces.
Here are the solutions to the try these problems:
1. a) T allele frequency = (88/125) + (1/2)*(37/125) = 0.7
t allele frequency = 1 - 0.7 = 0.3
b) TT genotype frequency = 0.49, Tt genotype frequency = 0.42, tt genotype frequency = 0.09
2. p = 200/300 = 0.667, q = 100/300 = 0.333
3. a) Recessive allele frequency = 1/2500 = 0.0004
b) Dominant allele frequency = 1 - 0.0004 = 0.9996
c) Heterozygous
The evolution of populations population geneticsStephanie Beck
This document discusses population genetics and the Hardy-Weinberg theorem. It defines key terms like population, gene pool, allele frequency, and microevolution. The Hardy-Weinberg theorem states that allele frequencies will remain constant between generations if a population is large, random mating occurs, there is no mutation, migration or natural selection. The document provides examples of how to use the Hardy-Weinberg equation to calculate allele frequencies and genotype proportions in populations.
The document discusses several mechanisms of evolution including natural selection, genetic drift, mutation, and gene flow. It explains the assumptions of Hardy-Weinberg equilibrium and how these mechanisms can lead to changes in allele frequencies over time and deviations from Hardy-Weinberg expectations. Examples are given of genetic drift, founder effects, bottlenecks, migration, and natural selection acting on populations.
The document discusses the classification of organisms and the binomial system of nomenclature. It explains that all organisms are classified based on their characteristics and relationships. Each organism is assigned a genus and species name, with the genus indicating the broader group it belongs to. Classification sorts organisms into a hierarchy of taxonomic ranks including species, genus, family, order, class, phylum, and kingdom. The example of chimpanzees and their taxonomic classification is provided to illustrate this system.
1) The document discusses Hardy-Weinberg equilibrium, which states that allele and genotype frequencies in a population remain constant from generation to generation if the population is large, randomly mating, and not experiencing selection, migration, mutation or genetic drift.
2) It provides an example showing how the frequencies of genotypes AA, Aa and aa are calculated based on the allele frequencies p and q in the parental generation.
3) The document explains that under Hardy-Weinberg equilibrium, the allele frequencies p and q will remain the same in subsequent generations, even as the specific genotype frequencies may change during mating.
The document discusses the Hardy-Weinberg principle, which states that allele and genotype frequencies in a population will remain constant from generation to generation if the population meets certain assumptions. These assumptions are that there is no natural selection, mutation, migration, genetic drift, or non-random mating occurring. The principle demonstrates that Mendelian inheritance in diploid populations will maintain genetic variation in the absence of evolutionary forces.
Here are the solutions to the try these problems:
1. a) T allele frequency = (88/125) + (1/2)*(37/125) = 0.72
t allele frequency = 1 - 0.72 = 0.28
b) TT genotype frequency = 0.72^2 = 0.52
Tt genotype frequency = 2*0.72*0.28 = 0.48
tt genotype frequency = 0.28^2 = 0.08
2. p = 200/300 = 0.667
q = 1 - p = 0.333
3. a) Recessive allele frequency = √(1/2500) = 0.02
b
This document discusses the Hardy-Weinberg law of genetic equilibrium. It states that in a large, randomly mating population, the frequencies of genotypes will remain constant from generation to generation in the absence of evolutionary influences like mutation, migration, genetic drift and non-random mating. The law establishes that the frequency of alleles A and a will be p and q, and the frequencies of genotypes AA, Aa and aa will be p^2, 2pq and q^2 respectively, where p + q = 1. The document provides examples of calculating genotype and gamete frequencies under Hardy-Weinberg equilibrium.
The document discusses the Hardy-Weinberg equation, which relates allele and genotype frequencies in a population at genetic equilibrium. It describes how the equation predicts that allele and genotype frequencies will remain constant from generation to generation if certain conditions are met, including no mutations, natural selection, migration, genetic drift or non-random mating. It provides examples of using the Hardy-Weinberg equation to calculate expected genotype and allele frequencies based on observed data from populations of horses and butterflies.
The document discusses the Hardy-Weinberg principle of population genetics. It states that the frequency of alleles in a population will remain constant over generations if the population is large, randomly mating, and not subject to mutations, gene flow, or selection pressures. It provides an example using cat coat color alleles to demonstrate calculating genotype frequencies based on observed phenotypes and applying the Hardy-Weinberg equations. Factors that can disrupt Hardy-Weinberg equilibrium and cause allele frequencies to change are also noted, including mutation, migration, natural selection, and genetic drift.
Hardy weinberg equilibrium and its consequences under different allelic syste...AnittaPulikanLionel
1) The document summarizes Hardy-Weinberg equilibrium, which states that gene and genotype frequencies in a population will remain constant from generation to generation if the population is large and there is no selection, migration, mutation, or non-random mating.
2) It describes the mathematical relationship between gene frequencies (p and q) and genotype frequencies (p^2, 2pq, q^2) known as the Hardy-Weinberg principle or the square law.
3) Several conditions must be met for a population to be in Hardy-Weinberg equilibrium including large population size, equal mating opportunities, no selection, migration, or mutation.
hardy weinberg genetic equilibrium by kk sahuKAUSHAL SAHU
INTRODUCTION
HISTORY
THE HARDY-WEINBERG LAW
DERIVATION
TERMINOLOGY
PROBLEMS
ASSUMPTION OF HAEDY –WEINBERG EQUILIBRIUM
REFERANCE
The Hardy–Weinberg principle, also known as the Hardy–Weinberg equilibrium, model, theorem or law.
States that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences.
These influences include mate choice, mutation, selection, genetic drift, gene flow and meiotic drive.
Because one or more of these influences are typically present in real populations, the Hardy–Weinberg principle describes an ideal condition against which the effects of these influences can be analyzed.
Population genetics deals with genetics at the level of populations rather than individuals. It considers the genetic composition of populations and how gene frequencies change over generations. The genetic constitution of a population is described by the proportion of individuals with each genotype and the transmission of genes from one generation to the next. Population genetics was first applied to genetic improvement in livestock. The Hardy-Weinberg law states that genotype and gene frequencies remain constant in a large, randomly mating population with no evolutionary influences. It provides a simple relationship between gene and genotype frequencies.
Foundations of Biological Sciences I Evolutionary Agents - 1 .docxbudbarber38650
Foundations of Biological Sciences I Evolutionary Agents - 1
A quick recap….
There are several terms that need to be clarified so that you can more easily follow the exercise. A gene is a
piece of DNA that directs the expression of a particular characteristic (trait). Genes are located on
chromosomes, and the location where a particular gene is found is referred to as the locus (plural: loci) of that
gene. An allele is a gene for which there is an alternative expression, which can lead to the alterative form of a
trait. For example, a diploid organism carries the allele “A” on one homologous chromosome, and the allele “A”
on the other. The genotype of this organism is then AA and it is said to be homozygous. An organism may also
carry two different alleles. For example on one chromosome it could carry the allele “A” and on the other it
could carry the allele “a”. The genotype of such an organisms is then Aa, and it is described as heterozygous for
this chromosomal locus.
The genotype of an organism is the listing of the two alleles for each trait that it possesses. The phenotype of an
organism is a description of the way a trait is displayed in the structure, behavior, or physiology of the organism.
Some alleles are dominant to others and mask the presence of other alleles. The dominant condition is indicated
by uppercase letters (e.g., “A”). The alleles that are masked are called recessive alleles. The recessive condition
is indicated by lowercase letters (e.g., “a”). When both dominants are present in the genotype (AA), the organism
is said to be homozygous dominant for the trait, and the organisms will show the dominant phenotype (trait
expression A). When both recessives are present in the genotype (aa), the organism is said to be homozygous
recessive for the trait, and the organisms will show the recessive phenotype (trait expression a). In the case of
complete dominance, the dominant allele completely masks the recessive allele, and an organism with a
heterozygous genotype (Aa) will show the dominant phenotype (trait expression A).
Evolutionary Agents
Evolution is a process resulting in changes in gene frequencies (= the genetic make-up) of a population over
time. The mechanisms of evolution include selection (which can cause change over time & adaptation), and
forces that provide variation and cause change over time (but not adaptation). Factors that change gene
frequencies over time are referred to as evolutionary agents.
A powerful way to detect the presence of evolutionary agents is the use of the Hardy–Weinberg model. This
model can be applied to traits that are influenced by several loci; the simplest case is for a trait that is regulated
by one locus with two alleles.
With the Hardy–Weinberg model, the frequency of genotypes in the population can be predicted from the
probability of encounters between gametes bearing the different alleles. With alleles R .
Similar to B.sc. agri i pog unit 4 population genetics (20)
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9
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2. 2
POPULATION GENETICS:
The study of the rules governing the
maintenance and transmission of
genetic variation in natural
populations.
3. 3
DARWINIAN EVOLUTION BY NATURAL SELECTION
Many more individuals are born than survive (COMPETITION).
Individuals within species are variable (VARIATION).
Some of these variations are passed on to offspring (HERITABILITY).
Survival and reproduction are not random. There must be a correlation
between fitness and phenotype.
4. 4
Gregor Mendel
The “rediscovery” of Mendel’s genetic studies
in 1902 by William Bateson completed the
missing model for the inheritance of genetic
factors.
Mendel published his work in the
Transactions of the Brunn Society of
Natural History in 1866.
6. 6
SEXUAL REPRODUCTION CONTRIBUTES TO VARIATION
Example – A Line Cross Experiment
Consider 2 diploid individuals with 3 loci and 2 alleles,
Parents: aabbcc x AABBCC
F1 progeny: AaBbCc
F2 progeny:
AABBCC AABBCc AABBcc
AABbCC AABbCc AABbcc
AAbbCC AAbbCc AAbbcc
AaBBCC AaBBCc AaBBcc
AaBbCC AaBbCc AaBbcc
AabbCC AabbCc Aabbcc
aaBBCC aaBBCc aaBBcc
aaBbCC aaBbCc aaBbcc
aabbCC aabbCc aabbcc
27
COMBINIATIONS
7. 7
Mechanisms of Evolution: Mendelian Genetics
in Populations
Genetic variation is the raw material of evolutionary change: how do we
measure it?
What are the forces that cause genetic changes within populations? That
is, what mechanisms cause evolutionary change?
8. 8
Population Genetics
Evolution can be defined as a change in gene frequencies through time.
Population genetics tracks the fate, across generations, of Mendelian
genes in populations.
Population genetics is concerned with whether a particular allele or
genotype will become more or less common over time, and WHY.
9. 9
A few things to keep in mind as we take
an excursion into population genetic
theory:
“Make things as simple as possible, but no simpler.”
---Einstein
“All models are wrong, some are useful.”
---Box
“No theory should fit all the facts because some of the facts are
wrong.”
---Bohr
10. 10
Some Definitions:
Population: A freely interbreeding group of individuals.
Gene Pool: The sum total of genetic information present in a
population at any given point in time.
Phenotype: A morphological, physiological, biochemical, or
behavioral characteristic of an individual organism.
Genotype: The genetic constitution of an individual organism.
Locus: A site on a chromosome, or the gene that occupies the
site.
Gene: A nucleic acid sequence that encodes a product with a
distinct function in the organism.
Allele: A particular form of a gene.
Gene (Allele) Frequency: The relative proportion of a particular
allele at a single locus in a population (a number between 0 and 1).
Genotype Frequency: The relative proportion of a particular
genotype in a population (a number between 0 and 1).
11. 11
The Gene PoolThe Gene Pool
Members of a species can
interbreed & produce fertile
offspring
Species have a shared
gene pool
Gene pool – all of the
alleles of all individuals in a
population
11
2
12. 12
The Gene PoolThe Gene Pool
Different species do
NOT exchange genes
by interbreeding
Different species that
interbreed often
produce sterile or less
viable offspring e.g.
Mule
12
13. 13
Assumptions:
1) Diploid, autosomal locus with 2 alleles: A and a
2) Simple life cycle:
PARENTS GAMETES ZYGOTES
(DIPLIOD) (HAPLOID) (DIPLOID)
These parents produce a large gamete pool (Gene Pool) containing
alleles A and a.
a A A a
a A A a A a a
a A A a a A a A A
a a A A a a a
a A a a A A
A a A
14. 14
Gamete (allele) Frequencies:
Freq(A) = p
Freq(a) = q
⇒ p + q = 1
Genotype Frequencies of 3 Possible Zygotes:
AA Aa aa
Freq (AA) = pA x pA = pA
2
Freq (Aa) = (pA x qa) + (qa x pA) = 2pAqa
Freq (aa) = qa x qa = qa
2
⇒ p2
+ 2pq + q2
= 1
15. 15
General Rule for Estimating Allele Frequencies
from Genotype Frequencies:
Genotypes: AA Aa aa
Frequency: p2
2pq q2
⇒Frequency of the A allele:
p = p2
+ ½ (2pq)
⇒Frequency of the a allele:
q = q2
+ ½ (2pq)
16. 16
Sample Calculation: Allele Frequencies
Assume N = 200 indiv. in each of two populations 1 & 2
Pop 1 : 90 AA 40 Aa 70 aa
Pop 2 : 45 AA 130 Aa 25 aa
In Pop 1 :
p = p2
+ ½ (2pq) = 90/200 + ½ (40/200) = 0.45 + 0.10 = 0.55
q = q2
+ ½ (2pq) = 70/200 + ½ (40/200) = 0.35 + 0.10 = 0.45
In Pop 2 :
p = p2
+ ½ (2pq) = 45/200 + ½ (130/200) = 0.225 + 0.325 = 0.55
q = q2
+ ½ (2pq) = 25/200 + ½ (130/200) = 0.125 + 0.325 = 0.45
17. 17
Main Points:
p + q = 1 (more generally, the sum of the
allele frequencies equals one)
p2
+ 2pq +q2
= 1 (more generally, the sum of
the genotype frequencies equals one)
Two populations with markedly different
genotype frequencies can have the same allele
frequencies
18. 18
PopulationsPopulations
A group of the same
species living in an area
No two individuals
are exactly alike
(variations)
More Fit individuals
survive & pass on their
traits
18
21. 21
Modern Synthesis TheoryModern Synthesis Theory
CombinesCombines DarwinianDarwinian
selectionselection andand MendelianMendelian
inheritanceinheritance
Population geneticsPopulation genetics --
study of genetic variationstudy of genetic variation
within a populationwithin a population
Emphasis onEmphasis on
quantitative charactersquantitative characters
(height, size …)(height, size …)
21
22. 22
Modern Synthesis TheoryModern Synthesis Theory
1940s – comprehensive1940s – comprehensive
theory of evolutiontheory of evolution (Modern(Modern
Synthesis Theory)Synthesis Theory)
Introduced by Fisher &Introduced by Fisher &
WrightWright
Until thenUntil then, many did not, many did not
accept that Darwin’s theoryaccept that Darwin’s theory
of natural selection couldof natural selection could
drive evolutiondrive evolution
22
S. Wright
A. Fisher
23. 23
Modern Synthesis Theory
• TODAY’S theory on evolution
Recognizes that GENES are responsible for the
inheritance of characteristics
Recognizes that POPULATIONS, not individuals,
evolve due to natural selection & genetic drift
Recognizes that SPECIATION usually is due to the
gradual accumulation of small genetic changes
23
24. 24
MicroevolutionMicroevolution
Changes occur in gene pools due to mutation,
natural selection, genetic drift, etc.
Gene pool changes cause more VARIATION in
individuals in the population
This process is called MICROEVOLUTION
Example: Bacteria becoming unaffected by
antibiotics (resistant)
24
26. 26
The Hardy-Castle-Weinberg Law
A single generation of random mating
establishes H-W equilibrium genotype
frequencies, and neither these frequencies
nor the gene frequencies will change in
subsequent generations.
Hardy
p2
+ 2pq + q2
= 1
27. 27
The Hardy-Weinberg PrincipleThe Hardy-Weinberg Principle
Used to describe a non-evolving
population.
Shuffling of alleles by meiosis and
random fertilization have no effect on the
overall gene pool.
Natural populations are NOT expected
to actually be in Hardy-Weinberg
equilibrium.
27
28. 28
The Hardy-Weinberg PrincipleThe Hardy-Weinberg Principle
Deviation from Hardy-Weinberg
equilibrium usually results in evolution
Understanding a non-evolving
population, helps us to understand how
evolution occurs
28
29. 29
5 Assumptions of the H-W Principle5 Assumptions of the H-W Principle
1. Large population size
- small populations have fluctuations in allele
frequencies (e.g., fire, storm).
2. No migration
- immigrants can change the frequency of an allele by
bringing in new alleles to a population.
3. No net mutations
- if alleles change from one to another, this will
change the frequency of those alleles
29
30. 30
5 Assumptions of the H-W Principle5 Assumptions of the H-W Principle
4. Random mating
- if certain traits are more desirable, then
individuals with those traits will be selected and
this will not allow for random mixing of alleles.
5. No natural selection
- if some individuals survive and reproduce at a
higher rate than others, then their offspring will
carry those genes and the frequency will change for
the next generation.
30
31. 31
The Hardy-Weinberg PrincipleThe Hardy-Weinberg PrincipleThe gene pool of a NON-EVOLVING population
remains CONSTANT over multiple generations
(allele frequency doesn’t change)
The Hardy-Weinberg Equation:
1.0 = p2
+ 2pq + q2
Where:
p2
= frequency of AA genotype
2pq = frequency of Aa
q2
= frequency of aa genotype
31
32. 32
The Hardy-Weinberg PrincipleThe Hardy-Weinberg Principle
Determining the Allele Frequency using Hardy-
Weinberg:
1.0 = p + q
Where:
p = frequency of A allele
q = frequency of a allele
32
33. 3333
Allele Frequencies Define Gene PoolsAllele Frequencies Define Gene Pools
As there are 1000 copies of the genes for color,
the allele frequencies are (in both males and females):
320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8
(80%) R
160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2
(20%) r
500 flowering plants
480 red flowers 20 white flowers
320 RR 160 Rr 20 rr
35. 35
IMPLICATIONS OF THE H-W PRINCIPLE:
1) A random mating population with no external forces acting on it will
reach the equilibrium H-W frequencies in a single generation, and
these frequencies remain constant there after.
2) Any perturbation of the gene frequencies leads to a new equilibrium
after random mating.
3) The amount of heterozygosity is maximized when the gene
frequencies are intermediate.
2pq has a maximum value of 0.5 when
p = q = 0.5
37. 37
FOUR PRIMARY USES OF THE H-W PRINCIPLE:
1) Enables us to compute genotype frequencies from generation to
generation, even with selection.
2) Serves as a null model in tests for natural selection, nonrandom
mating, etc., by comparing observed to expected genotype
frequencies.
3) Forensic analysis.
4) Expected heterozygosity provides a useful means of summarizing the
molecular genetic diversity in natural populations.
38. 38
DETECTING DEPARTURES FROM HWE
A χ2
-test (a standard goodness-of-fit test) can be used to detect
statistically significant departures from Hardy-Weinberg Equilibrium.
Step 1: Determine allele frequencies. (N = 100).
AA Aa aa
Observed: 30 60 10
p = 0.30 + 0.30 = 0.60 and q = 0.10 + 0.30 = 0.40
Step 2: Based on allele frequencies, calculate the expected
number of each genotype.
AA Aa aa
p2
N 2pqN q2
N
Expected: 36 48 16
39. 39
AA Aa aa
Observed: 30 60 10
Expected: 36 48 16
Step 3: Calculate χ2
test statistic.
χ2
= Σ [(O-E)2
/E]
= (30-36)2
/36 + (60-48)2
/48 + (10-16)2
/16 = 6.25
Step 4: Compare this result to critical value from the χ2
statistical
table. This test has 1 degree of freedom, so the critical value for α =
0.05 is 3.84.
6.25 > 3.84, so this is a significant departure from HWE!
DETECTING DEPARTURES FROM HWE
40. 40
IMPLICATIONS OF A STATISTICAL DEPARTURE FROM HWE
If the null hypothesis is true (i.e., we are in H-W equilibrium), we
would expect a sample of this size to show this much (or more) of a
departure from expectations (purely by chance sampling) less than
5 percent of the time.
One or more of the assumptions of the H-W principle are not
satisfied in this population.
Further research will be necessary to establish which assumption is
violated. [Excess of heterozygotes could be due to overdominant
selection, for example].
NOTE: A failure to detect a departure from H-W equilibrium does
not guarantee that the population satisfies all of the assumptions
of the model. The departure may simply not be statistically
detectable.
See Box 6.5 in F&H for more on X2
tests.
41. 41
EVOLUTIONARY THOUGHT AFTER DARWIN
By the 1870s, most scientists accepted the historical reality of
evolution (and this has been true ever since).
It would be at least 60 years after the publication of The Origin
of Species before natural selection would come to be widely
accepted.
People seemed to want life itself to be purposeful and creative,
and consequently did not find natural selection appealing.
Neo-Lamarckism -- inheritance of acquired
characteristics.
Orthogenesis -- variation that arises is directed toward a
goal.
Mutationism -- discrete variations are all that matter.
42. 42
Outcomes of the “MODERN
SYNTHESIS”
Populations contain genetic variation that arises by random
mutation.
Populations evolve by changes in gene frequency.
Gene frequencies change through random genetic drift, gene
flow, and natural selection.
Most adaptive variants have small effects on the phenotype so
changes are typically gradual.
Diversification comes about through speciation.
44. 44
MEASURING GENETIC VARIATION
IN NATURAL POPULATIONS
TWO COMMONLY USED MEASURES TO QUANTIFY GENETIC
VARIATION ARE:
P – the proportion of polymorphic loci (those that have 2
or more alleles)
H – the average heterozygosity = proportion of loci at
which a randomly chosen individual is heterozygous.
46. 46
Causes of MicroevolutionCauses of Microevolution
Genetic Drift
- the change in the gene pool of a small population due
to chance
Natural Selection
- success in reproduction based on heritable traits
results in selected alleles being passed to relatively more
offspring (Darwinian inheritance)
- Cause ADAPTATION of Populations
Gene Flow
-is genetic exchange due to the migration of fertile
individuals or gametes between populations
46
47. 47
Causes of MicroevolutionCauses of Microevolution
• Mutation
- a change in an organism’s DNA
- Mutations can be transmitted in gametes to offspring
• Non-random mating
- Mates are chosen on the basis of the best traits
47
49. 49
Factors that Cause Genetic DriftFactors that Cause Genetic Drift
•Bottleneck Effect
-a drastic reduction in population (volcanoes,
earthquakes, landslides …)
-Reduced genetic variation
-Smaller population may not be able to adapt to new
selection pressures
•Founder Effect
-occurs when a new colony is started by a few
members of the original population
-Reduced genetic variation
-May lead to speciation
51. 51
Loss of Genetic VariationLoss of Genetic Variation
•Cheetahs have little genetic variation in
their gene pool
•This can probably be attributed to a
population bottleneck they experienced
around 10,000 years ago, barely
avoiding extinction at the end of the
last ice age
54. 54
Modes of Natural SelectionModes of Natural Selection
• Directional Selection
- Favors individuals at one end of the phenotypic range
- Most common during times of environmental change
or when moving to new habitats
• Disruptive selection
- Favors extreme over intermediate phenotypes
- Occurs when environmental change favors an extreme
phenotype
54
55. 55
Modes of Natural SelectionModes of Natural Selection
Stabilizing Selection
- Favors intermediate over extreme phenotypes
- Reduces variation and maintains the cureent average
- Example: Human birth weight
55
60. 60
Heterozygote AdvantageHeterozygote Advantage
• Favors heterozygotes (Aa)
• Maintains both alleles (A,a) instead of removing less
successful alleles from a population
• Sickle cell anemia
> Homozygotes exhibit severe anemia, have
abnormal blood cell shape, and usually die before
reproductive age.
> Heterozygotes are less susceptible to malaria
60
61. 61
Other Sources of VariationOther Sources of Variation
• Mutations
- In stable environments, mutations often result in little or no
benefit to an organism, or are often harmful
- Mutations are more beneficial (rare) in changing environments
(Example: HIV resistance to antiviral drugs)
• Genetic Recombination
- source of most genetic differences between individuals in a
population
• Co-evolution
-Often occurs between parasite & host and flowers & their
pollinators
61
Main Points:
Since the Modern Synthesis we have a good understanding of the forces that act on populations to cause evolutionary change. These forces are: MUTATION, SELECTION, MIGRATION, and RANDOM GENETIC DRIFT.
It is the interaction of these forces with the GENETIC ARCHITECTURE in natural populations that determines the rate and trajectory of populations evolving in an adaptive landscape.
One pressing challenge for evolutionary biologists is to connect the processes that act on populations and within lineages with patterns of diversity between lineages.
Today I’m going to focus on the GENETIC ARCHITECTURE of natural populations.
In order to make the connection between evolution within lineages and diversity between we need to examine genetic variation at multiple levels……