Chapter_21_posted

Population and Evolutionary Genetics

Outline

• Population Genetics

  • Study of genetic variation within populations

• Genetic Variation

  • Presence of differences in individuals

• Hardy-Weinberg

  • Predictions, assumptions, implications

  • Determining allele frequencies

  • Equilibrium

  • Changes in allele frequencies over time

• Speciation

  • Formation of new and distinct species

• Evolutionary History

  • Study of how organisms change over time

Neo-Darwinism

• Historical Context

  • 1859: Publication of "On the Origin of Species"

    • Evidence that populations and species change over time due to natural selection

  • Early 1900s: Gregor Mendel's work on inheritance

    • Insights applied to population genetics

• Concept

  • Neo-Darwinism: Merge of population genetics and natural selection

Micro- vs. Macroevolution

• Definition of Evolution

  • Changes in genetic material through mutation

  • Alterations in allele frequencies in populations over time

• Microevolution

  • Changes within a population of a species

• Macroevolution

  • Major evolutionary developments resulting in new species or taxonomic groups

Population Genetics

• Focus Areas

  • Changes in genetic variation over time

  • Distribution of genotypes

  • Influence of natural forces (selection, mutation) on genetic variation

• Key Terms

  • Population: Individuals of the same species in a defined region that can interbreed

  • Gene Pool: Genetic information present in a population

Genetic Variation

• Significance

  • High levels of genetic variation found in most populations

  • Heterozygosity: Variation might be overlooked phenotypically

• Detection Methods

  • Artificial Selection: Breeding strategies to select for desired traits

  • DNA Sequence Analysis: Techniques used to analyze genetic variation

Genetic Variation in Humans

• Example: Cystic fibrosis gene (CFTR)

  • In European populations, ~1 in 44 individuals are heterozygous carriers

  • Over 1500 different mutations account for 67% of all alleles

The Hardy-Weinberg (H-W) Law

• Purpose

  • Describes allele and genotype frequencies in ideal populations

• Ideal Conditions

  • Infinite size, random mating, no evolutionary forces

Predictions and Equilibrium in H-W Law

• Predictions

  1. Allele frequencies do not change over time

  2. Genotype frequencies calculable: p² + 2pq + q² = 1

• Example Calculation

  • Given specific allele frequencies, genotype probabilities for AA, Aa, aa calculated

Changes in Hardy-Weinberg Frequencies

• Key Concept

  • If conditions change (selection, mutation), allele frequencies shift

• Application of H-W

  • Understanding allele frequency stability in idealized populations

Hardy-Weinberg Assumptions

• All genotypes have equal survival and reproduction

• No new alleles (mutations)

• No migration

• Infinitely large populations

• Random mating conditions

Implications of H-W Law

• Dominant traits may not increase in frequency

• Genetic variability is maintained over generations

• Knowing one genotype allows for prediction of others

Real-world Applications: CCR5 Gene and HIV Resistance

• CCR5 Receptor

  • Used by HIV to enter cells

  • Deletion mutation (Δ32) confers resistance

Determining Allele Frequencies: CCR5 Example

• Ways to determine allele frequency using observed data

  • Frequencies can be derived from observed genetic data representative of a population

Testing Hardy-Weinberg Equilibrium

• Concept: Compare observed and expected frequencies

• Discrepancies indicate violation of assumptions

Multiple Alleles and H-W Law

• ABO Blood Group Example

  • Different genotypes correspond to specific blood types

  • Incorporates three alleles: IA, IB, IO

• Hardy-Weinberg equation expands to accommodate multiple alleles

Natural Selection

• Mechanisms

  • Variations in phenotypes

  • Heritability of traits

  • Overproduction of offspring leading to competition

• Outcomes

  • Certain phenotypes are favored, becoming more common over generations

Types of Selection**

• Directional Selection: Favoring one extreme phenotype

  • Example: Beak sizes in Galapagos finches during drought

• Stabilizing Selection: Favoring intermediate phenotypes; reduces variance

  • Example: Birth weights in humans

• Disruptive Selection: Favors extremes, selects against intermediates

Other Evolutionary Forces

• Mutation: Introduces new alleles

  • Example calculation of mutation rates in populations

• Migration: Shifts allele frequencies due to gene flow

• Genetic Drift: Chance events affecting small populations; includes founder effects and bottlenecks

Non-random Mating and Inbreeding

• Types

  • Positive assortative mating: Similar genotypes mate more often

  • Negative assortative mating: Dissimilar genotypes preferentially mate

• Inbreeding: Increases homozygosity, risks for recessive disorders

Coefficient of Inbreeding

• Measures the probability of alleles being identical due to common ancestry

• Range from F=0 (no relation) to F=1 (completely related)

Speciation and Reproductive Isolation

• Definition of Species: Groups that can interbreed but are isolated from others

• Mechanisms

  • Prezygotic: Prevent mating (e.g., temporal, geographical)

  • Postzygotic: Affect viability or fertility of hybrids

Reconstructing Evolutionary History

• Examining genetic data to elucidate species relationships

• Molecular Clocks: Use mutation rates to estimate evolutionary timelines

Origin of Modern Humans

• Evidence of human origins and migrations revealed through archaeology

• Contested routes of movement illustrated through archaeological finds

Neanderthal and Modern Humans**

• Evidence for shared ancestry and interbreeding identified through genetic analysis

  • Neanderthals lived with Homo sapiens; some evidence suggests interbreeding

• Genetic traces in modern humans indicating Neanderthal heritage

Review and Conclusion

• Key concepts in population genetics, genetic variation, Hardy-Weinberg law, and implications in speciation and evolutionary history.