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.