Population Genetics and Natural Selection: Mechanisms and Applications

Population Genetics and Natural Selection

Introduction

Population genetics explores how genetic composition changes within populations over time, while natural selection is a primary mechanism driving evolutionary change. This section covers the processes and outcomes of natural selection, genetic drift, and their implications for evolution and agriculture.

The Process of Natural Selection

Genetic Variation and Fitness

• Genetic variation exists within populations due to differences in DNA sequences among individuals.

• Resources in the environment are limited, leading to competition among individuals.

• Fitness refers to an organism's ability to survive and reproduce in its environment. Individuals with greater fitness are more likely to pass their genes to the next generation.

• Natural selection can favor, disfavor, or conserve the genetic makeup of a population, depending on environmental pressures.

Modes of Natural Selection

Stabilizing Selection

• Acts against extreme phenotypes and favors average phenotypes.

• Reduces variation and maintains the status quo for a particular trait.

• Example: In a population, individuals with intermediate body size have higher survival and reproductive success than those with very small or very large body sizes.

Directional Selection

• Favors one extreme phenotype over others, causing the population's trait distribution to shift in one direction.

• Can lead to rapid evolutionary changes if the selective pressure is strong and persistent.

• Example: If larger body size confers a survival advantage, over time, the average body size in the population will increase.

Disruptive Selection

• Favors two or more extreme phenotypes over the average phenotype, resulting in a bimodal distribution.

• Can increase genetic diversity and may lead to speciation if the extremes become reproductively isolated.

• Example: In a population where both very small and very large body sizes are advantageous, but intermediate sizes are not, disruptive selection will increase the frequency of both extremes.

Summary Table: Modes of Natural Selection

Mode	Effect on Phenotype	Effect on Variation	Example
Stabilizing	Favors average	Reduces	Human birth weight
Directional	Favors one extreme	Shifts mean	Antibiotic resistance in bacteria
Disruptive	Favors both extremes	Increases	Beak size in Darwin's finches

Evolution by Natural Selection

Principles and Evidence

• Extensive research on natural populations supports the theory of evolution by natural selection.

• The environment shapes the evolution of anatomy, physiology, and behavior in organisms.

• Evolution by natural selection requires that traits are heritable (passed from parents to offspring).

Heritability: Essential for Evolution

Definition and Calculation

• Heritability (h2) is the proportion of total phenotypic variation in a trait that is attributable to genetic variance.

• Formula:

$h^2 = \frac{V_G}{V_P}$

• VG: Genetic variance

• VP: Phenotypic variance

• Phenotypic variance is the sum of genetic and environmental variance:

$V_P = V_G + V_E$

• Thus, heritability can also be expressed as:

$h^2 = \frac{V_G}{V_G + V_E}$

• High heritability means genetic factors strongly influence the trait; low heritability means environmental factors are more important.

Case Studies in Natural Selection

Stabilizing Selection for Egg Size in Ural Owls (Strix uralensis)

• Researchers studied variability in egg size among Ural owls.

• Questions addressed:

  • How much variation is due to genetic differences (heritability)?

  • How much is due to environmental influence (phenotypic plasticity)?

• Findings:

  • Egg size is highly heritable.

  • Egg size is undergoing stabilizing selection—most eggs are of intermediate size, which maximizes hatching success and fledgling survival.

Directional Selection: Soapberry Bugs and Host Plants

• Soapberry bugs feed on seeds from the Sapindaceae family, using slender beaks to pierce fruit walls.

• Variation in fruit wall thickness among host plants exerts selective pressure on beak length.

• Studies found a close relationship between fruit radius and beak length, demonstrating adaptation through directional selection.

Disruptive Selection: Darwin's Finches (Geospiza)

• Darwin's finches exhibit variation in beak size, which is important for feeding ecology and is highly heritable.

• On El Garrapatero, the G. fortis population is dominated by two groups: small-beaked and large-beaked birds.

• Disruptive selection and nonrandom mating contribute to this bimodal distribution.

Change Due to Chance: Genetic Drift

Mechanisms of Genetic Drift

• Genetic drift refers to random changes in allele frequencies, especially significant in small populations.

• Two main mechanisms:

  • Founder effect: A new population is established by a small number of individuals, leading to reduced genetic diversity.

  • Bottleneck effect: A large portion of a population is suddenly eliminated by a chance event, reducing genetic diversity.

• Habitat fragmentation can increase the impact of genetic drift by reducing population size and genetic diversity.

Genetic Variation in Island Populations

• Isolated and smaller island populations generally have lower genetic variation compared to larger, mainland populations.

• Reduced genetic variation limits the potential for populations to adapt and evolve in response to environmental changes.

Evolution and Agriculture

Artificial Selection

• Artificial selection is the intentional breeding of domesticated organisms to produce or maintain desirable traits.

• Examples include selecting for higher fruit yield, disease resistance, or specific physical characteristics in crops and livestock.

Unintended Evolutionary Consequences

• The use of chemicals in agriculture (e.g., pesticides, herbicides) can drive the evolution of resistance in plant and animal pests.

• This is an example of rapid evolution by natural selection in response to human-imposed selective pressures.

Additional info: The notes above expand on the provided slides and handwritten content, adding definitions, examples, and formulas for clarity and completeness. Case studies and summary tables are included for exam preparation.