BI111 Course Notes - Biological Diversity and Evolution

Course Overview

• Course: BI111: Biological Diversity and Evolution

• Department: Biology, Wilfrid Laurier University

• Term: Winter 2025

• Instructor: Dr. Tristan A.F. Long

  • Email: bi111@wlu.ca

  • Student Hours: Wednesdays (1:40-2:40 PM) in BA433 & Thursdays (1:30-2:30 PM) online or by appointment

  • Quizzes: Entrance quiz on Species & Speciation and Exit quiz on Population Genetics due Sundays @ 11:45

Learning Objectives

• Today’s Focus:

  1. Calculate allele and genotype frequencies

  2. Use allele frequencies to predict genotype frequencies under Hardy-Weinberg equilibrium (H-W equilibrium)

  3. Compare predictions to reality

Concepts of Genetic Variation

• Heritable Variation: Individuals in a population display genetic diversity that can be inherited.

• Simulated Genotypes in Populations:

  • Individuals are diploid and hermaphroditic

  • Alleles represented include recessive (b) and dominant (B) alleles (shown as hearts/diamonds for recessive & clubs/spades for dominant).

Hardy-Weinberg Principle

• A mathematical model predicting genotype frequencies under specific assumptions:

  1. Random Mating.

  2. Large Population Size (to avoid genetic drift).

  3. No Migration/Emigration/Mutation.

  4. No Selection (no differential reproductive success).

• Two alleles: Define p (frequency of b allele) and q (frequency of B allele), yields the equations:

  • Genotype frequencies:

  • Frequency of bb: p^2

  • Frequency of Bb: 2pq

  • Frequency of BB: q^2

Allele Frequencies Calculation

• Calculate frequencies by summing number of alleles from all individuals in the population:

  • Total alleles = Population Size x 2 (since diploid).

  • Compute p and q using the total count.

Expectations vs Reality

• After random mating, check if observed genotype frequencies match predicted ones. If they don't:

  • Identify the potential violation of H-W assumptions.

Genetic Drift

• The effect of small population size on genetic diversity. Bottleneck Effect: Loss of genetic variation can occur from significant reductions in population size.

• Simulations available to explore how genetic drift impacts allele frequencies.

Natural Selection

• Deleterious Alleles: Some harmful alleles may persist in the population if they are recessive and carry hidden effects.

• Explains why not all harmful alleles are eliminated from a population's gene pool.

Non-Random Mating

• Assortative Mating: Similar phenotypes mate more frequently.

• Disassortative Mating: Dissimilar phenotypes mate more frequently.

• Each type affects genotype frequencies, causing deviations from H-W equilibrium.

Quantitative vs Qualitative Traits

• Qualitative Traits: Distinct states (e.g., colour). Follow Mendelian inheritance.

• Quantitative Traits: Continuous variation (e.g., height). Often polygenic and influenced by environmental factors.

• Example of phenotypic distribution affected by natural selection through changes in allele frequencies.

Maintaining Genetic Variation

• Balancing Selection: Advantage for heterozygotes leads to stable frequencies of alleles despite selective pressures.

  • Example: Sickle-cell trait offers resistance to malaria, maintaining HbS allele in populations.

• Environmental influences can favour different phenotypes, further supporting the concept of balanced polymorphism.

Summary of Key Concepts

• Microevolution is driven by mutations, genetic drift, gene flow, natural selection, and non-random mating.

• Understanding these processes helps elucidate speciation mechanisms.

Resources for Further Study

• Video: "BI111 Beyond: Hardy Weinberg Calculations" via YouTube.

• Simulation Links: Explore genetic drift and evolution via provided online resources for interactive learning.

• Domesticated Species & Adaptation: Depending on the environment, different phenotypes can be adaptive or deleterious.

• Traits with a genetic basis that may disadvantage individuals in natural settings might be desirable to humans, leading to their favoring through selective breeding.

  • Example: The change from dehiscence (the shedding of ripened grains) to indehiscence (holding onto ripened grains).

• The genetic basis for speciation is not qualitatively different from the microevolutionary changes that occur within populations. The genetic variation present between and within populations serves as the ultimate basis for the origin of species.

Several factors can maintain genetic variation within populations:

• Balancing Selection: Selection for heterozygotes can stabilize allele frequencies despite selective pressures.

  • Example: Sickle-cell trait maintains HbS allele by providing resistance to malaria.

• Environmental Influences: Different environments can favor various phenotypes:

  • Riparian Environment: Abundant prey and predators favor non-aggressive phenotypes.

  • Arid-Land Environment: Limited prey and predators favor aggressive phenotypes.

• Genetic Drift: Affects genetic diversity, especially in small populations, through random changes in allele frequencies.

• Gene Flow: Movement of alleles between populations can introduce new genetic material, increasing variation.

• Mutation: New genetic variations are introduced into a population through changes in DNA sequences.

• Non-Random Mating: Assortative or disassortative mating can influence allele frequencies and maintain diversity.

Agents of Microevolutionary change

• Mutation: Introduction of new genetic variation.

• Genetic Drift: Loss of genetic variation due to the effect of small population size.

• Gene Flow:

  • Introduction of new genetic variation between populations through allele migration.

  • Can also lead to the loss of genetic variation within a population.

• Natural Selection: Changes in frequencies of alleles based on differential reproductive success.

• Non-random Mating: May change allelic frequencies as certain phenotypes mate more frequently than others, leading to a shift in genetic composition of the population.