Microevolution: Natural Selection and Genetic Drift

LABORATORY OBJECTIVES

  • The purpose of this laboratory is to enhance understanding of evolutionary mechanisms. After this lab and the associated readings, the student should be able to:

    • Define terms:

    • Evolution: A change in allele frequencies within a population over time.

    • Fitness: A measure of reproductive success defined as the number of surviving offspring an individual produces.

    • Compare and contrast:

    • Natural selection vs. genetic drift as mechanisms for evolution.

    • Describe factors necessary for natural selection to act in a population.

    • Explain how natural selection leads to adaptation in populations.

    • Discuss the differing impacts of genetic drift on large vs. small populations.

INTRODUCTION

  • Historical Context:

    • Charles Darwin wrote about evolution prior to understanding mutation or inheritance.

    • Modern evolution incorporates genetics, defining evolution through allele frequency changes.

  • Microevolution vs. Macroevolution:

    • Microevolution: Evolutionary changes at the population level across generations.

    • Macroevolution: Broader evolutionary changes resulting from accumulated microevolutionary processes.

  • Population Definition:

    • A population consists of individuals of the same species within a specific area capable of interbreeding.

  • Sources of Genetic Variation:

    • Genetic variation in sexually reproducing species arises from:

    • Mutation.

    • Independent assortment during gamete formation.

    • Crossing-over during gamete formation.

    • Random fertilization.

  • Gene Pool:

    • The gene pool is the total set of alleles for all genes in a population, subject to change over time through mechanisms such as mutation, migration, natural selection, and genetic drift.

NATURAL SELECTION

  • Darwin's Hypothesis:

    • Darwin introduced natural selection as a mechanism for evolutionary change, which occurs when:

    • There is genetic variation among individuals.

    • Individuals with different heritable traits experience differing reproductive success.

  • Selection Process:

    • Alleles that enhance survival and reproduction are "selected for", becoming more common.

    • Alleles detrimental to survival are "selected against" leading to a decrease in frequency.

  • Misinterpretation of "Survival of the Fittest":

    • The phrase misrepresents Darwin's concept of "fitness".

    • Biological Fitness Defined:

    • Defined as reproductive success, i.e., the number of surviving offspring produced.

  • Artificial Selection vs. Natural Selection:

    • Artificial selection: Humans select traits for breeding, while in natural selection, environmental factors determine advantageous traits.

    • Examples of artificial selection include domesticated animals and cultivated plants.

  • Adaptation Defined:

    • Adaptation is a heritable trait that enhances survival and reproduction, leading to increased frequency in future generations.

    • Example: Camouflage (cryptic coloration) enhances survival by helping individuals avoid predation.

GENETIC DRIFT

  • Random Process:

    • Genetic drift is a random change in allele frequencies due to chance events, differing from natural selection which is not random.

  • Population Size Impact:

    • Smaller populations are more affected by genetic drift, leading to significant allele frequency changes, potentially resulting in allele loss.

    • Example Scenario:

    • If a diploid individual with two alleles is randomly removed from a population of ten, it could drastically alter the gene pool, more so than in a population of one hundred.

EXERCISES

A. Natural Selection Simulation

Materials:

  • Plastic tray (“habitat”) for foraging.

  • Approximately 200 beads of a single color as prey.

  • Different foraging tools for each participant (e.g., forceps, plastic forks).

  • Plastic cups for each participant.

  • Writing tools for recording data.

Procedure:

  1. Organize into groups of six, each person using a different foraging tool. No more than two individuals should have the same tool.

  2. Pour beads into the habitat tray and spread evenly, monitored by the instructor.

  3. Forage for one minute, capturing beads one at a time and placing them in the plastic cup while following specific rules (e.g., not scraping beads).

  4. After the first round, count and record the number of beads captured.

  5. Identify successful predators; only individuals capturing the least will not reproduce. These individuals can select a different tool mirroring a successful predator.

  6. Repeat until only one predator type remains.

Data Collection Table:

  • Record the # of kills for each predator type across generations.

Discussion Questions:

  1. What was the surviving predator type, and why?

  2. If prey were tater tots instead of beads, would results differ? Consider attributes like color and patterns.

B. Genetic Drift Simulation

Materials:

  • Two plastic trays and colored beads (55 each of four colors).

  • Plastic cups for each participant.

Procedure:

  1. Create two populations (small: 5 beads of each color; large: 50 beads of each color).

  2. Simulate random mortality by removing beads blindfolded.

  3. For the large population, remove a total of 160 beads.

  4. Record the remaining colors and their respective percentages after each generation, and replenish to original sizes through reproduction.

  5. Repeat for small populations by removing 16 beads and recording percentages.

Data Collection Table:

  • Record remaining percentages of alleles through four generations.

Discussion Questions:

  1. Which population retained more alleles over generations?

  2. In habitat design for endangered species, is preserving one large habitat or several small habitats more beneficial for genetic diversity? Why?

  3. Match scenarios with evolutionary mechanisms:

    • Natural Selection Examples:
      a) Bright coloration of a poisonous snake.
      c) Increasingly large, sweet fruits by a tree.

    • Genetic Drift Examples:
      b) Albino coloration in a small population.
      d) Isolated population with uniform blood type.

NOTE ON CALCULATIONS

  • Percentage Calculations: For generations, combine the initial count of beads after rounds, multiplying the number of survivors by 4 to get new populations for the next generation.

  • E.g., if 10 red beads survive, next generation would total 10 + (10 * 4) = 50 red beads.