Microevolution: Natural Selection

Overview of Natural Selection

Natural selection is a fundamental mechanism of evolution, describing how certain traits become more or less common in a population due to differential survival and reproduction. Understanding the requirements and outcomes of natural selection is essential for studying evolutionary biology.

• Requirements for Natural Selection:

  1. Variation: Individuals in a population must exhibit differences in traits.

  2. Heritability: Traits must be heritable, meaning they can be passed from parents to offspring.

  3. Differential Survival and Reproduction: Some individuals must survive and reproduce more successfully than others due to their traits.

• Reproductive Fitness: Defined as an individual's ability to survive and reproduce, contributing genes to the next generation.

• Changing Frequencies: Natural selection can change allele frequencies in a population over time, leading to evolutionary change.

• Selection and Traits: Selection favors traits that increase fitness, resulting in an increase or decrease in trait frequency over generations.

• Statistical Tests: Scientists use statistical tests to determine if natural selection is acting on a trait by analyzing changes in trait frequencies.

• Figure Interpretation: Translating data from figures to text is a key skill in evaluating evolutionary trends.

Example: The peppered moth (Biston betularia) in England showed a shift in coloration due to industrial pollution, illustrating natural selection in response to environmental change.

Introduction to Genetic Terminology

Key Genetic Terms and Their Relationships

Genetics is the study of heredity and variation in organisms. Understanding basic genetic terminology is crucial for interpreting evolutionary processes and mechanisms.

• Nucleotide: The basic building block of DNA and RNA, consisting of a sugar, phosphate, and nitrogenous base.

• DNA (Deoxyribonucleic Acid): The molecule that carries genetic information in cells.

• Chromosome: A structure composed of DNA and proteins that contains many genes.

• Gene: A segment of DNA that codes for a specific protein or trait.

• Locus/Loci: The specific physical location of a gene on a chromosome.

• Phenotype: The observable characteristics or traits of an organism.

• Allele: Different forms of a gene found at the same locus.

• Genotype: The genetic makeup of an organism; the combination of alleles present.

• Homologous Chromosomes: Chromosome pairs, one from each parent, that have the same genes at the same loci.

Relationships: Genes are located on chromosomes, and each gene can have multiple alleles. The genotype determines the phenotype, and homologous chromosomes carry the same genes but may have different alleles.

Example: The gene for flower color in pea plants (Pisum sativum) can have alleles for purple or white flowers, resulting in different phenotypes.

Mechanisms of Microevolutionary Change

Processes That Alter Allele Frequencies in Populations

Microevolution refers to changes in allele frequencies within a population over time. Several mechanisms contribute to these changes, each with distinct effects on genetic diversity.

• Gene Flow: The movement of alleles between populations due to migration. Increases genetic diversity.

• Genetic Drift: Random changes in allele frequencies, especially in small populations. Includes bottleneck and founder effects.

  • Bottleneck Effect: A sharp reduction in population size due to environmental events, leading to loss of genetic variation.

  • Founder Effect: When a small group establishes a new population, resulting in reduced genetic diversity.

• Natural Selection: Differential survival and reproduction based on trait advantages, leading to adaptation.

Microevolution in Diploid Organisms: These mechanisms operate in organisms with two sets of chromosomes, affecting allele frequencies across generations.

Example: Genetic drift can cause rare alleles to disappear in a small population of island finches, while gene flow can introduce new alleles from migrating birds.

Case Studies in Evolution

Application of Evolutionary Mechanisms to Real-World Examples

Case studies help illustrate the practical implications of evolutionary theory by examining specific examples and research findings.

• Five Mechanisms of Evolution: Mutation, gene flow, genetic drift, natural selection, and non-random mating.

• Case Study: "Got Lactase? The Evolution of Genes and Culture"

  • Explores the evolution of lactase persistence in human populations and its relationship to cultural practices such as dairy consumption.

  • Highlights gene-culture coevolution and the impact of natural selection on human genetics.

• Case Study: "Pronghorns and Predators"

  • Examines the evolution of pronghorn speed as an adaptation to predation pressure.

  • Demonstrates natural selection acting on physical traits.

• Case Study: "Mutation – Elephant tusklessness"

  • Investigates the increase in tuskless elephants due to selective pressures from poaching.

  • Shows how mutation and selection interact in wild populations.

• Case Study: "Genetic Drift – Japanese warblers"

  • Analyzes genetic drift in bird populations isolated on islands, leading to changes in allele frequencies.

Additional info: Case studies often require reviewing articles, figures, and videos to understand the mechanisms and outcomes of evolutionary change.

Textbook Chapter Readings: Evolutionary Concepts

Key Questions and Learning Objectives from Chapters 19 and 21

Textbook readings reinforce core concepts in evolution, including natural selection, adaptation, and evidence for evolutionary change.

• Chapter 19:

  • Define natural selection and its requirements.

  • Distinguish between artificial and natural selection.

  • Explain the importance of variation in populations.

  • Describe how biogeography and fossil evidence support evolution.

• Chapter 21:

  • Describe genetic variation and its sources.

  • Explain how mutation, gene flow, and genetic drift impact populations.

  • Analyze the effects of selection and drift on allele frequencies.

Key Equations in Population Genetics

Hardy-Weinberg Principle

The Hardy-Weinberg equation describes the expected genotype frequencies in a non-evolving population.

• Equation:

Where p and q are the frequencies of two alleles in the population.

Summary Table: Mechanisms of Microevolution