Evolution Study Guide: Natural Selection & Adaptation

Evolution is the fundamental process that explains the diversity of life on Earth. This comprehensive study guide covers evolutionary theory, mechanisms of evolution, evidence for evolution, and modern applications in biology and medicine.

Historical Development of Evolutionary Theory

Pre-Darwinian Ideas

Lamarckism:

• Inheritance of acquired characteristics
• Use and disuse principle
• Example: Giraffes stretching necks to reach leaves

Problems with Lamarckism:

• No mechanism for inheritance of acquired traits
• Contradicted by genetics
• No evidence for directed change

Darwin's Theory of Evolution

Key Observations:

1. Variation exists within populations
2. Overproduction of offspring
3. Competition for limited resources
4. Differential survival and reproduction

Natural Selection:

• Definition: Process where organisms better adapted to environment survive and reproduce
• Requirements: Variation, heritability, differential fitness
• Result: Change in allele frequencies over time

Mechanisms of Evolution

Natural Selection

Types of Selection:

Directional Selection:

• Favors one extreme phenotype
• Example: Peppered moth color change during Industrial Revolution
• Formula: (\Delta p = sp(1-p)) where s is selection coefficient

Stabilizing Selection:

• Favors intermediate phenotypes
• Example: Human birth weight
• Result: Reduced variation around mean

Disruptive Selection:

• Favors both extremes
• Example: Beak size in finches
• Result: Bimodal distribution

Sexual Selection:

• Intrasexual: Competition within same sex
• Intersexual: Mate choice by opposite sex
• Example: Peacock tail feathers

Genetic Drift

Definition: Random change in allele frequencies due to chance

Types:

• Bottleneck effect: Population size reduction
• Founder effect: New population from small group

Formula: (\sigma_p = \sqrt{\frac{p(1-p)}{2N_e}})

Where:

• (\sigma_p) = Standard deviation of allele frequency
• (p) = Allele frequency
• (N_e) = Effective population size

Gene Flow

Definition: Movement of alleles between populations

Effects:

• Reduces genetic differences between populations
• Introduces new genetic variation
• Can prevent speciation

Mutation

Types:

• Point mutations: Single nucleotide changes
• Insertions/Deletions: Addition/removal of nucleotides
• Chromosomal mutations: Large-scale changes

Mutation Rate:

• Bacteria: 10⁻⁶ to 10⁻⁹ per generation
• Humans: 10⁻⁸ per nucleotide per generation

Evidence for Evolution

Fossil Record

Key Patterns:

• Transitional forms: Intermediate species
• Stratigraphic succession: Older fossils in deeper layers
• Extinction events: Mass extinctions and recoveries

Examples:

• Whale evolution: Land mammals to aquatic
• Horse evolution: Small forest browsers to large grazers
• Human evolution: Australopithecus to Homo sapiens

Comparative Anatomy

Homologous Structures:

• Similar structure, different function
• Example: Forelimbs of mammals
• Evidence: Common ancestry

Analogous Structures:

• Similar function, different structure
• Example: Wings of birds and insects
• Evidence: Convergent evolution

Vestigial Structures:

• Reduced function in current species
• Examples: Human appendix, whale pelvis
• Evidence: Evolutionary history

Comparative Embryology

Von Baer's Law:

• Early developmental stages are similar
• Differences appear later in development
• Example: Gill slits in vertebrate embryos

Molecular Evidence

DNA Sequence Comparison:

• Genetic distance: Number of differences
• Molecular clock: Rate of molecular change
• Phylogenetic trees: Evolutionary relationships

Protein Comparison:

• Amino acid sequences: Similar proteins across species
• Cytochrome c: Highly conserved protein
• Hemoglobin: Shows evolutionary relationships

Biogeography

Geographic Distribution:

• Continental drift: Explains distribution patterns
• Island biogeography: Species-area relationships
• Endemic species: Unique to specific regions

Population Genetics

Hardy-Weinberg Equilibrium

Assumptions:

• Large population size
• Random mating
• No mutation
• No migration
• No selection

Equations:

• (p + q = 1) (allele frequencies)
• (p^2 + 2pq + q^2 = 1) (genotype frequencies)

Example: If p = 0.7 (dominant allele), q = 0.3 (recessive allele)

• (p^2 = 0.49) (homozygous dominant)
• (2pq = 0.42) (heterozygous)
• (q^2 = 0.09) (homozygous recessive)

Evolutionary Applications

Medical Implications

• Antibiotic resistance: Natural selection for resistant bacteria
• Drug resistance: Evolution of drug-resistant pathogens
• Genetic disorders: Evolution of disease-causing mutations

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Keywords: evolution, natural selection, adaptation, evolutionary theory, population genetics.

Last Updated: July 12, 2025, 05:24 PM +04