How Populations Evolve

Introduction to Evolution

Evolution is the process by which the genetic composition of populations changes over generations. This leads to the diversity of life observed today and is driven by several mechanisms, including natural selection, genetic drift, and gene flow.

• Evolution: Change in allele frequencies in a population over time.

• Natural selection: Individuals with advantageous traits survive and reproduce more successfully, causing those traits to become more common.

• Example: Antibiotic-resistant bacteria survive treatment and reproduce, leading to a resistant population.

Charles Darwin and Natural Selection

• Charles Darwin: Developed the theory of evolution by natural selection.

• "Descent with modification": All species share common ancestors and accumulate changes over generations.

• Example: Finches on the Galápagos Islands evolved different beak shapes based on available food sources.

Artificial Selection vs. Natural Selection

Both processes change populations over time, but differ in their driving forces.

Genetic Drift and Gene Flow

• Genetic drift: Random changes in allele frequencies, especially in small populations.

• Gene flow: Movement of alleles between populations via migration.

• Example (drift): A natural disaster randomly kills individuals, altering allele frequencies.

• Example (flow): Animals migrate and breed with a new population, introducing new genes.

Bottleneck Effect vs. Founder Effect

• Bottleneck effect: A large population is drastically reduced, resulting in low genetic diversity.

• Founder effect: A small group starts a new population with limited genetic variation.

• Example (bottleneck): Natural disaster wipes out most individuals.

• Example (founder): A few individuals colonize an island.

Types of Selection

• Directional selection: Favors one extreme trait (e.g., larger beaks during drought).

• Disruptive selection: Favors both extremes, selects against intermediates (e.g., very small and very large fish survive).

• Stabilizing selection: Favors average traits, reduces variation (e.g., average birth weight in humans).

Sexual Selection vs. Natural Selection

• Sexual selection: Traits that increase mating success (e.g., bright feathers in birds).

• Natural selection: Traits that increase survival and reproduction (e.g., camouflage).

• Key difference: Sexual selection focuses on reproductive success; natural selection includes both survival and reproduction.

Evidence for Evolution

• Fossils: Show changes in species over time (e.g., whale ancestors with legs).

• Comparative anatomy: Homologous structures indicate common ancestry.

• Embryology: Similar early development stages among vertebrates.

• Molecular biology: DNA/protein similarities among related species.

• Homologous structures: Similar structure, common ancestry.

• Analogous structures: Similar function, evolved independently.

• Vestigial structures: Reduced, inherited from ancestors (e.g., whale pelvis bones).

Other Evolutionary Mechanisms

• Genetic drift: Random changes, especially in small populations.

• Gene flow: Migration introduces new alleles.

Fitness

• Fitness: The ability to survive and reproduce in a given environment.

• Fitness is relative and context-dependent.

• Example: A small animal that reproduces more may have higher fitness than a stronger animal that does not reproduce.

How Biological Diversity Evolves

What is a Species?

• Species: A group of organisms that can interbreed and produce fertile offspring.

• Example: Horses and donkeys produce mules, which are sterile, indicating they are different species.

Speciation

• Biological species concept: Species are groups that can interbreed and produce viable, fertile offspring.

• Reproductive isolation: Prevents gene flow between species.

• Prezygotic barriers: Prevent mating or fertilization (habitat, temporal, behavioral, mechanical, gamete incompatibility).

• Postzygotic barriers: Prevent hybrid offspring from surviving or reproducing (hybrid inviability, sterility, breakdown).

• Allopatric speciation: Geographic separation leads to divergence.

• Sympatric speciation: Divergence without geographic separation (e.g., polyploidy in plants).

Evolutionary Trees (Phylogenies)

• Show relationships among species.

• Node: Branch point, most recent common ancestor.

• Clade: Ancestor and all its descendants.

• Derived trait: Newer trait unique to a clade.

• Ancestral trait: Older trait shared more widely.

Cladograms

• Diagrams based on shared derived traits.

• Outgroup: Used for comparison, lacks derived traits of main group.

Importance of Classification

• Organizes biodiversity.

• Helps predict traits and understand evolution.

Viruses and Other Noncellular Infectious Agents

Why Aren’t Viruses Alive?

• Cannot reproduce independently.

• No metabolism.

• Not made of cells.

Viral Structure

• Capsid: Protein coat.

• Genetic material: DNA or RNA.

• Envelope: Lipid membrane from host cell (some viruses).

Viral Replication Cycles

• Lytic cycle: Virus replicates rapidly, lyses host cell.

• Lysogenic cycle: Viral DNA integrates into host genome, can remain dormant before activation.

Why Viruses Are Dangerous

• Hijack host cells for reproduction.

• Mutate quickly, making treatment difficult (e.g., flu virus changes annually).

• Envelope: Helps viruses enter cells and evade immune system.

• Receptor: Host cell protein that virus binds to.

• Host: Species or cell type a virus can infect.

• Bacteriophage: Virus that infects bacteria.

The Evolution of Microbial Life

Binary Fission

• Prokaryotes reproduce by splitting into two identical cells.

• Allows rapid population growth.

Prokaryotes Overview

• No nucleus; includes Bacteria and Archaea.

Nutrition Types

• Autotrophs: Make their own food (e.g., photosynthesis).

• Heterotrophs: Consume other organisms.

Bacteria and Disease

• Cause disease by damaging tissues or releasing toxins.

• Exotoxins: Secreted by bacteria.

• Endotoxins: Released when bacteria die.

Bacteria vs. Archaea

• Bacteria: Found everywhere.

• Archaea: Often in extreme environments (hot, salty, acidic).

Microbiome and Health

• Beneficial bacteria aid digestion; disruption can cause illness.

Microbial Metabolism

• Energy source: Phototrophs (light), chemotrophs (chemicals).

• Electron source: Organotrophs (organic), lithotrophs (inorganic).

• Carbon source: Autotrophs (CO2), heterotrophs (organic carbon).

• Oxygen use: Obligate aerobes (need O2), obligate anaerobes (harmed by O2), facultative anaerobes (can switch).

• ATP pathways: Respiration (electron transport chain), fermentation (no ETC).

Ecological Roles of Microorganisms

• Recycle nutrients, produce oxygen, aid digestion.

Antibiotic Resistance

• Bacteria evolve resistance via natural selection.

• Resistant bacteria survive antibiotics and reproduce, spreading resistance.

• Rapid reproduction and gene sharing accelerate resistance (e.g., MRSA).

Extremophiles (Archaea)

• Live in extreme conditions (thermal vents, salt lakes, acidic environments).

• Important for understanding early Earth.

Protists

• Eukaryotic organisms (have nucleus).

• Groups: animal-like (move), plant-like (photosynthesize), fungus-like (decompose).

• Closest relatives to plants: green algae.

The Evolution of Plants and Fungi

Major Plant Innovations

• Vascular tissue: Xylem (water/minerals up), phloem (sugars down); allows taller growth and colonization of drier habitats.

• Seeds: Protect and nourish embryos, allow dormancy and dispersal.

• Flowers: Specialized for reproduction, attract pollinators, often lead to fruit formation for seed dispersal.

• Pollen: Sperm-containing structure that can travel without water.

• Fruit: Mature ovary that protects seeds and aids dispersal.

Why Flowers Matter

• Increase reproductive efficiency and attract specific pollinators.

Fungi Structure and Function

• Composed of hyphae; mass of hyphae is called mycelium.

• Cell walls contain chitin.

• Form mutualisms (e.g., mycorrhizae with plant roots).

Fungi Reproduction

• Asexual (spores) and sexual (genetic variation).

Ecological Role of Fungi

• Decomposers, recycle nutrients, return nutrients to soil.

The Evolution of Animals

What is an Animal?

• Multicellular, heterotrophic, lack cell walls.

• Embryos go through developmental stages.

Symmetry and Body Plans

• Radial symmetry: Body arranged like a wheel (e.g., jellyfish).

• Bilateral symmetry: Left and right sides (e.g., humans).

Chordates and Mammals

• Chordates: Have a notochord (support structure).

• Mammals: Have hair and produce milk.

• Tetrapods: Four limbs; oldest are amphibians.

Animal Development

• Life cycle: Zygote → embryo → adult.

• Cephalization: Sensory organs concentrated at the head (in complex animals).

Body Cavities

• Coelom allows organs to grow and move independently, supports complex systems.

Major Evolutionary Trends

• Development of backbone, limbs, and amniotic egg (prevents drying out).

An Introduction to Ecology and the Biosphere

Climate Change and Abiotic Factors

• Climate change is driven by greenhouse gases (CO2), causing global warming and melting ice.

• Abiotic factors: Temperature, water, sunlight.

Adaptations

• Migration, hibernation, acclimation (short-term adjustment).

Biomes

• Freshwater vs. marine; land biomes depend on climate.

Human Impact

• Pollution, habitat destruction, climate change.

Solar Energy and the Greenhouse Effect

• Solar energy is the base of all food chains.

• Greenhouse effect: Traps heat in the atmosphere; excess leads to global warming.

Population Ecology

Population and Density

• Population: Group of same species in one area.

• Density: Number of individuals per area.

Growth Models

• Exponential growth: Rapid, unlimited growth.

• Logistic growth: Growth slows as it approaches carrying capacity.

• Limiting factors: Density-dependent (e.g., disease), density-independent (e.g., weather).

Pest Control

• Integrated Pest Management: Combines multiple safe methods for controlling pests.

Communities and Ecosystems

Biodiversity and Threats

• Biodiversity: Variety of life in an ecosystem.

• Threats: Habitat loss, pollution, climate change.

Key Concepts

• Niche: Role of a species in its environment.

• Competitive exclusion: No two species can occupy the same niche indefinitely.

Species Interactions

• Mutualism (+/+): Both benefit.

• Predation (+/-): One benefits, one harmed.

• Competition (-/-): Both harmed by shared resource use.

Food Webs and Energy Flow

• Food webs show energy flow through ecosystems.

• Biological magnification: Toxins increase in concentration up the food chain.

• Energy flows (lost as heat), matter recycles (nutrient cycles).

Importance of Biodiversity

• Increases ecosystem stability, provides resources (food, medicine).

Keystone Species

• Species with a large impact on ecosystem structure (e.g., wolves controlling deer populations).

Trophic Levels and Energy Transfer

• Producers → primary consumers → secondary/tertiary consumers.

• Only about 10% of energy is transferred to the next level.

Nutrient Cycles

• Carbon cycle: CO2 in air, photosynthesis, food webs, respiration, fossil fuel burning.

• Nitrogen cycle: Nitrogen fixation by bacteria, needed for proteins and DNA.

• Water cycle: Evaporation, condensation, precipitation, runoff, transpiration.

Flowering Plants

Roots vs. Shoots

• Roots: Absorb water and minerals.

• Shoots: Photosynthesis, support, reproduction.

Vascular System

• Xylem: Transports water and minerals upward.

• Phloem: Transports sugars throughout the plant.

Key Features

• Cuticle: Waxy layer preventing water loss.

• Cellulose: Main component of plant cell walls.

• Angiosperms: Flowering plants.

Pollination

• Animals (especially bees) help plants reproduce by transferring pollen.

Plant Growth

• Occurs at meristems (regions of active cell division).

Transpiration

• Water evaporates from leaves (mainly through stomata).

• Creates a pull that moves water upward in xylem (cohesion-tension mechanism).

• Stomata can close to reduce water loss, but this limits CO2 intake for photosynthesis.

Photosynthesis Review

• Occurs in chloroplasts; uses sunlight to make glucose.

The Working Plant

Transport of Materials

• Xylem: Moves water and minerals from roots upward (driven by transpiration).

• Phloem: Moves sugars from sources (leaves) to sinks (roots, fruits, growing shoots).

• Example: In summer, leaves send sugar to fruits; in spring, roots send stored sugars to new buds.

Plant Hormones

• Auxin: Stimulates elongation.

• Gibberellins: Promote growth.

• Cytokinins: Stimulate cell division.

• Ethylene: Promotes fruit ripening.

• Abscisic acid: Stress response.

Coevolution

• Two species evolve in response to each other (e.g., flowers and pollinators).

Evolution and Ecology

• Evolution explains trait origins; ecology explains organism interactions.

Plants and Animals: Interdependence

• Plants provide oxygen and food; animals provide CO2 and pollination.

Humans' Role

• Humans impact climate, biodiversity, and ecosystems.