How Populations Evolve

Introduction to Evolution

Evolution is the process by which the genetic makeup (allele frequencies) of a population changes over generations. This leads to species changing over time, a foundational concept in biology.

• 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: 19th-century naturalist who 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 who selects the traits and the purpose.

Genetic Drift and Gene Flow

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

• Gene flow: Movement of genes between populations via migration, increasing genetic variation.

• Example (drift): Random event kills individuals regardless of traits.

• Example (flow): Animals migrate and breed in a new area.

Bottleneck Effect vs. Founder Effect

• Bottleneck effect: Sudden reduction in population size, leading to low genetic diversity (e.g., natural disaster).

• Founder effect: Small group starts a new population with limited genetic variation (e.g., colonizing an island).

Types of Selection

• Directional selection: Favors one extreme trait; population shifts in one direction (e.g., larger beaks during drought).

• Disruptive selection: Favors both extremes; middle traits are selected against (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 increase chances of attracting mates (e.g., bright feathers in birds).

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

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

Evidence for Evolution

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

• Comparative anatomy: Homologous structures indicate common ancestry (e.g., forelimbs of mammals).

• Embryology: Similar early development stages among vertebrates.

• Molecular biology: DNA/protein similarities (e.g., humans and chimpanzees share many genes).

• Homologous: Similar structure due to common ancestry.

• Analogous: Similar function, evolved independently (e.g., bird vs. insect wings).

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

Other Mechanisms of Evolution

• Genetic drift: Random changes in allele frequency (includes bottleneck and founder effects).

• Gene flow: Movement of alleles between populations (migration).

Fitness

• Fitness: Ability to survive and reproduce; not about strength, but about passing on genes.

• Fitness is relative: Depends on the environment and comparison among individuals.

• Example: A small animal that reproduces a lot may have higher fitness than a strong one that does not reproduce.

How Biological Diversity Evolves

What is a Species?

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

• Example: Horses and donkeys produce a mule (not fertile) – thus, 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)

• Phylogenies: Diagrams showing evolutionary relationships.

• Node: Branch point (most recent common ancestor).

• Clade: Group including an ancestor and all its descendants.

• Derived trait: Newer trait unique to a clade.

• Ancestral trait: Older trait shared more widely.

Cladograms

• Cladograms: Diagrams based on shared derived traits.

• Outgroup: Species used for comparison that lacks derived traits of the main group.

Importance of Classification

• Organizes biodiversity

• Helps predict traits

• Enhances understanding of 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: Some viruses have a lipid membrane from the host

Viral Replication Cycles

• Lytic cycle: Virus makes many copies and bursts the host cell.

• Lysogenic cycle: Viral DNA integrates into host DNA and can remain dormant before becoming active.

Why Viruses Are Dangerous

• Hijack host cells to reproduce

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

• Envelope: Lipid membrane aiding entry and immune evasion

• 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

• Prokaryotic cell division: one cell splits 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: Released by living 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 lead to 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, regenerates NAD+)

Ecological Roles of Microorganisms

• Recycle nutrients

• Produce oxygen

• Help digest food

Antibiotic Resistance

• Bacteria evolve traits to survive antibiotics

• Natural selection: resistant bacteria survive and reproduce, spreading resistance

• Rapid spread due to fast reproduction and gene sharing

• Example: MRSA (methicillin-resistant Staphylococcus aureus)

Extremophiles (Archaea)

• Live in extreme conditions (hot, salty, acidic)

• 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

1. Vascular tissue: Xylem (water/minerals up), phloem (sugars down); allows growth in drier places.

2. Seeds: Protect and nourish embryo, allow dormancy and dispersal.

3. Flowers: Specialized for reproduction, attract pollinators, often lead to fruits for seed dispersal.

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

• Fruit: Mature ovary protecting seeds, aids dispersal

Why Flowers Matter

• Increase reproductive efficiency

• Attract specific pollinators

Fungi Structure and Function

• Made of hyphae (filaments); mass of hyphae = mycelium

• Heterotrophs (absorb nutrients); cell walls contain chitin

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

Fungi Reproduction

• Asexual (spores)

• Sexual (increases genetic variation)

Ecological Role of Fungi

• Decomposers (break down dead matter)

• Return nutrients to soil

The Evolution of Animals

What is an Animal?

• Multicellular, heterotrophic, no cell walls

• Embryos go through developmental stages

Symmetry

• Radial: Circular symmetry (e.g., jellyfish)

• Bilateral: Left/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

• Zygote → embryo → adult

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

Body Cavities

• True coelom allows organs to grow/move independently and 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

• Caused by greenhouse gases (CO2)

• Leads to 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

Solar Energy and the Greenhouse Effect

• Solar energy is the base of all food chains

• Greenhouse effect traps heat; excess leads to global warming

Population Ecology

Population and Density

• Population: Same species in one area

• Density: Individuals per area

Growth Models

• Exponential growth: Rapid, unchecked growth

• Logistic growth: Growth slows at carrying capacity

Limiting Factors

• Density-dependent: Disease, competition

• Density-independent: Weather, natural disasters

Pest Control

• Integrated Pest Management: Combines multiple safe methods

Communities and Ecosystems

Biodiversity and Threats

• Biodiversity: Variety of life

• Threats: Habitat loss, pollution, climate change

Key Concepts

• Niche: Role of an organism

• Competitive exclusion: No two species can share the same niche

Species Interactions

• Mutualism (+/+): Both benefit

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

• Competition (-/-): Both harmed

Food Webs and Energy Flow

• Food webs show energy flow

• Biological magnification: Toxins increase up the food chain

• Energy flows through ecosystems; matter recycles

Importance of Biodiversity

• Increases ecosystem stability

• Provides resources (food, medicine)

Keystone Species

• Species with a large impact on ecosystem (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 makes N usable for proteins/DNA

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

Flowering Plants

Roots vs. Shoots

• Roots: Absorb water and minerals

• Shoots: Photosynthesis

Vascular System

• Xylem: Transports water

• Phloem: Transports sugars

Key Features

• Cuticle: Waxy layer preventing water loss

• Cellulose: Main component of cell wall

• 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

Photosynthesis Review

• Occurs in chloroplasts

• Uses sunlight to make glucose

The Working Plant

Transport of Materials

• Xylem: Moves water/minerals upward from roots (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 sugar 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 together (e.g., flowers and pollinators)

Evolution and Ecology

• Evolution explains how traits arise

• Ecology explains how organisms interact

Plants and Animals

• Plants provide oxygen and food

• Animals provide carbon dioxide and pollination

Humans' Role

• Humans affect climate, biodiversity, and ecosystems