IB DP Biology HL Evolution and speciation Study Notes
IB DP Biology HL Evolution and speciation Study Notes
IB DP Biology HL Evolution and speciation Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of
- What is the evidence for evolution?
- How do analogous and homologous structures exemplify commonality and diversity?
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
A4.1.1 – Evolution as Change in the Heritable Characteristics of a Population
📖 Definition of Evolution
Evolution is the gradual change in heritable (genetic) characteristics of a population over generations.
- Acts on populations, not individuals
- Involves genetic variation, not acquired traits
- Driven by mechanisms like:
- Natural selection
- Mutation
- Genetic drift
- Gene flow
🔁 Darwinian Evolution vs Lamarckism
| Feature | Darwinian Evolution | Lamarckism |
|---|---|---|
| Mechanism | Natural selection | Inheritance of acquired traits |
| Traits Passed | Only genetically inherited traits | Acquired traits (e.g. muscle growth) |
| Modern View | Strongly supported by genetics and fossil evidence | Disproven by molecular biology |
| Example | Long-necked giraffes reproduced more | Giraffes stretched necks and passed it on |
🔬 Nature of Science (NOS): Evolution as a Scientific Theory
- Evolution by natural selection (Darwin & Wallace) is a scientific theory, not a guess.
- Well-supported by fossils, DNA, and real-time observation
- Explains diverse biological phenomena and predicts biodiversity patterns
- Science doesn’t “prove” theories like math – it seeks evidence-based explanations
❌ Common Misconceptions
| Myth | Scientific Fact |
|---|---|
| “Evolution is just a theory” | It’s a scientific theory based on strong evidence |
| “Individuals evolve” | Populations evolve, not individuals |
| “Evolution explains origin of life” | It explains how life changes, not how it began |
🧪 Evidence Supporting Evolution
- Fossil Record: Transitional forms show gradual change (e.g. Archaeopteryx)
- Comparative Anatomy: Homologous structures (e.g. limbs in humans, bats)
- Molecular Biology: Humans and chimpanzees share ~98.5% DNA
- Observed Evolution: Antibiotic resistance in bacteria, pesticide resistance in insects
🌱 Mechanisms of Evolution (Brief Overview)
| Mechanism | Description |
|---|---|
| Natural Selection | Favors traits that increase survival and reproduction |
| Mutation | Random changes in DNA — source of new variation |
| Gene Flow | Movement of genes between populations |
| Genetic Drift | Random changes in gene frequency in small populations |
| Sexual Reproduction | Shuffles genes and increases variation |
🧠 Key Definitions
- Adaptation: A heritable trait that enhances survival or reproduction
- Selective Pressure: An environmental force that drives natural selection
- Speciation: Formation of a new species from accumulated genetic change
- Population: Group of the same species capable of interbreeding in an area
A4.1.2 – Evidence for Evolution from DNA/RNA Base Sequences and Protein Amino Acid Sequences
🌍 DNA: The Universal Language of Life
DNA and RNA are made of nucleotide base sequences (A, T/U, C, G).
- All organisms use the same genetic code, suggesting a common origin
- Mutations over time cause small changes, which accumulate and can be compared
🧪 How Sequences Provide Evidence
- Closely related species share more similar DNA or protein sequences
- Distantly related species have more differences in their sequences
Example:
- Humans & Chimpanzees share ~98.5% of DNA
- Chickens and Turtles share fewer similarities
Hox Genes: Conserved Across Species
Hox genes control body plans in animals – found in flies, fish, frogs, and humans!
🌳 Phylogenetic Trees
- Diagrams that show evolutionary relationships based on sequence data
- The more similar the sequences, the closer the organisms on the tree
- Each branch point (node) shows a common ancestor
📌 Why This Matters
- Reconstruct evolutionary history
- Classify organisms more accurately
- Understand how species evolved over time
🔍 Protein Evidence
Proteins are made from amino acids coded by DNA. Examples include:
- Hemoglobin
- Cytochrome c
- Insulin
These proteins show conserved amino acid sequences across species.
Fewer differences in amino acid sequence = closer evolutionary relationship.
A4.1.3 – Evidence for Evolution from Selective Breeding of Domesticated Animals and Crop Plants
🔍 What is Selective Breeding?
Selective breeding (artificial selection) is the human-directed process of breeding individuals with desired traits to produce offspring with those traits.
It mimics natural selection, but instead of nature selecting, humans decide which traits are valuable.
Over many generations, this causes evolutionary changes within a population.
🧬 How It Works
- Traits are controlled by genes (alleles).
- Individuals with favorable alleles (e.g., high milk yield, larger fruit, specific colors) are chosen to reproduce.
- This increases the frequency of those alleles in the population.
- Over time, significant genetic and phenotypic changes occur — clear evidence of evolution.
🌿 Examples of Selective Breeding
- Livestock: Modern cows, pigs, and chickens have been bred to:
- Grow faster
- Yield more meat or milk
- Be docile and manageable
Compared to their wild ancestors, these animals look and behave very differently.
- Dogs: All dog breeds evolved from the gray wolf, yet they show enormous variation:
- Chihuahua vs. Great Dane
- Bred for specific roles: hunting, guarding, herding, companionship
Demonstrates how rapid and diverse evolution can be under strong selection.
- Crop Plants:
- Corn (maize) was bred from teosinte – a wild grass with tiny ears
- Modern crops like wheat, rice, and tomatoes are:
- Larger in size
- Better in taste
- More disease-resistant
All are genetically and visibly very different from their wild ancestors.
📈 What Does This Tell Us About Evolution?
| Insight | Explanation |
|---|---|
| Evolution Can Be Fast | Artificial selection causes significant changes in just a few generations. |
| Genetic Change Is Involved | Breeding alters allele frequencies, showing that evolution is a genetic process. |
| Humans Drive Evolution Too | Just as nature selects traits, humans influence evolution through breeding. |
🌱 Why It Matters
- Selective breeding is a real-time example of evolution.
- It shows that species are not fixed – they can change dramatically under selection.
- Applications include:
- Improving agriculture (better crops and livestock)
- Conserving endangered species
- Studying evolutionary mechanisms
🧠 Key Terms
| Term | Meaning |
|---|---|
| Selective Breeding | Human-controlled reproduction based on desired traits |
| Allele | A version of a gene |
| Phenotype | Observable characteristics |
| Genotype | Genetic makeup of an individual |
A4.1.4 – Evidence for Evolution from Homologous Structures
📌 What are Homologous Structures?
Homologous structures are anatomical features that are similar in structure but may have different functions, found in species that share a common ancestor.
These structures arise through divergent evolution. Similar bone arrangements suggest that different species evolved from the same ancestral form, even if they now serve different purposes.
Example: The Pentadactyl Limb
The pentadactyl limb (five-digit limb) is a classic example of a homologous structure shared by many vertebrates.
🧠 Basic Structure (Forelimbs & Hindlimbs):
- 1 bone (proximal): Humerus (arm) / Femur (leg)
- 2 bones (distal): Radius & Ulna / Tibia & Fibula
- Wrist/ankle bones: Carpals / Tarsals
- Five digits: Metacarpals & Phalanges / Metatarsals & Phalanges
Despite the same bone layout, these limbs have evolved for different functions in different species.
🐾 Functional Variations in Different Animals
| Animal | Limb Use | Function |
|---|---|---|
| Frog | Hindlimbs for jumping | Powerful leaps from land to water |
| Penguin | Forelimbs as flippers | Swimming through water |
| Bat | Forelimbs form wings | Flight |
| Horse | Limbs for running | Speed across land |
| Whale | Forelimbs as fins | Steering in water |
| Human | Arms and hands | Grasping, tool use, writing |
🌱 Why It’s Evidence for Evolution
Homologous structures show that different species have inherited a basic blueprint from a common ancestor.
Natural selection caused the same structure to be modified in response to different environments and lifestyles, demonstrating divergent evolution.
🧩 Vestigial (Rudimentary) Structures: More Evolutionary Clues
Vestigial organs are reduced or unused structures that were fully functional in ancestral species.
| Structure | Present In | Ancestral Function |
|---|---|---|
| Appendix | Humans | Digestion of cellulose in herbivores |
| Pelvic bones | Whales | Support legs in land ancestors |
| Wing bones | Ostriches, emus | Flight in ancestral birds |
| Eye spots | Cave-dwelling fish | Vision in ancestral surface-dwelling fish |
A4.1.5 – Convergent Evolution & Analogous Structures
📌 What is Convergent Evolution?
Convergent evolution is the process by which unrelated species evolve similar traits independently, often because they live in similar environments or face similar selective pressures.
These traits look or function similarly but did not arise from a common ancestor. The resulting structures are called analogous structures.
🧩 What Are Analogous Structures?
Analogous structures are body parts in different species that perform the same function but have different anatomical origins.
| Feature | Analogous Structure |
|---|---|
| Function | Same (e.g., flying, swimming) |
| Ancestry | Different (no common ancestor with that trait) |
| Developmental origin | Different body parts |
✈️ Examples of Analogous Structures
| Structure | Organisms Involved | Function | Evolutionary Origin |
|---|---|---|---|
| Wings | Birds and Insects | Flight | Bird wings = forelimbs; insect wings = body wall extensions |
| Fins/Tails | Fish and Whales | Swimming propulsion | Fish = bony tail; whales = vertebrate hindlimb remnants |
| Eyes | Humans and Octopuses | Vision | Evolved separately with similar features like lens & retina |
| Body Shape | Dolphins and Sharks | Streamlined swimming | Mammal vs cartilaginous fish anatomy |
🧠 How Convergent Evolution Happens
- Environmental Pressures: Similar habitats (e.g., aquatic or aerial) apply similar pressures.
- Selective Advantage: Traits that improve survival in those environments evolve.
- Independent Paths: Different species evolve the same functional solution independently.
🧪 Analogy vs Homology: What’s the Difference?
| Feature | Homologous Structures | Analogous Structures |
|---|---|---|
| Origin | Same ancestor | Different ancestors |
| Structure | Similar internal structure | Different internal structure |
| Function | May differ (e.g., human arm vs whale fin) | Always similar (e.g., wings for flight) |
| Example | Pentadactyl limb (humans, bats, whales) | Wings (insects and birds) |
A4.1.6 – Speciation by Splitting of Pre-existing Species
📖 What is Speciation?
Speciation is the formation of a new species when a population splits and evolves independently. It increases biodiversity by adding new species to Earth.
Speciation occurs only by splitting of pre-existing species – gradual changes alone do not count as speciation.
🌍 Key Mechanism: Splitting of a Species
All speciation begins with a barrier that prevents gene flow between two populations.
🔑 Main Types of Isolation Leading to Speciation:
| Isolation Type | Description |
|---|---|
| Geographic Isolation | Physical barriers (mountains, rivers, oceans) separate populations. |
| Ecological Isolation | Same area but different habitats or food preferences prevent interbreeding. |
| Behavioral Isolation | Different courtship behaviors or mating calls keep populations apart. |
| Reproductive Isolation | Genetic or anatomical differences prevent successful mating or fertile offspring. |
🔁 How Speciation Happens: Step-by-Step
- One species splits into two or more isolated populations.
- Each population evolves independently due to natural selection, mutation, and/or genetic drift.
- Over time, genetic differences accumulate.
- Eventually, populations can no longer interbreed – reproductive isolation has occurred.
- New species have formed.
🌱 Speciation ≠ Evolutionary Change
A population may evolve over time without becoming a new species. Speciation only occurs when populations can no longer reproduce together.
🌿 Biodiversity Impact
Speciation adds new species → increases biodiversity.
Extinction removes species → decreases biodiversity.
Net species diversity = Speciation – Extinction
💥 Rapid Speciation: Adaptive Radiation
In some cases, one species rapidly evolves into many to fill different ecological roles. This is known as adaptive radiation or explosive speciation.
Occurs when species colonize new environments with many unoccupied niches.
🧠 Summary Points
- Speciation occurs only by splitting of pre-existing species.
- Isolation + Time + Genetic Change = New Species
- Reproductive isolation is essential to the process.
- Speciation explains how life diversifies and expands species richness on Earth.
A4.1.7 – Roles of Reproductive Isolation and Differential Selection in Speciation
📌 What is Speciation?
Speciation is the process by which new species arise from pre-existing ones. It involves two essential steps:
- Reproductive isolation – populations are prevented from interbreeding.
- Differential selection – isolated groups evolve differently due to unique selective pressures.
1. Reproductive Isolation: Blocking Gene Flow
Reproductive isolation ensures that gene flow between populations stops, allowing them to evolve independently.
Mechanisms of Reproductive Isolation
| Type | Description |
|---|---|
| Geographic Isolation | Physical barriers (mountains, rivers, oceans) separate populations. |
| Ecological Isolation | Populations occupy different habitats or ecological niches. |
| Temporal Isolation | Different breeding times (e.g., seasons or times of day). |
| Behavioral Isolation | Different mating rituals or communication signals. |
| Mechanical Isolation | Incompatible reproductive anatomy prevents mating. |
| Gametic Isolation | Sperm and egg cannot fuse due to genetic incompatibility. |
2. Geographic Isolation: A Common First Step
Geographic isolation is one of the most common pathways to reproductive isolation. It prevents interbreeding due to physical separation. Over time, isolated populations experience different environmental conditions and evolve separately.
3. Differential Selection: Evolution in Isolation
Once populations are reproductively isolated, different selective pressures guide their evolution.
✅ Selective Pressures May Include: Predators, climate, food availability, disease, and competition.
These factors cause genetic divergence, eventually leading to the formation of new species.
🐒 Case Study: Bonobos vs. Common Chimpanzees
| Trait | Bonobos | Common Chimpanzees |
|---|---|---|
| Separation Mechanism | Congo River (geographic barrier) | Lived north of the Congo |
| Social Behavior | Peaceful, matriarchal | More aggressive, hierarchical |
| Selective Pressures | Different food sources, predators, and social structures | |
🦆 Extra Example: Steamer Ducks (Tachyeres)
They are a modern example of speciation due to isolation and selective pressures.
🧩 Summary: Key Concepts
| Concept | Meaning |
|---|---|
| Reproductive Isolation | Prevents gene flow → allows independent evolution |
| Differential Selection | Causes diverging traits due to unique environmental pressures |
| Geographic Isolation | Most common way to start the speciation process |
| Speciation | Occurs when isolated populations can no longer interbreed successfully |
Additional Higher Level
A4.1.8 – Differences and Similarities Between Sympatric and Allopatric Speciation
📖 What Is Speciation?
Speciation is the formation of new species from pre-existing ones. It requires reproductive isolation, which stops populations from interbreeding and allows them to evolve separately.
Reproductive isolation can be:
- Geographic (physical barriers)
- Behavioral (mating preferences)
- Temporal (different breeding times)
🌍 Allopatric Speciation (Allo = other, patric = place)
Definition:
Occurs when populations are geographically separated, preventing interbreeding.
Steps:
- Physical barrier (e.g., mountain, river) splits the population.
- Gene flow is blocked.
- Genetic differences accumulate through mutation, natural selection, and drift.
- Reproductive isolation becomes permanent.
- New species arise if they can no longer breed when reunited.
Example: Squirrels on either side of the Grand Canyon have evolved into distinct species due to geographic isolation.
🏞️ Sympatric Speciation (Sym = same, patric = place)
Definition:
Occurs without geographic separation — populations live in the same area but become reproductively isolated.
Causes:
- Behavioral isolation (e.g., different mating calls)
- Ecological isolation (e.g., different feeding habitats)
- Temporal isolation (e.g., different breeding times)
- Genetic/chromosomal changes (common in plants)
Example: Lake Malawi cichlids evolved into hundreds of species in the same lake due to behavior, color, and habitat differences.
⏰ Temporal Isolation Example – Winter Processionary Moth (Portugal):
- Winter form: Breeds in summer/autumn
- Summer form: Breeds in spring
- Result: No interbreeding → genetic divergence → potential speciation
🔄 Comparison Table: Allopatric vs Sympatric Speciation
| Feature | Allopatric Speciation | Sympatric Speciation |
|---|---|---|
| Geographic Isolation? | Yes | No |
| Gene Flow | Blocked by physical barriers | Prevented by behavioral, ecological, or temporal isolation |
| Rate of Occurrence | More common | Less common |
| Examples | Grand Canyon squirrels, Darwin’s finches | Lake Malawi cichlids, polyploid plants, moths |
| Mechanism | Natural selection + drift after separation | Reproductive isolation in the same area |
🧩 Key Takeaways
- Allopatric speciation is driven by physical separation.
- Sympatric speciation occurs without barriers – within the same geographic area.
- Both lead to genetic divergence and new species formation.
- Temporal, behavioral, and ecological isolation are key in sympatric pathways.
A4.1.9 – Adaptive Radiation as a Source of Biodiversity
📖 What Is Adaptive Radiation?
Adaptive radiation is the rapid evolution of a single ancestral species into many new species, each adapted to a different ecological niche.
Occurs when:
- New environments become available (e.g., colonization or mass extinction)
- Competition is low
- Genetic variation allows new adaptations
⚙️ Key Factors That Drive Adaptive Radiation
| Factor | Description |
|---|---|
| Ecological Opportunity | New environments with unfilled niches, e.g., after a mass extinction or on isolated islands. |
| Key Innovations | New traits (e.g., specialized beaks, wings) allow novel ways to exploit resources. |
| Reduced Competition | Fewer competitors allow divergence into different niches without strong competition. |
Examples of Adaptive Radiation
Darwin’s Finches (Galapagos Islands):
Descended from a single ancestor, evolved into over a dozen species. Beak shapes adapted for different diets such as:
- Seed-crushing
- Insect-eating
- Nectar-feeding
Allowed multiple species to coexist by reducing direct competition.
🌱 Brocchinia Bromeliads (Guiana Shield):
Evolved from a common ancestor into plants with different adaptations:
- Root-based nutrient uptake
- Insect-trapping mechanisms
- Water-storage leaf tanks
🌍 How Adaptive Radiation Boosts Biodiversity
- Fills ecological niches rapidly
- Reduces competition through niche partitioning
- Enables coexistence of closely related species
- Increases ecosystem complexity and resilience
🌳 Visual Analogy:
Think of adaptive radiation like a branching tree:
The trunk is the ancestral species.
Each branch is a new species evolving into a unique niche.
🧠 Key Takeaways
- One species can rapidly diversify into many through adaptive radiation.
- It’s a key driver of biodiversity, especially after mass extinctions or in isolated ecosystems.
- Relies on ecological opportunity, genetic variation, and reduced competition.
- Explains coexistence of similar species in complex ecosystems.
A4.1.10 – Barriers to Hybridization and Sterility of Interspecific Hybrids
🧩 What Is Interspecific Hybridization?
Interspecific hybridization occurs when two individuals from different species interbreed. Although hybrids may form, most:
- Are infertile (cannot reproduce)
- Have reduced fitness
This acts as a reproductive barrier, preventing gene flow and maintaining distinct species.
🚧 Barriers Preventing Mixing of Alleles Between Species
1. Hybrid Sterility
Even when mating produces offspring, hybrids are usually sterile.
| Cause | Explanation |
|---|---|
| Chromosomal Incompatibility | Different number/structure of chromosomes disrupts meiosis in hybrids. |
| Genetic Incompatibility | Genes from different species don’t function well together – affects fertility or development. |
| Hybrid Breakdown | Hybrids may be fertile, but their offspring are weak or infertile in the next generation. |
Example: The Mule
Mule = Male donkey × Female horse
Mules are viable but sterile – they cannot produce offspring.
This prevents mixing between horse and donkey gene pools.
Behavioral Barriers: Courtship Prevents Hybridization
Even before mating, behaviors ensure that individuals mate only within their species.
| Behavior Type | Role in Reproductive Isolation |
|---|---|
| Courtship rituals | Unique dances, displays, or signals ensure species recognition. |
| Vocalizations | Species-specific calls prevent mating with other species. |
| Synchronization cues | Timing of responses must match for successful mating. |
Example: Clark’s Grebe
This bird performs a complex courtship dance and only mates with individuals that perfectly match the behavior – avoiding hybridization with Western Grebe.
🧠 Why These Barriers Matter
- Prevent interbreeding between different species
- Preserve species integrity and distinct gene pools
- Allow evolutionary divergence and specialization
- Reinforce speciation through isolation
🧵 Summary Table
| Barrier Type | Example | Effect |
|---|---|---|
| Sterility | Mule (horse × donkey) | Hybrid cannot reproduce |
| Chromosome mismatch | Various interspecies hybrids | Meiosis fails – no gametes form |
| Behavioral isolation | Clark’s grebe | Courtship prevents hybridization |
| Genetic incompatibility | Developmental failures in hybrids | Offspring are weak or infertile |
A4.1.11 – Abrupt Speciation in Plants by Hybridization and Polyploidy
🧬 What Is Polyploidy?
Polyploidy occurs when a plant has more than two complete sets of chromosomes (e.g., 4n instead of 2n). It can cause immediate reproductive isolation and lead to rapid speciation – especially common in plants.
🌿 Two Main Types of Polyploidy in Plants
| Type | Description | Example |
|---|---|---|
| Autopolyploidy | Chromosome sets from one species double (e.g., 2n → 4n) | Arabidopsis arenosa (sand rock-cress) |
| Allopolyploidy | Hybridization between two species, followed by chromosome doubling | Persicaria maculosa (smartweed) |
🔁 Autopolyploidy (Genome Doubling in a Single Species)
A diploid (2n) individual undergoes whole genome duplication, forming a tetraploid (4n), which cannot reproduce with the original diploid population – resulting in instant reproductive isolation and sympatric speciation.
Example: Arabidopsis arenosa
Diploid forms are found in Eastern Europe. Autotetraploids originated in the Balkans and Western Carpathians and later spread across Western Europe, forming distinct lineages.
🌾 Allopolyploidy (Hybridization + Chromosome Doubling)
Occurs when two species hybridize to form a sterile hybrid, which later undergoes chromosome doubling to restore fertility. The result is a fertile hybrid species with sets of chromosomes from both parents – reproductively isolated from both.
Example: Persicaria maculosa
Formed via hybridization of P. foliosa and P. lapathifolia. Chromosome doubling allowed the sterile hybrid to become fertile, producing a genetically distinct plant with features from both parents.
🌍 Why Is This Important for Plant Evolution?
| Benefit | Explanation |
|---|---|
| Rapid Speciation | New species can form in a single generation via chromosome doubling. |
| Genetic Diversity | Hybrid species inherit genes from both parents — introducing new combinations. |
| Ecological Flexibility | Polyploids often tolerate extreme conditions and can colonize new habitats. |
🧠 Summary Table
| Process | Description | Reproductive Isolation Mechanism | Example |
|---|---|---|---|
| Autopolyploidy | Chromosome doubling within one species | Incompatible meiosis with diploid individuals | Arabidopsis arenosa |
| Allopolyploidy | Hybridization + chromosome doubling | Sterile hybrid becomes fertile only with similar polyploids | Persicaria maculosa |