IB DP Biology Diversity of organisms Study Notes
IB DP Biology Diversity of organisms 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 a species? and
- What patterns are seen in the diversity of genomes within and between species?
- 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
A3.1.1 – Variation Between Organisms as a Defining Feature of Life
🧬 What Is Variation?
Variation refers to the differences in characteristics among individuals of the same species. No two organisms (except clones or identical twins) are exactly alike.
Variation is fundamental to evolution and classification.
🌿 Types of Variation
| Type | Description | Heritability | Examples |
|---|---|---|---|
| Continuous | Traits vary gradually across a range | Inherited (polygenic) | Height, skin color |
| Discontinuous | Traits have distinct categories | Often genetic, single-gene | Blood type, flower color |
| Somatogenic | Acquired during lifetime | Not inherited | Tanning, muscle gain |
| Blastogenic | From genetic differences in gametes | Inherited | Eye color, genetic diseases |
| Meristic | Variation in number of body parts | Genetic or environmental | Petal number, digits |
| Substantive | Variation in size, shape, or color | Genetic or environmental | Hair color, fruit size |
🔬 Genetic vs Environmental Variation
- Genetic variation arises from DNA differences (e.g., mutations, meiosis).
- Environmental variation comes from external factors (e.g., sunlight, nutrition).
- Most traits are influenced by a combination of both.
📊 Why Variation Matters
- Basis of Evolution: Natural selection acts on variation – fitter traits are inherited.
- Basis of Classification: Organisms are grouped based on similarities and differences.
- Raw Material for Speciation: Discontinuous variation can lead to the formation of new species.
🧪 Inheritance vs Variation
| Term | Definition |
|---|---|
| Heredity | Passing of traits from parents to offspring |
| Variation | Differences between individuals in a population |
🌍 Levels of Biological Organization (Ecological Hierarchy)
Organism → Individual living being
Population → Same species in an area
Community → All species in an area
Ecosystem → Biotic + abiotic interactions
Biome → Large-scale ecosystems (e.g., tundra, desert)
Biosphere → All ecosystems on Earth
A3.1.2 – Species as Groups of Organisms with Shared Traits
🧬 What Is a Species?
A species is a group of organisms that share key structural (morphological), genetic, and functional traits, and typically:
- Interbreed to produce fertile offspring (in sexually reproducing organisms)
- Share a common gene pool
- Are reproductively isolated from other such groups
🔍 The Morphological Species Concept (Linnaean Concept)
This is the oldest and most widely used way to define species, originally developed by Carl Linnaeus in the 18th century.
Definition: A species is a group of organisms that look similar and can be distinguished from other groups based on shared, observable traits (morphology).
🧠 Key Features of the Morphological Species Concept
| Aspect | Description |
|---|---|
| Focus | Physical traits like shape, size, structure |
| Method | Classification based on anatomy (e.g., bones, wings, flowers) |
| Used When | DNA or reproductive data is unavailable (e.g., fossils) |
| Developed By | Carl Linnaeus — “Father of Taxonomy” |
📌 Examples
- Butterflies with different wing patterns → separate species
- Plants with distinct leaf shapes and flower structures
- Fossils → grouped as species based solely on bone structure
⚠️ Limitations of the Morphological Concept
| Limitation | Explanation |
|---|---|
| Convergent evolution | Unrelated species may evolve similar traits (e.g., sharks & dolphins) |
| Cryptic species | Genetically distinct but visually identical |
| Intraspecific variation | Same species may vary greatly (e.g., dog breeds) |
| Subjectivity | “Visible similarity” is open to interpretation |
🧬 How It Compares to Other Species Concepts
| Concept | Focus | Example |
|---|---|---|
| Morphological | Shared physical traits | Fossils, extinct organisms, museum specimens |
| Biological | Ability to interbreed & produce fertile offspring | Most commonly used today |
| Phylogenetic | Shared evolutionary ancestry | Based on DNA & evolutionary trees |
A3.1.4 – Biological Species Concept
🧬 What Is a Species?
Biological Definition
According to the biological species concept, a species is:
“A group of organisms that can interbreed and produce fertile offspring under natural conditions.”
🌱 Key Characteristics of a Species
| Feature | Description |
|---|---|
| Interbreeding | Members of the species can successfully mate with one another |
| Fertile offspring | The resulting offspring can also reproduce |
| Reproductive isolation | Species are genetically and reproductively isolated from other groups |
🧠 Example:
A donkey × zebra = zedonk
But the zedonk is sterile → not same species
❗ Challenges with the Biological Species Concept
| Limitation | Explanation |
|---|---|
| Asexual organisms | Many organisms (e.g., bacteria, some fungi) reproduce asexually, so interbreeding is not applicable |
| Fossil species | Fossils do not reveal reproductive behaviors; we can’t test whether extinct organisms could interbreed |
| Hybridization | Some distinct species (e.g., lions × tigers) can interbreed and produce fertile hybrids in rare cases |
| Ring species | Populations of species that can interbreed with neighboring groups but not with distant ones in the same ring |
| Geographical isolation | Organisms may not interbreed simply due to physical separation—not because they’re genetically different |
🔍 Other Species Concepts (Competing Theories)
| Concept | Definition |
|---|---|
| Morphological species concept | Based on physical characteristics (shape, size, structure) |
| Ecological species concept | Based on ecological niche and role in the environment |
| Phylogenetic species concept | Based on genetic lineage and evolutionary history (DNA comparisons) |
A biological species is a group of organisms that can breed and produce fertile offspring.
But it doesn’t apply well to asexual organisms, fossils, or hybrid species.
Other species definitions help overcome these limitations, such as morphological or phylogenetic concepts.
A3.1.5 – Difficulties Distinguishing Between Populations and Species During Speciation
🧬 What Is Speciation?
Speciation is the process by which one species splits into two or more new species.
It occurs when populations of the same species stop interbreeding. Over time, they accumulate genetic, physical, and behavioral differences. These changes build up gradually, not suddenly.
🌍 Population vs. Species
| Term | Definition |
|---|---|
| Population | A group of organisms of the same species, living and interacting in the same area |
| Species | A group of organisms that can interbreed and produce fertile offspring |
🔄 Speciation and Reproductive Isolation
When two populations stop interbreeding, they accumulate differences. These differences may be due to:
- Geographical separation (e.g., a mountain, river)
- Behavioral isolation (e.g., mating calls, times)
- Genetic mutations
Over time, the populations may become so different that they can no longer interbreed, even if they come back into contact → they become separate species.
⚠️ Why It’s Hard to Distinguish Populations from Species
| Reason | Explanation |
|---|---|
| Gradual change | Speciation is a continuous process; there’s no clear line between one species and two |
| Hybrid formation | Sometimes diverging populations can still produce hybrids, making it unclear if they’re truly separate species |
| Geographical isolation | Populations may not interbreed due to distance, not because they are genetically incompatible |
| Subjectivity | Biologists may disagree on whether diverging groups are two species or one with variation |
🧠 Example:
Two bird populations live on opposite sides of a valley and never meet.
Over thousands of years, they develop different songs, colors, and behaviors.
If brought together:
- If they can mate → same species
- If they can’t → different species
- If they produce infertile hybrids → late stages of speciation
Speciation is a gradual process of divergence between non-interbreeding populations.
It’s often hard to decide whether two populations are still the same species or have become different species.
Reproductive isolation over time leads to genetic divergence, but boundaries aren’t always clear.
A3.1.6 – Diversity in Chromosome Numbers of Plant and Animal Species
🧬 What Are Chromosomes?
- Chromosomes are thread-like structures made of DNA and protein found in the nucleus of eukaryotic cells.
- They carry genetic information in the form of genes.
- Most organisms have chromosomes in pairs – one from each parent.
🔢 Diploid Number (2n)
- The diploid number is the total number of chromosomes in a somatic (body) cell.
- Diploid cells always have an even number of chromosomes because chromosomes are in pairs.
- The haploid number (n) is half the diploid number, found in gametes (sperm/egg).
🌱Chromosome Number Diversity
| Species | Diploid Chromosome Number |
|---|---|
| Humans | 46 |
| Chimpanzees | 48 |
| Fruit fly | 8 |
| Dog | 78 |
| Horse | 64 |
| Wheat (hexaploid) | 42 |
🧠 Key Concept
Chromosome number does not correlate directly with complexity.
For example, a fern can have over 500 chromosomes, far more than a human!
🔄 Evolutionary Insight
Humans and chimpanzees have very similar DNA but a different number of chromosomes.
This is due to a fusion event in human chromosome 2 (two ancestral chromosomes joined to form one).
Chromosome number can change over time through:
- Fusion (joining of two chromosomes)
- Fission (splitting of one into two)
- Duplication or deletion of chromosome segments
Diploid cells always contain an even number of chromosomes.
Different species have different chromosome numbers, even if closely related (e.g., humans = 46, chimps = 48).
Chromosome number is not linked to organism complexity, and changes can occur during evolution.
A3.1.7 – Karyotyping and Karyograms
🧬 What is Karyotyping?
Karyotyping is the process of organizing and visualizing chromosomes of an organism, usually from a mitotic cell in metaphase.
Chromosomes are stained, photographed, and arranged in a standard format: largest to smallest, with sex chromosomes at the end.
🧾 What is a Karyogram?
A karyogram (or karyotype image) is the visual chart showing homologous chromosome pairs, ordered by:
- Length
- Banding pattern (from special stains like Giemsa)
- Centromere position (metacentric, submetacentric, acrocentric)
🔍 Uses of Karyograms
| Purpose | Explanation |
|---|---|
| Sex determination | Identify XX (female) or XY (male) chromosomes |
| Diagnosing disorders | Identify extra/missing chromosomes (e.g. Down syndrome = 3 copies of chr. 21) |
| Evolutionary comparison | Compare chromosomes between species |
🧪 Application: Origin of Human Chromosome 2
The Hypothesis:
Human chromosome 2 was formed by the fusion of two ancestral chromosomes (12 and 13) that are still separate in great apes (e.g. chimpanzees, gorillas).
✅ Evidence Supporting the Fusion Hypothesis:
| Evidence Type | Observation |
|---|---|
| Banding pattern | Human chromosome 2 matches chimpanzee chromosomes 12 + 13 in pattern |
| Telomeres in middle | Internal telomere sequences (normally at ends) found in the middle of chromosome 2 |
| Two centromeres | Traces of a second, inactive centromere found on human chromosome 2 |
| DNA similarity | High similarity of gene sequences between human chr. 2 and chimpanzee 12+13 |
⚖️ Nature of Science (NOS): Testable vs Non-Testable Statements
| Type | Example |
|---|---|
| Testable Hypothesis | “Human chromosome 2 formed by the fusion of two ancestral chromosomes.” |
| Non-Testable Statement | “Human chromosome 2 was created by divine design.” |
Karyotyping classifies chromosomes by length, banding pattern, and centromere location.
Human chromosome 2 likely formed by fusion of two ancestral ape chromosomes.
This is supported by internal telomeres, two centromere remnants, and matching banding patterns.
Hypotheses in biology must be testable using observable evidence.
A3.1.8 – Unity and Diversity of Genomes Within Species
🧬 What is a Genome?
A genome is the entire set of genetic material in an organism.
It includes:
- All of an organism’s DNA
- Both coding regions (genes) and non-coding regions
🌱 Unity Within a Species
All individuals in a species share most of their genome.
For example:
Humans share ~99.9% of their DNA with each other.
This shared genetic blueprint ensures:
- Similar body structures
- Biochemical processes (e.g., enzymes, hormones)
- Inherited traits typical of that species
🌿 Sources of Genetic Diversity
Despite the unity, small variations exist. These include:
Single-Nucleotide Polymorphisms (SNPs):
- A SNP (“snip”) is a change in a single DNA base at a specific position.
- Example: One person may have G instead of A at a location.
- There are millions of SNPs across the genome.
Insertions/Deletions (Indels):
- Small stretches of DNA added or deleted, which may affect gene function or expression.
Copy Number Variations (CNVs):
- Sections of DNA may appear in multiple copies in some individuals.
🧪 Significance of Genetic Variation
| Feature | Importance |
|---|---|
| Personal traits | Eye color, height, metabolism, immunity |
| Disease susceptibility | SNPs may influence risks for diseases like diabetes, cancer |
| Drug response (pharmacogenomics) | Explains why individuals react differently to medications |
| Forensics and ancestry | DNA fingerprinting relies on individual genetic variations |
All members of a species share a common genome, but small variations like SNPs, indels, and CNVs create genetic diversity.
These variations are critical for personal traits, disease resistance, and evolutionary adaptation.
A3.1.9 – Diversity of Eukaryote Genomes
🧬 What Is a Genome?
A genome is the complete set of DNA in an organism, including all of its genes and non-coding sequences.
Eukaryotic genomes are stored in linear chromosomes inside a membrane-bound nucleus.
📏 Genome Size Variation
Genome size = Total amount of DNA, usually measured in base pairs (bp) or megabases (Mb).
Genome size is NOT directly related to organism complexity (known as the C-value paradox).
Example:
- A plant (e.g. lily) can have a genome much larger than a human’s.
- Humans: ~3.2 billion base pairs
- Amoeba: over 100 billion base pairs
🧬 Base Sequence Variation
Eukaryotic genomes differ widely in their DNA sequences, including:
- Differences in gene number
- Regulatory sequences
- Amount and types of non-coding regions
Even closely related species (e.g., humans and chimpanzees) show noticeable sequence variation.
🧬 Within vs Between Species
| Comparison | Description |
|---|---|
| Within a species | Genomes are highly similar (e.g. humans share ~99.9% of their DNA) |
| Between different species | Genomes show significant variation in both size and sequence content |
🧪 Importance of Genome Diversity
- Explains evolutionary divergence
- Helps classify organisms based on molecular data
- Reveals unique adaptations in different organisms
- Used in comparative genomics to find gene functions and ancestral relationships
Eukaryotic genomes vary widely in size and DNA sequence.
Variation between species is much greater than within a species and reflects evolutionary changes, adaptations, and genetic complexity.
A3.1.10 – Comparison of Genome Sizes
📏 What is Genome Size?
Genome size refers to the total amount of DNA in one copy of a genome.
It is usually measured in base pairs (bp), kilobases (kb), or megabases (Mb).
🌱Genome Size ≠ Organism Complexity
A larger genome size does not mean a more complex organism.
This is known as the C-value paradox – many simple organisms have more DNA than complex ones.
🔬 Examples from Different Taxonomic Groups
| Organism | Genome Size (Mb) | Notes |
|---|---|---|
| Escherichia coli | ~4.6 | Prokaryote; small genome; efficient coding |
| Drosophila | ~180 | Insect; moderate genome size |
| Homo sapiens | ~3,200 | Large genome; many non-coding regions |
| Lily (Lilium) | ~90,000 | Plant; huge genome, much of it is repetitive non-coding DNA |
| Amoeba dubia | ~290,000 | Largest known genome; single-celled protist |
💡 Note: Genome size can vary dramatically even among closely related species.
📚 Application: Extracting Data from Databases
Bioinformatics databases like Ensembl, NCBI Genomes, and the Genome Size Database provide:
- Genome size data
- Taxonomic classification
- Comparison tools for multiple species
Skill Tip:
When using a database:
- Sort by group (plants, animals, fungi, protists)
- Compare average sizes
- Look for exceptions to the genome size vs. complexity trend
Genome size does not predict organism complexity.
Simple organisms can have larger genomes due to repetitive, non-coding DNA.
Databases help scientists compare genome sizes across species to study evolutionary relationships.
A3.1.11 – Current and Potential Future Uses of Whole Genome Sequencing
🧬 What is Whole Genome Sequencing (WGS)?
Whole genome sequencing involves determining the complete DNA sequence of an organism’s genome at a single time.
📈 Trends in Genome Sequencing
Speed increasing: What once took years can now be done in hours or days.
Costs decreasing:
- 2003 (Human Genome Project): ~$3 billion
- Today: <$200 for a full human genome (and dropping)
This is due to advances in next-generation sequencing (NGS) technologies.
✅ Current Uses of Whole Genome Sequencing
| Application Area | Description |
|---|---|
| Evolutionary Research | Comparing genomes across species to study common ancestry, mutation rates, and speciation |
| Medical Diagnosis | Identifying genetic mutations responsible for inherited diseases like cystic fibrosis, BRCA (breast cancer risk), etc. |
| Infectious Disease Tracking | Sequencing viruses (e.g., SARS-CoV-2) to monitor variants and outbreaks |
| Agricultural Genetics | Understanding plant and livestock genomes to improve yield, disease resistance, etc. |
🔮 Potential Future Uses of Whole Genome Sequencing
| Application Area | Future Benefit |
|---|---|
| Personalized Medicine | Tailoring medical treatments based on a person’s unique genetic profile |
| Pharmacogenomics | Predicting how individuals will respond to specific drugs |
| Gene Therapy Development | Designing treatments to correct faulty genes |
| Preventive Healthcare | Detecting genetic risks for diseases before symptoms appear |
| Synthetic Biology | Creating new organisms or systems based on genetic blueprints |
📌 NOS (Nature of Science) Link
Technological advances in sequencing have rapidly transformed theories of evolution, classification, and medical practice, showing how science progresses through improvements in tools and methods.
Whole genome sequencing is faster and cheaper than ever.
It is currently used in evolutionary biology, disease diagnosis, and agriculture.
In the future, it will revolutionize personalized medicine, drug response, and gene therapy.
A3.1.12 – Difficulties Applying the Biological Species Concept
🔍 What Is the Biological Species Concept (BSC)?
A species is defined as a group of organisms that can interbreed and produce fertile offspring.
❗ Limitations of the BSC
The BSC works well for sexually reproducing animals, but it fails in some important cases:
1. Asexually Reproducing Species (e.g., bacteria, some plants, fungi)
| Feature | Why BSC Doesn’t Apply |
|---|---|
| No mating occurs | There’s no interbreeding, so we can’t use reproductive compatibility to define species |
| Reproduction by cloning | Offspring are genetically identical to parent-no way to test fertility of hybrids |
Example: Bacteria reproduce by binary fission, not by mating.
2. Horizontal Gene Transfer in Bacteria
| Feature | Impact on Species Definition |
|---|---|
| Bacteria can exchange genes across species lines | Makes boundaries between species blurry |
| Genes can be transferred via plasmids, viruses, or direct contact (conjugation) | Causes genetic mixing between unrelated bacteria |
| New traits can arise suddenly | Confuses attempts to define species based on shared traits |
This contradicts the BSC, which assumes species evolve independently without gene mixing from others.
Examples of BSC Failure
| Organism Type | Problem with BSC |
|---|---|
| Bacteria | Asexual + Horizontal gene transfer = messy species lines |
| Bdelloid rotifers | Anciently asexual animals—no interbreeding, yet considered a species |
| Dandelions | Many reproduce asexually but still classified into species |
BSC doesn’t work for organisms that don’t breed sexually (like bacteria, fungi, and some plants).
In bacteria, horizontal gene transfer further blurs species lines by mixing genes across populations.
A3.1.13 – Chromosome Number as a Shared Trait Within a Species
🧬 Key Concept
All members of the same species typically have the same number of chromosomes in their diploid cells.
🔗 Why Is Chromosome Number Important?
- During sexual reproduction, chromosomes must pair up properly during meiosis.
- This allows normal gamete formation and the production of fertile offspring.
- If two organisms have different chromosome numbers, meiosis is disrupted, and offspring are likely to be infertile or non-viable.
❌ Cross-Breeding Problems Between Species with Different Chromosome Numbers
| Scenario | Outcome |
|---|---|
| Closely related species with same chromosome number | May produce fertile offspring |
| Different chromosome numbers | Offspring usually sterile or non-viable |
Example: Horse and Donkey
| Parent | Chromosome Number |
|---|---|
| Horse | 64 |
| Donkey | 62 |
| Mule (offspring) | 63 (odd number) |
Because 63 chromosomes can’t pair properly during meiosis, mules are sterile.
Same species = same chromosome number
Different chromosome numbers = poor chromosome pairing during meiosis → Offspring are likely infertile (e.g., mules)
This is why chromosome number is a key reproductive barrier in defining species.
A3.1.14 – Engagement with Local Species to Develop a Dichotomous Key
🌿 What is a Dichotomous Key?
A dichotomous key is a tool used to identify organisms by answering a series of yes/no or either/or questions about their observable features.
🧪 What Students Should Be Able to Do
- Observe and record distinguishing features of local plant or animal species
- Use these traits to construct a dichotomous (two-option) decision tree
- Apply the key to identify unknown specimens
🪴 Example Features to Look For (Plants)
- Leaf shape (e.g. lobed vs unlobed)
- Flower color or number of petals
- Stem texture (hairy vs smooth)
- Arrangement of leaves (alternate vs opposite)
🐛 Example Features to Look For (Animals)
- Number of legs (e.g. 6 vs 8)
- Body covering (fur, scales, feathers)
- Presence of wings or antennae
- Shape or segmentation of body
🔍 Simple Example – Insects
| Step | Question | Go To |
|---|---|---|
| 1 | Does the insect have wings? | Yes → Step 2; No → Step 3 |
| 2 | Are the wings transparent? | Yes → Bee; No → Butterfly |
| 3 | Does the insect have antennae? | Yes → Ant; No → Spider |
Dichotomous keys use clear binary choices
Help in the classification and identification of local species
Encourage hands-on observation and engagement with biodiversity in your own environment
A3.1.15 – Identification of Species from Environmental DNA Using Barcodes
🧬 What is Environmental DNA (eDNA)?
Environmental DNA (eDNA) refers to genetic material collected from environmental samples like soil, water, or air — without needing to isolate the organisms themselves.
Organisms leave behind DNA through:
- Skin cells
- Mucus
- Feces
- Pollen
- Shed tissues
🧪 What is DNA Barcoding?
DNA barcoding is a technique that uses short, standardized regions of DNA to identify and classify species – like scanning a product barcode in a store.
| Type | Common Barcode Genes |
|---|---|
| Animals | COI gene (cytochrome oxidase I) |
| Plants | rbcL or matK genes |
🔍 How It Works
- Sample collection (e.g. pond water or forest soil)
- Extract DNA from the sample
- Amplify barcode region using PCR (polymerase chain reaction)
- Compare the sequence to a known DNA barcode database (e.g. BOLD, GenBank)
- Identify the species present in the habitat
🌱 Advantages of Using eDNA and Barcoding
- Non-invasive – No need to capture or see the organism
- Detects rare or elusive species
- Helps estimate species richness and biodiversity
- Works even with fragmented DNA
- Faster and cheaper than traditional methods
📌 eDNA barcoding allows scientists to quickly survey biodiversity in an environment.
Uses short DNA markers to identify species from soil, water, or air samples.
Revolutionizes conservation biology, habitat monitoring, and invasive species detection.