IB Biology SL: Theme A - Unity & Diversity
A3.1 - Diversity of Organisms
Comprehensive Study Guide for IB Biology Students
📚 Introduction
The diversity of life on Earth is one of the most fundamental characteristics of biology. Scientists estimate there are approximately 8.7 million terrestrial species and 2.2 million oceanic species on our planet. This topic explores how organisms vary, how we classify them, and the mechanisms that create new species over time.
Understanding diversity requires examining variation at multiple levels: from genetic differences within populations to the spectacular variety of life forms across ecosystems.
🧬 Variation Between Organisms
What is Variation?
Variation refers to the differences in characteristics between organisms of the same species or between different species. It is a defining feature of life and the raw material for evolution.
Sources of Genetic Variation
- Mutations: Random changes in DNA sequences that introduce new alleles into populations
- Meiosis: The process of sexual reproduction that creates genetic recombination through crossing over and independent assortment
- Random Fertilization: The combination of gametes from two parents creates unique genetic combinations
- Gene Flow: Migration of individuals between populations introduces new alleles
Types of Variation
1. Continuous Variation
Characteristics that show a range of values between two extremes, typically influenced by multiple genes (polygenic) and environmental factors.
Examples: Height, skin color, weight, intelligence
2. Discontinuous (Discrete) Variation
Characteristics that fall into distinct categories with no intermediates, typically controlled by a single gene or a small number of genes.
Examples: ABO blood groups, attached/unattached earlobes, ability to roll tongue
Environmental vs. Genetic Factors
Most characteristics result from an interaction between genetic makeup (genotype) and environmental influences. For example:
- Plant height is determined by genes but also affected by water, nutrients, and sunlight
- Human intelligence has a genetic component but is influenced by education, nutrition, and environment
- Body mass is influenced by both genetic predisposition and diet/exercise
🦎 Understanding Species
Definition of Species
A species is defined as a group of organisms that can interbreed to produce fertile offspring and share common characteristics. Members of the same species:
- Share similar morphological (structural) features
- Can reproduce with one another to produce viable, fertile offspring
- Are reproductively isolated from other species
- Share a common gene pool and evolutionary history
The Biological Species Concept
The most widely accepted definition focuses on reproductive compatibility. However, this concept has limitations:
- Cannot be applied to asexually reproducing organisms
- Difficult to test for extinct or geographically separated populations
- Some species can produce hybrids (e.g., horse × donkey = mule, but mules are sterile)
Shared Traits Within Species
Organisms within a species share:
- Common ancestry and evolutionary history
- Similar genetic makeup (typically >97% DNA similarity)
- Morphological characteristics (body structure and appearance)
- Biochemical similarities (proteins, enzymes, metabolic pathways)
- Behavioral patterns (courtship, feeding, communication)
🔬 Carolus Linnaeus and Taxonomy
The Father of Taxonomy
Carolus Linnaeus (1707-1778) was a Swedish botanist who revolutionized biological classification. He developed a hierarchical system for organizing living organisms that is still used today, with modifications.
His major work, Species Plantarum (1753), established the foundation for modern taxonomy and introduced the binomial nomenclature system.
The Linnaean Hierarchy
Linnaeus organized organisms into a hierarchical classification system with seven main taxonomic levels (from broadest to most specific):
- Kingdom - Broadest category (e.g., Animalia, Plantae)
- Phylum - Major body plan (e.g., Chordata)
- Class - General characteristics (e.g., Mammalia)
- Order - More specific grouping (e.g., Primates)
- Family - Related genera (e.g., Hominidae)
- Genus - Closely related species (e.g., Homo)
- Species - Most specific (e.g., Homo sapiens)
Memory aid: "King Philip Came Over For Good Soup" or "Dear King Philip Came Over For Good Spaghetti"
Importance of Linnaean Classification
- Universal system: Provides a common language for scientists worldwide
- Reflects relationships: Groups organisms based on shared characteristics and evolutionary relationships
- Organizational clarity: Makes identifying and studying organisms more manageable
- Evolutionary insights: Helps understand ancestry and relationships between species
📝 Binomial Nomenclature System
The Two-Name System
Binomial nomenclature is a formal system for naming species using two Latin or Latinized words. This system replaced the confusion of common names that varied by language and region.
Each species has a unique scientific name consisting of:
- Genus name (generic epithet) - First word, always capitalized
- Species name (specific epithet) - Second word, always lowercase
Rules for Writing Scientific Names
✓ Rule 1: Capitalize the Genus
The first word (genus) must always begin with a capital letter.
✓ Rule 2: Lowercase the Species
The second word (species) must always be written in lowercase letters.
✓ Rule 3: Italicize or Underline
Both words must be written in italics (typed) or underlined (handwritten).
✓ Rule 4: Use Latin Forms
Both names should use Latin grammatical forms, though they may be based on other languages.
✓ Rule 5: Abbreviation Rules
After first use, the genus may be abbreviated to its first letter followed by a period (e.g., H. sapiens).
Examples of Binomial Nomenclature
Common Name | Scientific Name | Breakdown |
---|---|---|
Human | Homo sapiens | Homo = man; sapiens = wise |
Domestic Dog | Canis familiaris | Canis = dog; familiaris = domestic |
African Lion | Panthera leo | Panthera = panther; leo = lion |
Tiger | Panthera tigris | Panthera = panther; tigris = tiger |
Domestic Cat | Felis catus | Felis = cat; catus = domestic |
Note: Lions and tigers belong to the same genus (Panthera) because they share many characteristics, but they are different species.
Advantages of Binomial Nomenclature
- Universal recognition: Scientists worldwide use the same names regardless of language
- Avoids confusion: Common names can refer to different species in different regions
- Shows relationships: Species in the same genus share the same first name
- Precise identification: Each species has only one correct scientific name
- Stable nomenclature: Names don't change based on location or language
🌍 Speciation: The Origin of New Species
What is Speciation?
Speciation is the evolutionary process by which populations evolve to become distinct species. It occurs when populations become reproductively isolated and accumulate genetic differences over time.
For speciation to occur, populations must experience:
- Reproductive isolation - Prevention of gene flow between populations
- Genetic divergence - Accumulation of different mutations and alleles
- Natural selection or genetic drift - Different evolutionary pressures
Types of Speciation
1. Allopatric Speciation (Geographic Isolation)
Allopatric speciation is the most common type of speciation. It occurs when populations are separated by geographical barriers, preventing gene flow between them.
Process of Allopatric Speciation:
- Geographical separation: A physical barrier divides a population (e.g., mountain range, river, ocean, canyon)
- No gene flow: The separated populations cannot interbreed
- Different environments: Each population experiences different selection pressures
- Genetic divergence: Different mutations arise, and natural selection favors different traits
- Reproductive isolation: Over many generations, populations become so different they can no longer interbreed
- New species formed: Two distinct species now exist where there was once one
🔍 Example: Darwin's Finches
On the Galápagos Islands, finch populations became geographically isolated on different islands. Each island had different food sources, leading to different selection pressures. Over time, beak shapes evolved differently on each island, eventually creating multiple distinct species from a common ancestor.
Types of Geographical Barriers:
- Mountain ranges dividing populations
- Bodies of water (rivers, lakes, oceans)
- Deserts or inhospitable terrain
- Human-made barriers (roads, canals, cities)
- Continental drift separating landmasses
2. Sympatric Speciation (No Geographic Isolation)
Sympatric speciation occurs when populations become reproductively isolated without geographical separation. This is less common than allopatric speciation and typically involves genetic changes or behavioral differences.
Mechanisms of Sympatric Speciation:
A. Polyploidy (Common in Plants)
Organisms have extra sets of chromosomes (e.g., \(3n\) triploid, \(4n\) tetraploid). Polyploid individuals cannot breed with diploid \((2n)\) individuals, creating instant reproductive isolation.
B. Behavioral Isolation
Changes in courtship behaviors, mating calls, or display rituals prevent interbreeding even when populations live together.
C. Temporal Isolation
Populations breed or flower at different times (seasons, times of day), preventing gene flow despite living in the same area.
D. Ecological Isolation
Populations exploit different resources or habitats within the same area (e.g., different host plants, feeding at different depths in a lake).
🔍 Example: Cichlid Fish in African Lakes
In Lake Victoria, hundreds of cichlid species evolved sympatrically through ecological specialization and sexual selection (females preferring males with specific colorations), all within the same lake system without geographical barriers.
Comparing Allopatric and Sympatric Speciation
Feature | Allopatric | Sympatric |
---|---|---|
Geographic Barrier | Required (physical separation) | Not required (same location) |
Frequency | Most common type | Less common |
Time Required | Usually gradual (thousands of years) | Can be rapid (especially polyploidy) |
Main Mechanism | Different selection pressures in isolated areas | Genetic changes, behavioral changes, polyploidy |
Examples | Darwin's finches, Grand Canyon squirrels | Polyploid plants, cichlid fish |
🧬 Karyotyping and Karyograms
What is Karyotyping?
Karyotyping is the process of analyzing an individual's complete set of chromosomes. It involves photographing chromosomes during metaphase of cell division, then arranging them in a standard format called a karyogram (or karyotype).
A karyogram displays chromosomes organized by size, shape, and banding patterns, allowing identification of chromosomal abnormalities.
The Karyotyping Process
- Cell collection: Sample obtained from blood, amniotic fluid, or tissue biopsy
- Cell culture: Cells are cultured in a laboratory to increase their number
- Mitosis stimulation: Cells are stimulated to divide and arrested at metaphase (when chromosomes are most condensed)
- Staining: Chromosomes are stained with specific dyes (e.g., Giemsa stain) to create visible banding patterns
- Microscopy: Chromosomes are photographed under a microscope
- Arrangement: Chromosomes are cut from the photograph and arranged in homologous pairs
- Analysis: The karyogram is examined for abnormalities in number, size, or structure
Chromosome Classification in Karyograms
Chromosomes are classified based on three main features:
1. Length/Size
Chromosomes are numbered from 1 to 22 (autosomes) in order of decreasing size, with chromosome 1 being the largest and chromosome 22 being the smallest.
Sex chromosomes (X and Y) are listed separately.
2. Centromere Position
The centromere is the constricted region where sister chromatids attach. Position determines chromosome shape:
- Metacentric: Centromere near the middle (arms roughly equal length)
- Submetacentric: Centromere off-center (arms slightly unequal)
- Acrocentric: Centromere near one end (one arm much shorter)
- Telocentric: Centromere at the very end (rare in humans)
3. Banding Patterns
When stained, chromosomes show characteristic light and dark bands (G-bands). These patterns are:
- Unique to each chromosome
- Used to identify specific chromosomes
- Help detect structural abnormalities (deletions, duplications, translocations)
Human Chromosome Numbers
Normal human cells contain:
46
Total Chromosomes
Diploid number \((2n = 46)\)
23
Pairs of Chromosomes
22 autosomal pairs + 1 sex pair
Autosomes:
22 pairs of chromosomes (44 total) that are the same in males and females
Sex Chromosomes:
Females: XX (two X chromosomes)
Males: XY (one X and one Y chromosome)
Important: Human karyograms show 46 chromosomes (diploid). Gametes (sex cells) contain only 23 chromosomes (haploid, \(n = 23\)).
Chromosome Number Diversity Across Species
Different species have different numbers of chromosomes:
Organism | Diploid Number (2n) |
---|---|
Human (Homo sapiens) | 46 |
Chimpanzee (Pan troglodytes) | 48 |
Domestic Dog (Canis familiaris) | 78 |
Fruit Fly (Drosophila melanogaster) | 8 |
Garden Pea (Pisum sativum) | 14 |
Rice (Oryza sativa) | 24 |
Key Point: Humans have 46 chromosomes while our closest relatives, chimpanzees, have 48. This difference arose from the fusion of two ancestral chromosomes to form human chromosome 2.
Clinical Applications of Karyotyping
Karyotyping is used to detect chromosomal abnormalities:
Numerical Abnormalities (Aneuploidy)
- Down Syndrome (Trisomy 21): Three copies of chromosome 21 (47 chromosomes total)
- Edwards Syndrome (Trisomy 18): Three copies of chromosome 18
- Patau Syndrome (Trisomy 13): Three copies of chromosome 13
- Turner Syndrome (Monosomy X): Only one X chromosome in females (45,X)
- Klinefelter Syndrome: Extra X chromosome in males (47,XXY)
Structural Abnormalities
- Deletions: Part of a chromosome is missing
- Duplications: Part of a chromosome is copied
- Translocations: Part of one chromosome is transferred to another
- Inversions: A chromosome segment is reversed
🧬 Unity and Diversity of Genomes
What is a Genome?
A genome is the complete set of genetic information (DNA) in an organism. It includes:
- All genes (coding sequences that produce proteins)
- Non-coding DNA (regulatory sequences, introns, repetitive DNA)
- Mitochondrial DNA (in eukaryotes)
- Chloroplast DNA (in plants)
The human genome contains approximately 3 billion base pairs and around 20,000-25,000 protein-coding genes.
Unity of Genomes Within Species
All members of the same species share most of their genome, which explains why organisms of the same species:
- Have the same basic body structure and organization
- Share similar biochemical pathways and proteins
- Exhibit species-specific behaviors
- Can interbreed and produce fertile offspring
🔍 Example: Human Genome Unity
Any two humans share approximately 99.9% of their DNA sequences. This high similarity ensures all humans have the same basic anatomy, physiology, and biochemistry. Despite sharing 99.9% of our DNA, the remaining 0.1% (about 3 million base pairs) accounts for all the genetic variation that makes each person unique.
Diversity of Genomes Within Species
While species share most genetic information, small variations create diversity among individuals. These variations include:
1. Single-Nucleotide Polymorphisms (SNPs)
SNPs (pronounced "snips") are variations at a single nucleotide position in the DNA sequence.
Example of an SNP:
Individual A: ...AAGCCTAGCTAAG...
Individual B: ...AAGCCTACCTAAG...
The highlighted position shows a G→C substitution
Significance: There are millions of SNPs throughout the human genome. Most SNPs have no effect, but some can influence traits like eye color, disease susceptibility, or drug metabolism.
2. Insertions and Deletions (Indels)
Small stretches of DNA (usually 1-50 base pairs) that are added or removed. These can affect gene function or expression levels.
3. Copy Number Variations (CNVs)
Large sections of DNA (1,000+ base pairs) that vary in copy number between individuals. Some people may have multiple copies of certain DNA segments while others have fewer.
4. Structural Variations
Larger-scale rearrangements including inversions, translocations, and duplications of chromosome segments.
Significance of Genetic Variation
Area | Importance of Genetic Variation |
---|---|
Individual Traits | Determines physical characteristics (height, eye color, hair texture), metabolic rates, and personality traits |
Disease Susceptibility | SNPs and CNVs influence risk for diseases like diabetes, cancer, heart disease, and Alzheimer's |
Pharmacogenomics | Explains why individuals respond differently to medications; enables personalized medicine |
Evolution | Provides raw material for natural selection; allows populations to adapt to changing environments |
Forensics | DNA fingerprinting relies on individual genetic variations for identification |
Ancestry | Genetic variations trace human migration patterns and evolutionary relationships |
Genome Comparisons Between Species
Comparing genomes across species reveals evolutionary relationships:
Species Comparison | DNA Similarity |
---|---|
Human vs. Human | 99.9% |
Human vs. Chimpanzee | ~98.8% |
Human vs. Mouse | ~85% |
Human vs. Fruit Fly | ~60% |
Human vs. Banana | ~50% |
Key Insight: Higher DNA similarity indicates closer evolutionary relationships. The high similarity between humans and chimpanzees reflects our recent common ancestor (approximately 6-7 million years ago).
🎯 Key Concepts Summary
✓ Variation
Differences between organisms arise from genetic factors (mutations, meiosis) and environmental influences, existing as continuous or discontinuous types.
✓ Species
Groups of organisms that can interbreed to produce fertile offspring, sharing similar characteristics and a common gene pool.
✓ Linnaean Classification
Hierarchical system organizing life from Kingdom to Species, providing universal framework for biological classification.
✓ Binomial Nomenclature
Two-part Latin naming system (Genus species) ensuring universal, precise identification of organisms worldwide.
✓ Speciation
Formation of new species through reproductive isolation—allopatric (geographic barriers) or sympatric (same location).
✓ Karyotyping
Analysis of chromosome number and structure, revealing abnormalities through examination of size, banding patterns, and centromere position.
✓ Genomes
Complete genetic information; species members share 99%+ DNA, while small variations (SNPs, indels, CNVs) create individual diversity.
👨🏫 About the Author
Adam Kumar
Co-Founder @ RevisionTown
Math & Science Expert specializing in various international curricula including IB, AP, GCSE, IGCSE, and more. Dedicated to helping students excel in their academic journey through comprehensive, accessible educational resources.
This comprehensive guide covers all essential concepts for IB Biology SL Theme A3.1 - Diversity of Organisms. For more resources and study materials, visit RevisionTown.