Theme A — Unity and diversity

Theme A — Unity and diversity — IB Biology SL Revision Notes & Topical Questions

5 min read 13 parts Lesson 1 of 11 Unit quiz

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Lesson summary

Theme A — Unity and diversity — IB BIOLOGY SL notes: Summarise how life could have arisen, how cells are studied, how evolution and speciation produce diversity, and how biodiversity is lost.Cell = the smallest unit of self-sustaining life, enclosed by a plasma membrane and stori…

Viruses are not cells
Concentration of reactants
Hydrothermal vents
Meteorites
Primordial soup
Significance

Part 1 of 10Cells and the functions of life

Cells and the functions of life

Cell

Cell the smallest unit of self-sustaining life, enclosed by a plasma membrane and storing genetic information in DNA

The functions of life are metabolism, growth, response to stimuli, homeostasis, reproduction, excretion and nutrition.

Viruses are not cells

Viruses are not cells they lack metabolism, need no nutrition and replicate only by using host-cell components

Part 2 of 10Conditions on early Earth

Condition	Early Earth	Modern Earth	Relevance
Free oxygen	Almost none	About 21%	Organic molecules were not oxidised
Main gases	CO₂, N₂, H₂O, and proposed CH₄ and NH₃	N₂ and O₂	Supplied C, H and N
Ozone layer	Absent	Present	High UV energy reached the surface
Energy	Lightning, UV, volcanic heat	Mainly sunlight	Drove synthesis reactions
Temperature	Hot and variable	Mild	Faster reactions

Conditions on early Earth

Concentration of reactants

Concentration of reactants oceans and tidal pools concentrated the reactants

Hydrothermal vents

Hydrothermal vents deep-sea alkaline vents offered heat, minerals and chemical gradients

Meteorites

Meteorites carried amino acids to the Earth

Part 3 of 10Steps to the first cell

Four steps were needed
- organic molecules, polymers, replication and a compartment

Stage	Problem solved	Model	Limitation
Simple organics	Raw materials	Miller-Urey; meteorites	Atmosphere uncertain
Polymers	Complex molecules	Clay surfaces, vent minerals	Polymers tend to hydrolyse
Self-replicating molecule	Inheritance	Ribozymes, RNA world	RNA is fragile
Membrane	Compartment	Fatty acid vesicles	Getting molecules inside

1Vesicles

Vesicles fatty acids self-assemble into vesicles with hydrophobic tails inwards
The membrane lets the internal chemistry differ from the surroundings.

2RNA

RNA stores a base sequence and folds into ribozymes that catalyse reactions
It could replicate and catalyse before DNA or proteins existed.
DNA later took over storage because it is chemically more stable.

Part 4 of 10LUCA and endosymbiosis

LUCA

LUCA the last universal common ancestor, a single-celled organism that lived about 4 billion years ago

Evidencethe universal genetic code, shared genes and the same ribosome structure.
Gene studies suggest it lived near hydrothermal vents.
Stromatolites and microfossils show early prokaryotes.
Other early life forms probably existed but left no descendants.

Endosymbiosis

Endosymbiosis a large cell engulfed an aerobic bacterium that became the mitochondrion
A later engulfed photosynthetic bacterium became the chloroplast.
These organelles have a double membrane, their own circular DNA, 70S ribosomes and divide by binary fission.

Part 5 of 10Studying cells

Electron microscope

Electron microscope uses electron beams and has far higher resolution than a light microscope

Artefact

Artefact a feature in a prepared specimen caused by processing and not by the living cell

Measuring a cell with a graticule

Place a stage micrometer on the stage and align the scales.
Calculate the value of one graticule division.
Replace the micrometer with the specimen and count the divisions across the cell.
Multiply the divisions by the value of one division.

Making a temporary slide

Making a temporary slide cut a thin layer and place it flat, add a drop of stain, and lower the coverslip at an angle
Begin with the lowest-power objective and focus before switching to higher power.

Karyogram

Karyogram an image of an individual's chromosomes arranged in pairs by size and centromere position

Part 6 of 10Key chemistry terms

1Solvent

Solvent a liquid in which solutes dissolve, and water dissolves polar and ionic substances

2Purine

Purine a double-ring base, such as adenine or guanine

3Coding sequence

Coding sequence the part of a gene that carries the instructions for a protein

Part 7 of 10Evidence for evolution

Evidence	What is compared	Example	Shows	Limitation
Molecular sequences	DNA, RNA or amino acid sequences	Humans and chimpanzees share almost identical cytochrome c	Common ancestry, and relatedness can be measured	Mutation rates vary
Selective breeding	Domesticated varieties and wild ancestors	Wild cabbage gave kale, broccoli and cauliflower	Selection can change species quickly	Humans choose the trait
Homologous structures	Bone plans of limbs	Bat wing, whale flipper and human arm	Divergence from a shared ancestor	Similarity may be convergent

Evidence for evolution

Convergent evolution

Convergent evolution unrelated lineages independently evolve similar features under similar selection pressures

Homologous structures

Homologous structures share structure from a common ancestor, such as the pentadactyl limb in a bat and a whale

Analogous structures

Analogous structures share function but differ in origin, such as bird and insect wings

Part 8 of 10Speciation

Geographical isolation
- the Congo River may have separated the ancestors of bonobos and chimpanzees
- The river stopped gene flow.
- Different selection pressures acted on the two populations.
- They eventually became reproductively isolated.

Barrier	Geographical	Behavioural	Seasonal
Example	Snapping shrimps either side of Panama	Lacewings with different mating songs	Plants flowering in different months
Permanent?	Not necessarily, as populations could rejoin	Yes, once the signals differ	Yes, once the timings no longer overlap

Lamarck

Lamarck early 1800s, proposed inheritance of characteristics acquired by use and disuse, such as the giraffe's stretched neck
His idea is now rejected apart from minor epigenetic effects.

Darwin

Darwin mid 1800s, explained change by random variation
Longer-necked variants fed and bred more.

Part 9 of 10Biodiversity loss

Species richness

Species richness the number of different species in an area

Agricultural runoff reduces aquatic biodiversity

Excess fertiliser washes into a lake.

Nitrate and phosphate boost algal growth.

Algae block light and plants die.

Decomposers use oxygen.

Dissolved oxygen falls.

Fish and invertebrates die.

Captive breeding and reintroduction

Identify a species with a critically low wild population.

Capture the remaining individuals.

Breed them using studbooks to limit inbreeding.

Restore and protect the habitat.

Release the animals gradually with monitoring.

Track survival and breeding in the wild.

Part 10 of 10More from the specification

Carbon

Element with four valence electrons that forms the backbone of organic molecules.

Radiation

Transfer of heat as infrared energy from a warm object.

Theory

A well-tested explanation supported by a large body of evidence.

Miller-Urey experiment

A 1950s laboratory simulation showing amino acids can form from simple gases with an energy input.

Molecular clock

Use of sequence differences, assuming a constant mutation rate, to date divergence.

Key terms

Term	Meaning
Primordial soup	Early oceans rich in simple organic compounds.
LUCA	The last universal common ancestor, a single-celled organism that lived about 4 billion years ago.
Artefact	A feature in a prepared specimen caused by processing and not by the living cell.
Convergent evolution	Independent evolution of similar features in unrelated lineages under similar selection pressures.
Species richness	The number of different species in an area.
Karyogram	An image of chromosomes arranged in pairs by size and centromere position.
Endosymbiosis	The origin of mitochondria and chloroplasts from engulfed bacteria living inside a larger cell.

Avoid these mistakes

Saying that the Miller-Urey experiment created life — it only made simple organic molecules

Thinking convergent species are closely related — they evolved similar features independently

Saying that reproductive isolation always needs a physical barrier — it can also be behavioural or seasonal

Thinking Lamarck's acquired characteristics are inherited — they are not, so variation must arise by mutation and be selected

Last updated: 18 October 2026