Unit Overview

Theme D: Continuity and Change

IB Biology HL· 5 min read 📊 n/a

1. Unit at a Glance

This unit follows a layered learning structure: core (standard level) topics first establish foundational concepts of continuity and change across every biological scale, then additional higher level (AHL) topics deepen your understanding of molecular genetics and evolutionary processes for HL certification.

You will progress from molecular mechanisms of genetic transfer up to large-scale macroevolution, building connections between each level to understand how stability and change interact across the tree of life.

Below are all sub-topics for this unit, ordered to build your understanding progressively:

Core: Molecules

Explore how molecular structure and interactions support genetic continuity and drive change at the molecular level.

★★⏱ 10 min

Core: Cells

Examine how cell division and cell processes maintain continuity and introduce variation in populations of cells.

★★⏱ 9 min

Core: Organisms

Analyze how life cycles and reproductive strategies maintain continuity of species and generate variation among individuals.

★★★⏱ 10 min

Core: Ecosystems

Investigate how continuity in ecosystem structure is balanced by change through succession and species turnover.

★★★⏱ 11 min

AHL: Nucleic acid structure

Dive deeper into the molecular structure of DNA and RNA that underpins accurate genetic transfer.

★★★⏱ 8 min

AHL: DNA replication extensions

Explore advanced mechanisms of DNA replication and its regulation in eukaryotes.

★★★★⏱ 10 min

AHL: Transcription and translation extensions

Cover post-transcriptional modification, gene expression regulation and advanced translation concepts.

★★★★⏱ 12 min

AHL: Meiosis and variation

Detail how meiosis generates genetic variation through crossing over and independent assortment.

★★★⏱ 9 min

AHL: Inheritance extensions

Learn complex inheritance patterns including polygenic inheritance, linkage and gene mapping.

★★★★⏱ 11 min

AHL: Mutation and gene editing

Explore types of mutation and modern biotechnological methods for targeted gene modification.

★★★★⏱ 10 min

AHL: Speciation

Examine processes of reproductive isolation and how new species form from ancestral populations.

★★★⏱ 9 min

AHL: Plant reproduction

Study the structure and function of reproductive organs and life cycles in flowering plants.

★★★⏱ 10 min

AHL: Macroevolution

Explore large-scale evolutionary patterns, mass extinctions and trends over geological time.

★★★★⏱ 8 min

2. Common Pitfalls

Wrong move:

Confusing genetic continuity with a complete lack of change

Why:

Students often assume continuity means no variation across generations

Correct move:

Frame continuity as reliable transfer of genetic information, which still generates variation that drives long-term change

Wrong move:

Treating core and AHL topics as disconnected concepts

Why:

AHL topics are extensions of core foundations, not unrelated new material

Correct move:

Connect every AHL topic back to the core concepts you learned earlier in the unit

3. Quick Reference Cheatsheet

Concept	Key Summary
Central Dogma	Genetic information flows: $D N A \to R N A \to P r o t e in$
Main Sources of Variation	Mutation, meiosis (crossing over/independent assortment), random fertilization
Hardy-Weinberg Equilibrium	$p^{2} + 2 pq + q^{2} = 1$ , $p + q = 1$ ; describes allele frequencies in non-evolving populations
DNA Base Pairing	A pairs with T (2 H-bonds), G pairs with C (3 H-bonds)
Meiosis Outcome	1 diploid parent cell → 4 genetically distinct haploid daughter cells
Biological Species Concept	Groups of interbreeding populations that produce fertile offspring, reproductively isolated from other groups
Macroevolution Patterns	Includes stasis, mass extinction, adaptive radiation, and gradual change over geological time

What's Next

Begin your study of this unit with the first core sub-topic, which lays the foundational molecular context for understanding continuity and change across all biological scales. Once you complete all sub-topics in this unit, you can move on to the next unit overview to continue your IB Biology HL progress.