Unit Overview
Theme D: Continuity and Change
IB Biology SLΒ· 5 min read π 23-27% of total exam score
1. Unit at a Glance
This unit is structured around two connected core themes: genetics (how genetic information functions at molecular and organismal levels) and evolution (how genetic change over time produces adaptation and biodiversity). We start at the molecular level, walking through how DNA is copied, read, and modified, before moving to inheritance of traits, then building up to patterns of change across entire populations and species.
This unit includes the following sub-topics:
DNA replication and transcription
Covers DNA structure, semi-conservative replication, and transcription of DNA to mRNA.
β β β± 12 min
Translation and gene expression
Explains how mRNA is translated into proteins and how gene expression is regulated.
β β β± 10 min
Mutation and gene technology
Explores mutation causes and effects, plus modern biotechnologies like PCR and CRISPR.
β β β β± 15 min
Inheritance
Covers Mendelian and non-Mendelian inheritance patterns and how to solve genetic cross problems.
β β β β± 18 min
Natural selection
Explains the mechanism of natural selection and key evidence supporting evolution.
β β β± 12 min
Adaptation and speciation
Describes how adaptations evolve and new species arise via reproductive isolation.
β β β β± 11 min
Evolutionary history and biodiversity
Covers organism classification and evolutionary relationship reconstruction via cladistics.
β β β β β± 14 min
2. Common Pitfalls
Wrong move:
Confusing transcription and translation locations in eukaryotes
Why:
This common mix-up leads to easy lost points on multiple choice and short answer questions
Correct move:
Transcription occurs in the nucleus; translation occurs at ribosomes in the cytoplasm
Wrong move:
Assuming all mutations are harmful
Why:
This misunderstanding undermines understanding of genetic variation, the raw material for evolution
Correct move:
Most mutations are neutral; only a small subset are harmful or beneficial
Wrong move:
Mixing up homologous and analogous structures
Why:
This error leads to incorrect evolutionary tree construction in cladistics questions
Correct move:
Homologous structures share common ancestry; analogous structures share function but not ancestry
3. Quick Reference Cheatsheet
Concept | Key Summary |
|---|---|
Central dogma of molecular biology | DNA RNA Protein |
Semi-conservative DNA replication | Each new DNA molecule has one original strand + one new strand |
Monohybrid cross genotypic ratio | dominant : recessive phenotype for heterozygote Γ heterozygote |
Conditions for natural selection | Variation, heritability, differential survival and reproduction |
Biological species definition | Groups that can interbreed to produce fertile, viable offspring |
Most parsimonious cladogram | The tree with the fewest evolutionary character changes is most likely correct |
Base substitution vs frameshift mutation | Substitution changes at most one amino acid; frameshift changes all downstream amino acids |
What's Next
Begin this unit by learning about the core molecular processes of DNA replication and transcription, the foundation for all genetic and evolutionary concepts covered in Theme D. After completing all sub-topics in this unit, you will move on to the next theme covering ecology and environmental interactions.