# Theme D: Continuity and Change

> IB Biology SL · IB SL Biology 2025 Syllabus
> Source: https://www.owlsprep.com/study/ib-biology-sl-u4-overview/
> Weight: 23-27% of total exam score

This unit explores how genetic information is passed between generations, expressed to produce traits, and modified over time to drive evolution and shape Earth's biodiversity.

**Prerequisites:** [Unit 1: Cell Biology](https://www.owlsprep.com/study/ib-biology-sl-u1-overview/); [Unit 2: Molecular Biology](https://www.owlsprep.com/study/ib-biology-sl-u2-overview/)

## Learning objectives

- Explain how genetic information is stored, copied, and expressed in living organisms
- Analyze inheritance patterns and predict outcomes of genetic crosses
- Describe causes and effects of mutation, genetic variation, and modern gene technology
- Evaluate evidence for evolution by natural selection and speciation
- Classify biodiversity and reconstruct evolutionary relationships between groups

## 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](https://www.owlsprep.com/study/ib-biology-sl-u4-dna-replication-and-transcription/) — Covers DNA structure, semi-conservative replication, and transcription of DNA to mRNA.
- [Translation and gene expression](https://www.owlsprep.com/study/ib-biology-sl-u4-translation-and-gene-expression/) — Explains how mRNA is translated into proteins and how gene expression is regulated.
- [Mutation and gene technology](https://www.owlsprep.com/study/ib-biology-sl-u4-mutation-and-gene-technology/) — Explores mutation causes and effects, plus modern biotechnologies like PCR and CRISPR.
- [Inheritance](https://www.owlsprep.com/study/ib-biology-sl-u4-inheritance/) — Covers Mendelian and non-Mendelian inheritance patterns and how to solve genetic cross problems.
- [Natural selection](https://www.owlsprep.com/study/ib-biology-sl-u4-natural-selection/) — Explains the mechanism of natural selection and key evidence supporting evolution.
- [Adaptation and speciation](https://www.owlsprep.com/study/ib-biology-sl-u4-adaptation-and-speciation/) — Describes how adaptations evolve and new species arise via reproductive isolation.
- [Evolutionary history and biodiversity](https://www.owlsprep.com/study/ib-biology-sl-u4-evolutionary-history-and-biodiversity/) — Covers organism classification and evolutionary relationship reconstruction via cladistics.

## Common pitfalls

- **Wrong:** Confusing transcription and translation locations in eukaryotes
  - Why it fails: This common mix-up leads to easy lost points on multiple choice and short answer questions
  - Correct: Transcription occurs in the nucleus; translation occurs at ribosomes in the cytoplasm
- **Wrong:** Assuming all mutations are harmful
  - Why it fails: This misunderstanding undermines understanding of genetic variation, the raw material for evolution
  - Correct: Most mutations are neutral; only a small subset are harmful or beneficial
- **Wrong:** Mixing up homologous and analogous structures
  - Why it fails: This error leads to incorrect evolutionary tree construction in cladistics questions
  - Correct: Homologous structures share common ancestry; analogous structures share function but not ancestry

## Cheatsheet

| Concept | Key Summary |
| --- | --- |
| Central dogma of molecular biology | DNA $\rightarrow$ RNA $\rightarrow$ Protein |
| Semi-conservative DNA replication | Each new DNA molecule has one original strand + one new strand |
| Monohybrid cross genotypic ratio | $3:1$ dominant : recessive phenotype for heterozygote × 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.

- [DNA replication and transcription](https://www.owlsprep.com/study/ib-biology-sl-u4-dna-replication-and-transcription/)
- [Translation and gene expression](https://www.owlsprep.com/study/ib-biology-sl-u4-translation-and-gene-expression/)
- [Mutation and Gene Technology](https://www.owlsprep.com/study/ib-biology-sl-u4-mutation-and-gene-technology/)

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