# Evolutionary history and biodiversity

> IB Biology SL · Theme D: Continuity and Change
> Source: https://www.owlsprep.com/study/ib-biology-sl-u4-evolutionary-history-and-biodiversity/

This module explores how we reconstruct evolutionary relationships between organisms, connect fossil evidence to modern biodiversity, and use classification systems that reflect shared ancestry. You will learn to build and interpret phylogenetic trees.

**Prerequisites:** [Basic principles of evolution by natural selection](https://www.owlsprep.com/study/ib-biology-sl-u3-evolution-by-natural-selection/)

## Learning objectives

- Reconstruct and interpret evolutionary relationships using cladistic methods
- Evaluate evidence from the fossil record and comparative morphology for evolution
- Distinguish between species concepts for different use cases
- Explain how modern classification reflects evolutionary history

## Evidence from Fossils and Morphology

**Fossil Record** — The total collection of discovered fossils that preserves remains or traces of organisms from past geological time

*Example:* Fossilized bones of the early hominid *Australopithecus afarensis*

The fossil record provides direct chronological evidence of evolutionary change, showing how organisms have diversified over millions of years. Transitional fossils, like *Archaeopteryx*, provide key links between major groups (dinosaurs and birds). Comparative morphology also reveals shared ancestry: homologous structures have the same basic structure, but are adapted for different functions across groups.

**Worked example:** Explain how the pentadactyl limb provides evidence for common ancestry across mammals.

1. 1. Define the pentadactyl limb: it is a conserved five-digit limb structure found across most tetrapods (mammals, birds, amphibians, reptiles).
2. 2. Note that the structure is adapted for very different functions across groups: human grasping, bat flight, whale swimming, and horse running, but the underlying bone arrangement is nearly identical.
3. 3. This similarity cannot be explained by convergent evolution, since the functions are completely different. The only parsimonious explanation is that all these mammals inherited the pentadactyl limb from a shared common ancestor.

## Cladistics and Identifying Clades

**Clade** — A group of organisms consisting of a common ancestor and all of its living and extinct descendants

*Example:* All modern birds form a single clade descended from a shared dinosaur ancestor

Cladistics groups organisms based on shared derived characteristics: traits that are new to a clade and inherited by all its members. Originally built from morphological data, modern cladistics now uses molecular data (DNA/amino acid sequences) to get more accurate estimates of relatedness. Molecular data often revises old classification groups that were based on morphology alone.

**Worked example:** Given the trait table below, identify which two groups are most closely related:  
| Group | Vascular tissue | Seeds | Flowers |
|-------|-----------------|-------|---------|
| Moss  | No              | No    | No      |
| Fern  | Yes             | No    | No      |
| Pine  | Yes             | Yes   | No      |
| Rose  | Yes             | Yes   | Yes     |

1. 1. Count the number of shared derived traits between each pair of groups. Derived traits are new traits that evolve in the lineage leading to a clade.
2. 2. Rose and pine share two derived traits (vascular tissue, seeds) that are not present in fern or moss. Rose shares only one derived trait (vascular tissue) with fern, and zero with moss. Pine also shares only one derived trait with fern.
3. 3. A higher number of shared derived traits indicates more recent shared ancestry, so rose and pine are the most closely related pair.

> **note**
>
> Traditional classification groups are not always clades. For example, the traditional group 'reptiles' excludes birds, even though birds descend from the same common ancestor as other reptiles.

## Interpreting Phylogenetic Trees

**Phylogenetic Tree** — A branching diagram that represents hypothesized evolutionary relationships between groups. Cladograms are phylogenetic trees where branch lengths do not represent time; other trees use branch length to show evolutionary time or mutation rate.

Rooted trees have a single common ancestor at the base, and each branching node represents a speciation event. Relatedness between two groups is determined by how recent their most recent common ancestor (MRCA) is, not by similarity of traits or the order of tips on the tree.

**Worked example:** In a rooted tree: [Lamprey (outgroup)] → node → [Shark] → node → [Salamander] → node → [Lizard] → node → [Goat, Human]. What is the relationship between Human and the other groups?

1. 1. To find relatedness, trace back along each lineage from the tips until you reach the shared node (MRCA).
2. 2. The MRCA of Human and Goat is the most recent node on the human lineage, meaning Human shares a more recent ancestor with Goat than with any other group in the tree.
3. 3. Conclusion: Human is most closely related to Goat, followed by Lizard, then Salamander, then Shark.

**Check your understanding**

Test your understanding of relatedness

1. Two species that share a more recent common ancestor are always:

   - A. More similar in morphology than older lineages
   - B. More closely related than those with an older MRCA
   - C. Always in the same genus
   - D. Both living species

   *Why:* Relatedness in phylogenetics is explicitly defined by how recent the shared common ancestor is, not morphology, taxonomic rank, or whether both species are still alive.

## Classification and Species Concepts

Modern classification is aligned with evolutionary relationships, so taxa correspond to clades. Binomial nomenclature (Genus species) gives every species a unique universal name, allowing scientists to communicate clearly about biodiversity. Multiple species concepts are used, depending on the context.

**Worked example:** Why cannot the biological species concept be used to classify extinct fossil organisms?

1. 1. Recall the biological species concept defines a species as a group of organisms that can interbreed to produce fertile, viable offspring.
2. 2. Applying this definition requires observing reproduction and testing for fertility, which is impossible for extinct fossil organisms that can no longer reproduce.
3. 3. Paleontologists instead use the morphological species concept (based on shared physical traits) or phylogenetic species concept (based on evolutionary relatedness) to classify fossil species.

## Common pitfalls

- **Wrong:** Assuming the order of tips at the end of a cladogram indicates relatedness
  - Why it fails: Tip order can be rotated around any node without changing evolutionary relationships. Only node positions matter.
  - Correct: Always trace back from tips to find the most recent common ancestor to determine relatedness.
- **Wrong:** Using analogous structures to group organisms into clades
  - Why it fails: Analogous structures evolve via convergent evolution, not shared ancestry, so they do not reflect relatedness.
  - Correct: Only use homologous derived traits or molecular sequence data to build and interpret cladograms.
- **Wrong:** Assuming all traditional taxonomic groups are true clades
  - Why it fails: Many traditional groups are paraphyletic: they exclude some descendants of the common ancestor.
  - Correct: Modern cladistics only recognizes monophyletic groups (true clades) as valid taxa.
- **Wrong:** Claiming the fossil record is incomplete so it is not reliable evidence for evolution
  - Why it fails: The fossil record is expected to be incomplete, since fossilization requires very specific rare conditions.
  - Correct: Recognize the fossil record is incomplete but provides consistent, verifiable evidence of evolutionary change over time.

## Cheatsheet

| Key Concept | Definition | Exam Note |
| --- | --- | --- |
| Clade | Common ancestor + all descendants | Must be monophyletic |
| Derived Trait | New trait inherited by all clade members | Used to build cladograms |
| Homologous | Same origin, different function | Indicates shared ancestry |
| Analogous | Different origin, same function | Convergent evolution, ignore for cladistics |
| MRCA | Most Recent Common Ancestor | Defines degree of relatedness |
| Biological Species Concept | Interbreeding produces fertile offspring | Cannot use for fossils/asexuals |

## What's next

Understanding evolutionary history and classification is the foundation for studying all of biodiversity, and connects directly to other core topics in IB Biology SL. This sub-topic builds on your understanding of evolution by natural selection, and provides the framework for learning about specific groups of organisms and their evolutionary adaptations. Mastery of phylogenetic tree interpretation is a frequent, high-weight exam question that relies on applying the core concepts you explored here. Next, you can deepen your understanding of how new species form, explore the full classification of all living kingdoms, or connect these concepts to molecular genetics.

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