IB Biology 5.4 Cladistics Notes

Introduction

Cladistics is a method of classifying organisms based on their evolutionary relationships. This approach groups organisms into clades, which include all descendants of a common ancestor. Cladistics is distinct from traditional taxonomy, which focuses on naming and classifying organisms based on shared characteristics. This study note will delve into the intricacies of cladistics, its principles, applications, and the impact of modern sequencing technologies on this field.

Clades and Cladistics

What is a Clade?

A clade is a group of organisms that includes a single common ancestor and all its descendants. Clades can be large or small, depending on the level of common ancestry being studied. For example, the clade of mammals includes all modern mammals and their extinct relatives back to their last common ancestor.

Principles of Cladistics

Cladistics classifies organisms based on homologous characteristics (traits inherited from a common ancestor) rather than analogous characteristics (traits that evolved independently). The goal is to construct a cladogram, a tree-like diagram that shows the evolutionary relationships among various species.

Homologous vs. Analogous Characteristics

• Homologous Characteristics: Traits inherited from a common ancestor. Example: The limb bones of humans, birds, and whales.
• Analogous Characteristics: Traits that evolved independently in different lineages. Example: The wings of bats and butterflies.

Constructing Cladograms

Nodes and Branches

• Node: Represents a common ancestor where two branches diverge.
• Branch: Represents the evolutionary path from a common ancestor to a descendant species.