---
title: "Outgroup — AP Biology Definition & Exam Guide"
description: "The outgroup is the lineage least closely related to the rest of a phylogenetic tree, used as a reference to root the tree. Key to Topic 7.9 and the AP exam."
canonical: "https://fiveable.me/ap-bio/key-terms/outgroup"
type: "key-term"
subject: "AP Biology"
---

# Outgroup — AP Biology Definition & Exam Guide

## Definition

The outgroup is a species or group of organisms that is used as a reference point when constructing a phylogenetic tree. It is chosen because it is believed to be less closely related to the other organisms being studied.

## Review

### Related Terms

- Ingroup: The ingroup refers to the group of organisms that are more closely related to each other than they are to the outgroup.
- [Phylogenetic Tree](/ap-bio/key-terms/phylogenetic-tree): A phylogenetic tree is a diagram that shows the evolutionary relationships between different species or groups of organisms.
- [Common Ancestor](/ap-bio/key-terms/common-ancestor): Common ancestor refers to an organism from which two or more different species evolved.