# Explain how horizontal gene transfer (HGT) challenges the traditional model of a bifurcating tree of life as a representation of evolutionary relationships among prokaryotes.

> IB DP Biology Higher Level (2023 syllabus) — A3.1 Diversity of organisms · Explain · 4 marks

## Mark scheme (4 marks)

1. The traditional tree of life model depicts evolution as a branching (bifurcating) process in which lineages diverge from common ancestors without exchanging genetic material.
2. HGT involves the transfer of genetic material between organisms that are not in a parent–offspring relationship, including between distantly related or different species of prokaryotes.
3. Because HGT introduces genes from unrelated lineages, a single organism's genome may contain genes with different evolutionary histories, making it impossible to represent all relationships as a single branching tree.
4. A web/network of life, rather than a tree, has been proposed as a more accurate model because it can represent reticulate (non-vertical) connections between prokaryotic lineages caused by HGT.

## Key terms

- [horizontal gene transfer (HGT)](https://www.gradenine.co.uk/glossary/horizontal-gene-transfer-hgt)
- [bifurcating tree of life](https://www.gradenine.co.uk/glossary/bifurcating-tree-of-life)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-horizontal-gene-transfer-hgt-1f98974b) · Published by Druglandscape Ltd.