Explain how horizontal gene transfer (HGT) challenges the traditional model of a bifurcating tree of life as a representation of evolutionary relationships among prokaryotes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The traditional tree of life shows evolution as a branching, bifurcating process where lineages diverge from common ancestors without exchanging genetic material.
Horizontal gene transfer (HGT) moves genes between organisms that are not in a parent–offspring relationship, often between distantly related or different prokaryotic species.
Because HGT introduces genes from unrelated lineages, a single organism’s genome can contain genes with different evolutionary histories, so all relationships cannot be shown in one single branching tree.
Therefore a web or network model, rather than a tree, is proposed for prokaryotes, as it can represent the reticulate, non‑vertical connections created by HGT.
Horizontal gene transfer (HGT) moves genes between organisms that are not in a parent–offspring relationship, often between distantly related or different prokaryotic species.
Because HGT introduces genes from unrelated lineages, a single organism’s genome can contain genes with different evolutionary histories, so all relationships cannot be shown in one single branching tree.
Therefore a web or network model, rather than a tree, is proposed for prokaryotes, as it can represent the reticulate, non‑vertical connections created by HGT.
Examiner tips
- Use the word ‘bifurcating’ and ‘branching’ for the tree model. Mention that HGT occurs between non‑parent‑offspring and often distant species. Explain that genes can have different histories, breaking a single tree. State that a network or web model is the alternative.
- common_mistakes
- :
- Confusing vertical inheritance with HGT. Saying HGT only occurs within species. Failing to link HGT to the need for a network model.
Mark scheme (4 marks)
- The traditional tree of life model depicts evolution as a branching (bifurcating) process in which lineages diverge from common ancestors without exchanging genetic material.
- 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.
- 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.
- 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 in this question
horizontal gene transfer (HGT) · bifurcating tree of life
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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