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Biology LibreTexts

20.3D: Web, Network, and Ring of Life Models

  • Page ID
    13537
  • To more accurately describe the phylogenetic relationships of life, web and ring models have been proposed as updates to tree models.

    Learning Objectives

    • Describe the web, network, and ring of life models of phylogenetic trees

    Key Points

    • A phylogenetic model that resembles a web or a network was proposed since eukaryotes evolved not from a single prokaryotic ancestor, but from a pool of many species that were sharing genes by HGT mechanisms.
    • A phylogenetic model that resembles a ring was proposed in which species of all three domains, Archaea, Bacteria, and Eukarya, evolved from a single pool of gene-swapping prokaryotes.
    • Phylogenetic models will continue to evolve as phylogeneticists remain highly skeptical of the current tree, web, and ring models.

    Key Terms

    • web of life: a phylogenetic model that resembles a web or a network more than a tree
    • ring of life: a phylogenetic model where all three domains of life (Archaea, Bacteria, and Eukarya) evolved from a pool of primitive prokaryotes

    The recognition of the importance of Horizontal gene transfer (HGT), especially in the evolution of prokaryotes, has caused some to propose abandoning the classic “tree of life” model. In 1999, a phylogenetic model that resembles a web or a network more than a tree was proposed. The hypothesis is that eukaryotes evolved not from a single prokaryotic ancestor, but from a pool of many species that were sharing genes by HGT mechanisms. Some individual prokaryotes were responsible for transferring the bacteria that caused mitochondrial development in the new eukaryotes, whereas other species transferred the bacteria that gave rise to chloroplasts. This model is often called the “web of life.” In an effort to save the tree analogy, some have proposed using the Ficus tree with its multiple trunks as a phylogenetic tree to represent the evolutionary role for HGT.

    image
    Figure \(\PageIndex{1}\): Phylogenetic web of life model: In the (a) phylogenetic model proposed by W. Ford Doolittle, the “tree of life” arose from a community of ancestral cells, has multiple trunks, and has connections between branches where horizontal gene transfer has occurred. Visually, this concept is better represented by (b) the multi-trunked Ficus than by the single trunk of the oak, similar to the tree drawn by Darwin.

    Others have proposed abandoning any tree-like model of phylogeny in favor of a ring structure. The ” ring of life ” is a phylogenetic model where all three domains of life evolved from a pool of primitive prokaryotes. Using the conditioned reconstruction algorithm, it proposes a ring-like model in which species of all three domains (Archaea, Bacteria, and Eukarya) evolved from a single pool of gene-swapping prokaryotes. This structure is proposed as the best fit for data from extensive DNA analyses; the ring model is the only one that adequately takes HGT and genomic fusion into account. However, phylogeneticists remain highly skeptical of this model.

    image
    Figure \(\PageIndex{1}\): Phylogenetic ring of life model: According to the “ring of life” phylogenetic model, the three domains of life evolved from a pool of primitive prokaryotes.

    In summary, the “tree of life” model proposed by Darwin must be modified to include HGT. This does not mean a tree, web, or a ring will correlate completely to an accurate description of phylogenetic relationships of life. A consequence of the new thinking about phylogenetic models is the idea that Darwin’s original conception of the phylogenetic tree is too simple, but made sense based on what was known at that time.

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