Posted by: paulgarner | September 8, 2010

The pattern of life: tree, mosaic, network or something else?

“What is the pattern of life?” is a question that has faced students of nature from the earliest times. Aristotle proposed a linear arrangement of organisms, ranking them from simple to complex, while Darwin replaced the Scala Naturae with an evolutionary tree and banished the idea of orthogenesis from biology altogether.

Ever since Darwin, the phylogenetic relationships between organisms have been represented by a single bifurcating tree of life. This tree-like pattern is said to be the result of the process by which an ancestral species splits into two descendant species – each of which then forms a new branch on the evolutionary tree.

Creationists have also been interested in the pattern of life and what it can tell us about the origin and history of living organisms. Some have suggested that abundant homoplasies – similarities that contradict a given evolutionary tree – weaken the argument that there is a unique tree-like pattern of life (e.g. Wise 1998). But if the similarities we observe in living things are not arranged in an evolutionary tree-like pattern, then how are they arranged? Perhaps the answer is some kind of complex mosaic or network rather than a tree, but so far this non-evolutionary pattern has remained elusive.

Those interested in exploring these kinds of questions may want to consult a themed collection of papers just published in Biology and Philosophy, guest edited by M. A. O’Malley. The fifteen papers in this edition address various aspects of the tree of life concept and the challenges that it has faced from within mainstream biology in recent years, especially as a result of the lateral gene transfer thought to have been rampant in the early history of life.

I don’t have access to this journal so I haven’t read any of the papers yet, but the whole issue seems worth a look. The contents are as follows:

  • The tree of life: introduction to an evolutionary debate – O’Malley, Martin and Dupré
  • The attempt on the life of the Tree of Life: science, philosophy and politics – Doolittle
  • The series, the network, and the tree: changing metaphors of order in nature – Rieppel
  • Why was Darwin’s view of species rejected by twentieth century biologists? – Mallet
  • Ernst Mayr, the tree of life, and philosophy of biology – O’Malley
  • Microbiology and the species problem – Ereshefsky
  • The myth of bacterial species and speciation – Lawrence and Retchless
  • Natural taxonomy in light of horizontal gene transfer – Andam, Williams and Gogarten
  • Evaluating Maclaurin and Sterelny’s conception of biodiversity in cases of frequent, promiscuous lateral gene transfer – Morgan
  • Symbiosis, lateral function transfer and the (many) saplings of life – Bouchard
  • Lifeness signatures and the roots of the tree of life – Malaterre
  • Gene sharing and genome evolution: networks in trees and trees in networks – Beiko
  • Testing for treeness: lateral gene transfer, phylogenetic inference, and model selection – Velasco and Sober
  • Trashing life’s tree – Franklin-Hall
  • On the need for integrative phylogenomics, and some steps toward its creation – Bapteste and Burian


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