Darwin’s Tree of Life – mere culture and convenience
There is no doubt that we are drawn to organization that involves hierarchy. Carl Linnaeus (1707-1778) famously devised a scheme for plant classification according to their reproductive characteristics. He established the idea of grouping organisms into a hierarchy of progressively shared traits. In his Systema naturae (1735) Linnaeus decreed that this logical organization of living things should involve increasingly larger categories. Thus, all similar individuals would be members of a species, similar species could be lumped into a genus. Similar genera were clumped into a family, similar families into an order, similar orders into a class, similar classes into a phylum or division. These groups were supposed to be exclusive. An organism was supposed to be a member of only one group. In Linnaeus’ view, this logical arrangement of organisms reflected God’s character and wisdom.
But all that changed with Darwin. He suggested that a spontaneous process of development had occurred that resulted in diversification of organisms from a single life form into all the creatures we see today. Darwin’s tree of life would have developed from descent with modification or evolution from a common ancestor.
As the era of molecular biology arrived, biologists had great hopes that trees based on chemical compounds would match those calculated from physical characteristics. However bad news soon followed. Some scientists even came to the conclusion that “Molecular phylogeneticists [specialists] will have failed to find the ‘true tree’ not because their methods are inadequate or because they have chosen the wrong genes, but because the history of life cannot be represented as a tree.”1
As a result of this situation, scientists have developed new explanations for the failure of different molecules inside a group of organisms, to suggest the same tree of life. They now suggest that some genes have been transferred from one set of organisms into another other unrelated organisms. This popular explanation is now termed horizontal gene transfer or HGT.
Assumptions about horizontal gene transfer have greatly complicated any conclusions about descent with modification (evolution) and have drastically resulted in very different organisms being clumped together. Thus, new plots of relationships often look very different from former popular drafts of the tree of life.
A recent essay entitled: “The Past, present and future tree of life” by Cedric Blais and John M. Archibald, demonstrates why the tree of life concept is in serious trouble2: “The issues are not merely empirical but also pragmatic, hinging on what scientists value, what they research and the tools they need. Data alone cannot resolve such a debate – a fact that provides an opportunity to rethink our objects of study and methods, and to experiment with new possibilities. We must not ask which is the truer picture of evolution – trees or networks – but rather what is it that we are trying to picture in the first place, and why.”3 They suggest the issue is pragmatic, not a search for truth.
What conclusions do the scientists feel confident about? Some specialists think they may be able to distinguish a “statistical tree of life” which does not represent any specific past history. “The continued use of the tree of life for classification is thus as much a reflection of its practical convenience and historical and cultural inertia as it is a commitment to natural classification.”4 The authors are skeptical of the reality of the tree of life because “much of the evidence for ancient relationships is inconclusive at best.”5 The tree of life may not be evident from the data, but in a pragmatic sense it “is still a source of insight into evolutionary history.”6 Evidently there is no clear evidence for an evolutionary tree of life. But scientists like the idea anyway for its evolutionary implications!
Perhaps it is time that Darwin’s “tree of life” is recognized for what it is, an idea that was based on wishful thinking. That is not good enough for a supposed scientific explanation for the relationships of all life forms. Many Christians, on the other hand, see the logical relationships of organism characteristics as reflecting the sovereign purposes of God. The reason we can separate life forms into hierarchical organization is because God demonstrates his wisdom and logical character by designing life this way.
- Ford Doolittle. 1999. Science 284: p. 2124-2128. See p. 2124.
- Cedric Blais and John M. Archibald. 2021. Current Biology 31, R314-R321, April 12.
- R314. Emphasis mine.
- R318. Emphasis mine.
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