In the 1980s, David Penny and his colleagues pooled together their collective knowledge of biology, statistics, and philosophy to address Karl Popper’s criticism that “Darwinism is not a testable scientific theory, but a ‘metaphysical research program’—a possible framework for testable scientific theories." While he was personally convinced that evolution was a fact, Popper was concerned that the theory of evolution boiled down to a tautology: in the struggle for existence, the fittest will survive. But who are the fittest? The ones with the highest survival rate, of course. In other words, “survival of the survivors.” For something to be a scientific theory, it must in principle be vulnerable to being proven wrong by outside evidence. Tautologies are true by definition, regardless of the evidence.
Penny figured that this problem with the theory was qualitative and linguistic, and that he could develop a method that would quantitively falsify the theory of evolution if it were untrue. He turned to the model of the tree of life. He looked at 11 species, and according to the math there are (2n - 5)!!, or 34,459,425 different evolutionary trees you could construct with just 11 species. He then compared 5 different genes across the species. His reasoning was that if the differences between the genes of these species were not the product of gradual mutations and binomial speciation, then there would be no consensus between the genes about which of the 34 million trees best represents the relationship between the species. If the tests did not agree across different genes and species, that would be the end of the theory of evolution.
As a matter of fact, not only was there a consensus between the five genes across his sample of species, but this experiment has been replicated across hundreds of genes in thousands of species, all forming the consensus that there is just one tree of life relating the lineage and common ancestry of all life on Earth.
Postscript: The above picture is taken from a book called The Society of Mind by Marvin Minsky. The book outlines a theory of how natural intelligence can emerge from unintelligent systems working together. It would be an otherwise dry and textbookish read if it weren't for humorous illustrations like this one scattered throughout.
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