Ch. 14 - Proof and disproof; science and politics
The first edition of this book [1978] contained a short chapter with the heading 'Proof and disproof,' applying my (naive) understanding of some of the ideas current in philosophy of science of the 1970s to evolutionary theory. The chapter was troublesome in various ways, but in rewriting the book I eventually decided to leave it much as it was, because I still think the ideas are interesting, and to add a commentary ('Science and politics').
14.2 Is evolution science?
[The general theory of evolution] must be unique and unrepeatable, like the history of England. Before Darwin, species were generally thought to be fixed and immutable, each with some discoverable and universal essence, like the elements or chemical compounds. Darwin explained species as temporary, local things, each with a beginning and an end depending on contingencies of history. So the general theory of evolution is a historical theory, about unique events - and unique events are, by some definitions, not part of science for they are unrepeatable and so not subject to test. ... Yet biologists have enormous advantages over historians. First, they have a coherent, and scientific, theory of genetics, and their interpretations must be consistent with it. Second they have one basic tool, homology. And third they have the universal scientific principle of ... Occam's razor....
The general theory of evolution is thus neither fully scientific (like physics, for example) nor unscientific (like history). Although it has no laws it does have rules, and it does make general predictions about the properties of organisms. It therefore lays itself open to disproof. Darwin cited several sorts of observations which would, in his view, destroy his theory. In this he was certainly more candid than his opponents. The potential tests Darwin mentioned are:
- 'If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down';
- 'certain naturalists believe that very many structures have been created for beauty in the eyes of man, or for mere variety. This doctrine, if true, would be absolutely fatal to my theory';
- 'if it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory'.
Darwin's potential tests may strike the reader as pretty feeble, or as tests of natural selection rather than evolution, but many discoveries, not forseen by Darwin, provide more severe tests of the theory. These include Mendelian genetics, the real age of the earth, the universality of DNA and the genetic code and (most recently and spectacularly) the evidence from DNA sequences of innumerable 'vestigial organs' at the molecular level. Evolution has survived all of these with flying colours. Darwin could not possibly have predicted that the hereditary material (of which he knew nothing) would turn out to be littered with rubbish, with 'rusting hulks' like the delta haemoglobin psuedogene found in Old World monkeys, or with meaningless repeated sequences like the shared Alu sequences in apes and humans. An interesting argument is that in the law courts (where proof 'beyond reasonable doubt' is required), cases of plagiarism or breach of copyright will be settled in the plaintiff's favour if it can be shown that the text (or whatever) is supposed to have been copied contains errors present in the original. Similarly, in tracing the texts of ancient authors, the best evidence that two versions are copies from another or from the same original is when both contain the same errors. ... Shared psuedogenes, or shared Alu sequences, may have the same significance - like shared misprints they can have come about only by shared descent. ...
14.3 Alternative theories
Using Popper's criterion we must conclude that evolutionary theory is not testable in the same way as a theory of physics, chemistry, or genetics, by experiments designed to falsify it. But the essence of scientific method is not to test a single theory to destruction; it is to test two (or more) rival theories, like Newton's and Einstein's, and to accept the one that passes more or stricter tests until a better theory turns up. We must look at evolution theory and natural selection theory in terms of performance against the competition.
The belief that all organisms are related by descent and have diverged through a natural, historical process has only one main competitor, creation theory, though there are different stories of how the Creator went about His work. All creation theories are purely metaphysical. They make no predictions about the activities of the Creator, except that life as we know it is the result of His plan. Since we do not know the plan, no observation can be inconsistent with it. ...
14.5 Science and politics
This chapter is so far much as it was in the first edition, though it then included less on neutral change in DNA and nothing on psuedogenes and other molecular misprints, which were then undiscovered. As a result it was perhaps a bit more agnostic than the version you have just read. What follows is a commentary on some events since. ...
[In 1981 the Natural History Museum opened a Origin of the Species exhibit, which created a controversy over how it should present the theory of evolution. The controversy raged on in the pages of Nature.] Popper had contributed to this debate in 1980 with a letter to New Scientist that included 'some people think that I have denied scientific character to the historical sciences, such as ... the history of the evolution of life on Earth. ... This is a mistake ... historical sciences have in my opinion scientific character: their hypotheses can in many cases be tested. ... some people would think that the historical sciences are untestable because they describe unique events. However, the description of unique events can very often be tested by deriving from them testable predictions or retrodictions.' [ellipses are Patterson's] Popper gave no example of such a prediciton, but one often quoted (attributed to J.B.S. Haldane) is that finding human bones in Carboniferous rocks (about 350 million years old) would falsify the general theory of evolution. ...
In 1978 Popper wrote that the Darwinian theory of common descent 'has been well tested' (he did not say how) and 'That the theory of natural selection may be so formulated that it is far from tautological. In this case it is not only testable, but it turns out to be not strictly universally true. ... Yet in every particular case it is a challenging research programme to show how far natural selection can possibly be held responsible for the evolution of a particular organ or behavioural programme.' [ellipses are Patterson's]
14.6 Summary
...I will close by summing up my own opinion on the truth of evolution. In terms of mechanism, or causes of evolutionary change, the neutral theory of molecular evolution is a scientific theory; it can be put in law-like form: changes in DNA that are less likely to be subject to natural selection occur more rapidly. This law is tested every time homologous DNA sequences are compared, and explains observations (summarized in Chapters 9 and 10) that are otherwise inexplicable. But neutral theory assumes (or includes) truth of the general theory - common ancestry or Darwin's 'descent with modification' - and 'misprints' shared between species, like the psuedogenes or reversed Alu sequences are (to me) incontrovertible evidence of common descent. I see the general historical theory, common descent, as being as firmly established as just about anything else in history. ... The most difficult problems in evolutionary biology are in deciding or discovering the extent to which positive natural selection is responsible for the diversity of life.
Colin Patterson, Evolution, 2nd Ed. (1999)