Brief Outline
Additional comments on the evidence for evolution.
The strongest argument for descent with modification is what William Whewell called consilience of induction -- the concurrence of many separate lines of evidence and argument into a single, integrated explanatory framework. Consilience of induction is what made Darwin famous. In his Origin, he explained how a host of diverse observations, including hierarchical classification, comparative anatomy (homology), biogeography, the fossil record, and adaptation, could each lead to the same coherent theory of descent with modification.
Consilience defined
Many independent sets of observations all suggest the same conclusion, that present kinds of organisms are descended with modification from organisms that lived in the distant past. These lines of evidence include:
None of these would, by itself, constitute overwhelming evidence for the transmutation of species. But, taken all together, the fact that every one of these various phenomena can be accounted for by the same explanation is quite powerful. The coincidence of these independent lines of evidence constitutes consilience of induction for descent with modification.
Consilience of induction for natural selection as the mechanism of evolution.
Similarly, several independent lines of reasoning or empirical observation all suggest that natural selection is the mechanism for descent with modification. These lines of evidence include:
In other words, a causal mechanism underlying descent with modification could be induced, without calling upon any new or mysterious principles, from the empirical facts of heredity, variation, reproductive excess (from Malthus), and "survival of the fittest". The strongest evidence for Darwin's mechanism is exactly those empirical observations. If mutation did not provide an endless supply of novel variants, if such variation was not heritable, if all offspring survived, if hereditary variation in traits made no difference for survival, in any of these cases evolution by natural selection would be disproven. But all of these are facts which have been abundantly confirmed. With this causal mechanism, the consilience grew ever so much stronger. Now not only could so much be explained through descent with modification, but descent with modification could itself be explained by processes which were reliably observable and readily appreciated already in Darwin's time.
Current limitations of the evidence for evolution.
With the massively supported concept of descent with modification, and with irrefutable confirmation (one might say logical necessity) of natural selection, the only real question in Darwin's time concerned the sufficiency of the latter to explain the former. Although everyone agreed that selection could eliminate the unfit, could selection really create the fit as well? And could mutation provide an adequate supply of new variation sufficient to sustain the process indefinitely? Without an adequate theory for inheritance or for the origin of variation, these questions could not be answered in the nineteenth century. To some extent, they remain today, as a motive for continuing research within the established theoretical frameworks of genetics, developmental biology, and evolution.
At the present time there is simply NO empirical support whatsoever for any alternative to evolution as an explanation for the history of life. No alternative for decent with modification, nor any for mutation and selection as essential [and presumably sufficient] causal mechanisms. Nevertheless, difficulties remain. "The central problem with the [modern evolutionary] synthesis is its failure to show (or to provide distinct signs) that natural selection of random mutations could account for observed levels of adaptation" [ref 1].
Challenge to the evolutionary explanation.
The only noteworthy challenge to modern evolutionary theory comes from outside science, from the various forms of "creationism" which all stem from religious apologetics. (That source is not always obvious, since some creationist literature avoids any mention of theology.) Most forms of creationism include two central ideas. The first is an assertion that mutation and natural selection are inadequate to explain descent with modification. (Typically this is combined with an assertion that descent with modification has not even occurred, but that linkage is not always present nor is it logically necessary.) The second assertion is that some higher power or intelligent designer (i.e., God) must be invoked to explain adaptive "design". What that power might be (in cases where God is not mentioned), how that power might operate, or how its operation might be empirically detected, are all questions that are typically not discussed. The primary "evidence" cited for creationism is the presumed inadequacy of evolution to explain complex adaptation and macroevolution. Any and all gaps in current knowledge thereby become "evidence" that invalidates evolutionary theory and "proves" the creative power of God. This line of reasoning is sometimes caricatured as a "god of the gaps" argument, and is arguably even worse as theology than it is as "science".
A recent incarnation of the "god of the gaps" argument is called "intelligent design". The book Darwin's Black Box, by Michael Behe [ref 2], provides an excellent exposure to such argument. The phrase "black box" is used in science to describe a mechanism which forms part of an explanation but whose internal workings are (currently) unknown. The metaphor comes from practical engineering, where various mechanical and/or electrical components often come sealed in black cases. Such devices can be used without knowing (or at least without caring) about their internal workings. For most people, most everyday machines are "black boxes". We understand what they do (e.g., the workings of a refrigerator make it cold inside; the workings in a digital wristwatch cause the little numbers to tell the time), but we usually do not need to know precisely how the mechanism operates. Anyway, many explanatory concepts in science start out as black boxes. For example, throughout the 1800s atoms and molecules were black boxes. Chemists believed that they were real, and could correctly describe the composition of molecules from atoms. But they did not know how these atoms were held together (i.e., by means of the quantum mechanics of electron orbitals), nor could they prove directly that discrete atoms and molecules even existed. Similarly, for the first half of the Twentieth Century, the gene was a black box. By mid-century the behavior of the gene was known and its necessary properties could be inferred (e.g., Schrödinger's What Is Life). But not until research after Watson and Crick could we understand the "black box" that is now known as DNA with its transcription and translation mechanisms. The developmental program and the mechanisms of mutation are still black boxes for modern science. Most of evolutionary history is also a black box, and likely to remain so. At its heart, Behe's "intelligent design" theory simply presumes that things which are mysteries now will remain unknowable no matter how much we learn about these boxes' contents. But instead of accepting such problems as opportunities for further discoveries about genetics and evolution, "intelligent design" theory simply declares the problems intrinsically unsolvable.
Summary of additional comments
Ridley omits from his Chapter 3 presentation two philosophical principles that are both extremely important in scientific reasoning: Occam's Razor and Consilience of Induction.
Because of abundant evidence that the mechanism (natural selection) operates and also abundant evidence that the process (evolution) has happened, there is a STRONG PRESUMPTION that the one accounts for the other. HOWEVER, note that (in our current state of knowledge) it must still be PRESUMED that there is a sequence of spontaneous, undirected mutations that will lead along a naturally selectable route from one ancestral set of adaptations (e.g., those of a paleozoic fish) to another, transmuted descendent set (e.g., those of any modern bird or mammal).
Plausible "just-so" stories can be told to make this presumption seem plausible, but we do not yet have a theory relating genetics, mutations, adaptations, and fitness that is adequate to explain the transformation in any detail.
Such concerns are at the heart of Mivart's old "incipient stages of complex adaptations" argument (Ridley, p. 12), and also lie behind the outdated "acquired characters", "orthogenesis", and "hopeful monsters" hypotheses. But because there is no substantial evidence for any of these alternatives, the initial presumption stands as the most promising candidate for a necessary and sufficient explanation of evolution.
One of the most conspicuous modern versions of "scientific creationism", "intelligent design" theory, rejects the presumption of adequacy for mutation and natural selection and in its place postulates the NECESSITY for some intelligent agency to account for the acquisition of complex adaptations. However, because there is (1) absolutely no positive evidence for any such agency (except adaptation itself) and (2) there is also no positive evidence for inadequacy of the conventional mechanism (apart from current ignorance concerning the genetics of complex adaptations, as noted above), "intelligent design" theory is itself rejected by most scientists.
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