General introduction contextualizes research presented in the project and presents the three main conversation partners that are taken into account. First, it explains crucial stages of the development of evolutionary theory – from Darwin’s proposal, through the neo-Darwinian contribution and the two stages of the twentieth century evolutionary synthesis, until the most recent expanded evolutionary synthesis. Second, it lists foundational categories in the Aristotelian-Thomistic metaphysics that become relevant in the context of contemporary evolutionary biology. Finally, it refers to the classical theology of creation as grounding a constructive model of the most up-to-date Thomistic version of theistic evolution that will be developed in the book. Introduction ends with a general plan of the project.

On November 24, 1859, the English naturalist Charles Robert Darwin published On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life . In that book (Darwin 1859), he argued that all organisms, living and dead, were produced by a long, slow, natural process, from a very few original organisms. He called the process “natural selection,” later giving it the alternative name of “the survival of the fittest.” This first chapter is devoted to presenting (without critical comment) the argument of the Origin, very much with an eye to the place and role of natural selection. As a preliminary, it should be noted that the Origin, for all it is one of the landmark works in the history of science, was written in a remarkably “user-friendly” manner. It is not technical, the arguments are straightforward, the illustrative examples are relevant and easy to grasp, the mathematics is at a minimum, meaning non-existent. Do not be deceived. The Origin is also a very carefully structured piece of work (Ruse 1979a). Darwin knew exactly what he was doing when he set pen to paper.

Now we come to the elephant in the room. Darwin’s theory was incomplete. When the theory was completed, would natural selection prove to be that effective? Although he threw in a lot of assorted, presumed-relevant facts, no one, starting with Darwin, had much idea about the nature of variation – how it comes, what form it takes, how regular it is. And, without this knowledge, given that natural selection supposedly works on this variation, it is hard to make definite judgments about its effectiveness; especially since Darwin stressed that, although variation has causes, it is random in the sense of not appearing according to need. When he was not pushing the Lamarckian alternative, he was adamant that it is selection alone that is responsible for adaptation.

Turn now to those who think natural selection is vastly overrated as a cause of evolutionary change. It is at best a clean-up process after the real creative work has been done. It is little surprise that these critics come from within the organismic model, implicitly or explicitly. At the scientific level, we have encountered already the most (and properly) distinguished of them all, the American population geneticist Sewall Wright. Remember his “shifting balance theory,” where the key lay in genetic drift, as gene levels fluctuated randomly in small subpopulations, and then, when new adaptive features appeared, the subpopulations rejoined the larger group (probably the species), and through a form of group selection the new feature spread through the whole group. This is highly Spencerian – infused with a solid dose of Bergsonian vitalism – as equilibrium is disturbed and then regained at a higher level, part of an overall progressive process, presumably ending in humankind.

A little arbitrarily, but not entirely without reason, let us take 1959, the 100th anniversary of the Origin, as the date when the Darwinian paradigm finally came into its own. Natural selection and Mendelian genetics, now rapidly becoming molecular genetics, gave the explanation of the tree of life. If we continue to think in Kuhnian terms, what now of normal science? We should expect to see the subbranches of the consilience come into their own, as practitioners moved forward, theoretically, experimentally, and in nature, raising and solving their problems. And in major respects we do see exactly this.

In 1866, Thomas Hardy, raised a sincere member of the Church of England, wrote his sonnet “Hap.” It expressed the anxiety about – “fear of” is not too strong a term – the world into which natural selection has pitched us. No longer can we rely on a Good God to care for us, to suffer for us, to make possible eternal life. In the non-progressive world of Darwinian evolution, all is meaningless.

Among the many books authored by Peter Bowler, the eminent historian of evolutionary biology, three stand out: The Eclipse of Darwinism (1983); The Non-Darwinian Revolution: Reinterpreting a Historical Myth (1988); and Darwin Deleted: Imagining a World without Darwin (2013). Bluntly, he says: “there is now a substantial body of literature to convince anyone that the part of Darwin’s theory now recognized as important by biologists had comparatively little impact on late nineteenth century thought” (Bowler 1988, ix). I cannot say Bowler is entirely wrong. Indeed, in The Darwinian Revolution: Science Red in Tooth and Claw (1979), I contributed to this “body of literature,” and my book was quite openly a synthesis of the state of Darwinian play in the second half of the nineteenth century. But is this the end of the story, and if it is, why is it the end of the story? Today, as Bowler also recognizes, we accept the finding of natural selection as a major scientific achievement, up there with relativity theory. Let us pick up on this paradox.

Natural selection. I am an evolutionist, which means that, to understand the present, we must dig into the past. That holds for culture as much as for biology. So, taking my own advice, where do we end up? Or, more precisely, where do we start off? As always, when dealing with Western culture, we begin with the Greeks, Plato and Aristotle. Neither of them was an evolutionist. Indeed, rather like the Buddhists, they believed that the (physical) world is eternal: no beginning, no end. But they did have much to say of great interest to our inquiry.

Time to pull back and get a little more conceptual. We need to ask some penetrating questions about the nature, the scope, the truth-value of natural selection. Finding answers, the quest begins in the past. Charles Darwin was a graduate of the University of Cambridge. The greatest British scientist of them all, Isaac Newton, was also a graduate of the University of Cambridge, and his spirit, his achievements, his reputation, infused every discussion about science, including about the life sciences. In his Principia, Newton started with his three laws of motion, together with his law of gravitational attraction, and then went on to infer, deductively, the pertinent terrestrial laws, those of Galileo, and the pertinent celestial laws, those of Copernicus affirming the heliocentric nature of the Universe and those of later thinkers, especially Kepler on planetary motion. It was a given that the ambitious young Charles Darwin would want to show Kant dead wrong. There could be a Newton of the blade of grass, and that Newton was going to be Charles Darwin.

When a new cause is introduced into science, as often as not it is accepted without trouble. Few, if any, had worries about the Watson–Crick double helix and the subsequent working out of the genetic code. Genetics was put on a molecular causal basis. However, it is not uncommon for there to be opposition. Huygens’ wave theory of light was an outsider for nearly two centuries. Sometimes worries are ongoing. One doubts that, as long as there are those interested in mental health, Freud’s Oedipus Complex is going to be happily accepted by all. There have been, continue to be, and probably always will be disputes, often bitter, about its causal status. As we have seen, natural selection did not have an altogether easy birth. But as time went by, things seem to have improved. Newton and Leibniz all over again.

Rapid evolution can be observed happening in nature when selection is unusually strong. We are all familiar, these days, with the evolution of antibiotic resistance in bacteria and the evolution of pesticide resistance in insects. Less familiar, but also very rapid, is the evolution of resistance to heavy metals in populations of plants that have adapted to growing on the spoil-heaps surrounding zinc and lead mines. These cases of unusually strong selection and consequently rapid evolution are all associated with human modification of the environment. The classic case study of evolution happening – industrial melanism in moths – also fits into this category.

Evo-devo has come a long way since its origins a mere four decades ago. Many exciting things have been discovered, and there will be many more discoveries to come in the years ahead. I have tried, in this book, to give you a flavour of this new branch of science. Here, I summarize what I think are its most important conclusions so far and the most important challenges that lie ahead.

In the previous chapter we looked at several different kinds of developmental bias. One of our conclusions was that there are both specific biases, such as the numbers of centipede trunk segments and mammalian neck vertebrae, and general biases, such as the tendency for variant developmental trajectories – and in particular viable ones – to be clustered close to the ancestral trajectory. For example, in the case of snails we noted that the forms of developmental repatterning that were generally available for natural selection to act on were slight quantitative modifications of the pattern of development of the snail that was the ancestor of the clade concerned – an example being developmental trajectories leading to differences in adult shell size. Acknowledging this form of bias entails accepting that evolution of body form does not usually take place via radical-effect macromutations. This is interesting because we saw in Chapter 2 that from the late nineteenth century to the mid-twentieth century, prominent biologists who had a specific interest in the evolution of development, such as William Bateson, D’Arcy Thompson, and Richard Goldschmidt, took a macromutational approach.

Although today we call the scientific study of the relationship between evolution and development ‘evo-devo’, neither that term, nor its longer counterpart ‘evolutionary developmental biology’, existed before about 1980. Yet the study of the relationship between the two great creative processes of the living world has a much longer history – effectively starting in the nineteenth century, the first century in which there was a well-articulated theory of evolution (first Lamarck’s, then Darwin’s). We generally refer to evo-devo’s nineteenth-century antecedent as ‘comparative embryology’. Although in the subsequent period from about 1900 to 1980 there were further studies of the relationship between evolution and development, there is no collective term for this endeavour, because mainstream developmental biology and evolutionary biology were largely separate undertakings during that stretch of time. The few biologists who tried to deal with the two together over this 80-year period might be described as mavericks. Each of them produced interesting bodies of work, but these did not really link up to form a scientific discipline.

Body-plan features that have been discussed so far include symmetry, segmentation, skeletons, and limbs. When these are encountered in different phyla, are they homologous or convergent? There are examples of both of these, plus examples where the answer is not yet clear. Bilateral symmetry of the overall body plan seems to have originated just once. So the fact that vertebrates and arthropods are both bilaterally symmetrical is due to their having inherited that body layout from their last common ancestor; in other words, their bilaterality is homologous. However, although vertebrates and arthropods both have skeletons (whereas animals belonging to many other phyla do not) these represent convergent rather than homologous skeletons – this is clear from the fact that one is ‘endo’, the other ‘exo’. Turning to segments and limbs, the fact that both vertebrates and arthropods have these component parts is hard to interpret with certainty one way or the other. The reason for this is our lack of knowledge of that ancient animal that we call the urbilaterian, or ‘Urby’ for short. Direct evidence of this creature will probably never be forthcoming, since it was almost certainly small and soft-bodied, and has left us with no fossils from which to infer its living form. Instead, we can only make rather indirect inferences based on the point in the animal evolutionary tree at which we think bilaterality arose. However, indirect inference is better than nothing, so here goes.

Here, I list ten important issues where I think that there is a significant risk of misunderstandings among those who are new to the field. After each potential misunderstanding, there is a statement of the correct situation, as I perceive it. Many of these issues are related to the rationale underlying the emergence of evo-devo as a (relatively) new discipline.

Our starting point for discussion of evolutionary pattern is the word ‘clade’. This was introduced by the British biologist Julian Huxley (grandson of Darwin’s bulldog T. H. Huxley) in the 1940s. It means a taxonomic group of a particular kind: one that includes all the descendants of a particular ancestral species, and no others. This kind of group can also be called monophyletic. When the German taxonomist Willi Hennig founded the new approach to taxonomy that we now call cladistics, in the 1950s and 1960s, the idea of a clade was central. For those not familiar with cladistics, Hennig’s main concern was that the evolutionary trees that were used through much of the literature of evolutionary biology confounded two things: closeness of ancestry and similarity in body form.

The two great creative processes of biology are evolution and development. You and I, as adult human beings, are products of both. Evolution took about four billion years to make the first human from a unicellular organism that emerged from the primordial soup. Development, in the form of embryogenesis together with its post-embryonic counterpart, takes less than 20 years to produce an adult human from a different unicellular organism – a fertilized egg or zygote. By this measure, development operates more than 200 million times faster than evolution. However, despite their very different timescales, the two great creative processes of biology are intrinsically interwoven. Evo-devo is the scientific study of this interweaving. Its full name is evolutionary developmental biology, but because this is an unwieldy phrase it is almost universally referred to by its nickname.

It is constructive to approach this issue from a historical perspective. Some aspects of animal relatedness have been known for a long time – centuries – while some have only been established in the last few decades. And others remain to be worked out or confirmed. A useful starting point for this historical approach is the 1817 four-volume work Le Règne Animal (The Animal Kingdom) by the French comparative anatomist Georges Cuvier, who divided the kingdom into four embranchements (branches): vertebrates, molluscs, articulates (outwardly segmented animals), and radiates (radially symmetrical animals). We should note here that Cuvier was an anti-evolutionist; he was opposed to the evolutionary theories of his fellow Frenchmen Jean-Baptiste Lamarck and Étienne Geoffroy Saint-Hilaire, and he did not live to see the publication of Darwin’s Origin of Species. However, many non-evolutionists prior to Darwin (from Aristotle onwards) made good attempts at the classification of animals, even though the fruits of their labours would not be given an evolutionary interpretation until later. Here, I will discuss Cuvier’s suggested groups as being evolutionary ones, even though that is not how he saw them.