A common creationist claim is that transitional species, intermediates between one form and another which are predicted by evolutionary theory, do not and could not exist. Their reasoning is usually that transitional forms, by the logic of evolution, could not exist because an organism in the process of transition would not be fully suited to either its former environment or the one it was evolving to live in, and would be at a disadvantage in both. To bolster their arguments, creationists frequently assert that complex structures such as wings or eyes could not have evolved, because they would not be advantageous until they were completely assembled. A “half-built” wing or eye would provide no advantage at all, and thus could not be selectively favored for further improvement. (Intelligent-design advocate Michael Behe bases his entire argument on this idea, though he shrinks it down to the molecular level.)
To the layperson unacquainted with how evolution works, this argument may sound superficially convincing. However, on detailed examination, it does not hold up. Like many other creationist objections to evolution, it is heavily oversimplified, and the very details which it overlooks are its stumbling block.
The first thing to note is that evolution does not require living things to be suited to only one environment. There are specialist organisms that are exquisitely well-adapted to their particular environmental niche, but there are also generalists that can switch between different ways of making a living as the circumstances demand. (Indeed, one of the most successful generalist species ever to evolve is our own – Homo sapiens.) Natural selection can drive a species toward either greater specialization or greater flexibility, depending on what circumstances dictate.
With this in mind, an answer to the first objection can be stated concisely: Transitional forms can exist because, by definition, they are suited to both their environments. Just because a species is adapting to a new environment, a new niche, does not mean it must lose the adaptations or abilities that made it successful in its previous niche. Even this is an oversimplification – there are and were many transitional species that did not need to survive in two different environments, but were intermediate between two species that both dwelled in the same environment. For example, the species Acanthostega, thought to be ancestral to modern amphibians, lived in the same murky swamps as both its fish ancestors and its amphibian descendants.
Another example of a transitional form well-suited to both its environments is Archaeopteryx, intermediate between birds and dinosaurs and thought to have some capability for flight. Its skeleton, which is basically that of a small theropod dinosaur except for the feathers, indicates that it probably would have competed on the ground with its non-flying relatives for resources. However, its capacity for flight, no matter how limited, would have given it a significant selective advantage as well – whether to find food, such as flying insects, to escape from predators, or both.
The creationists may object that these are some of the easiest examples, and that there are other kinds of transitions where it is not nearly so obvious how an intermediate could have survived. The appropriate response to such claims is the biologist’s maxim often dubbed “Orgel’s Second Rule”: “Evolution is cleverer than you are.” Our imaginations do not place limits on what nature can do, as scientific investigation of the world has revealed time and time again. Evolution has had hundreds of millions of years to test different solutions, and often arrives at results that human beings could never have anticipated.
In one iconic example, creationist Duane Gish used to ridicule the idea of whales and dolphins evolving from land mammals by presenting a slide of an absurd creature that was half cow and half whale, and asking rhetorically how such a malformed beast could ever survive. This tactic reliably provoked gales of laughter from his creationist audiences. Several years later, the fossil Ambulocetus was discovered in present-day Pakistan, along the shores of an ancient waterway called the Tethys Sea. Ambulocetus, whose name literally means “the walking whale”, was a marvelous example of evolutionary transition – a four-legged creature with a mouth full of sharp teeth that would have made it a predator to be reckoned with on the land, and also possessing large, flipper-like feet and a sinuous spine that made it well-suited to life in the water. Gish’s much-mocked example of transition turned out not to be so impossible after all.
The second half of the creationist argument states that complex biological structures, such as eyes and wings, could not have evolved because they would not be any good until they were complete, and a partially formed version would not present any selective advantage and could not continue evolving. As in the previous case, it is deceptively simple and plausible-sounding, but also faulty. It ignores a crucial point about how evolution operates, and works only by overextension of an analogy beyond the bounds of its applicability.
The error in this position is that it assumes a complex structure could only work in its modern form, but this is not true at all. The analogy is deliberately misleading. Creationists would like people to think of the evolution of an eye as analogous to the construction of a house – and a half-built house is obviously no good to anyone. But when building a house, the builders know from the beginning what their ultimate objective is and move only towards it, even if it means that the intermediate stages will not function as a house.
This analogy does not represent evolution. Evolution has no specific long-term goals, no foresight, and did not “know” from the beginning that anything like a modern eye would eventually develop. At each step, evolution can only move in directions of increased fitness and functionality, and so it does not and cannot produce functionless structures that evolve into functional structures. Instead, it produces simpler but still functional structures that, over time, adapt to become more complex and better suited to their environments.
To make the house-building analogy more similar to evolution, postulate that we start not with an empty lot but a primitive structure such as a tepee, and set the rule that every intermediate stage must also serve as a functional shelter. With these constraints, the tepee might grow taller and wider over time. At some point it might develop a more sophisticated and sturdier support structure, making it a more secure shelter but also allowing for further improvement, such as proper walls. For better protection against the elements, it might “evolve” a thatched or a shingled roof, or insulated walls. In time, something close to a modern house might appear, and while every step along the way could be considered “half-built” compared to the end product, every “half-built” stage would also serve as a house, albeit a more primitive one.
Using the standard method, a half-built house is no good for shelter. But using evolution’s method, a “half-built” eye is good for vision. As seen in the above example, the key is to think of a “half-built” eye not as a modern eye with half its parts missing, but as a more primitive but still functional eye whose capability for vision is simply not as well refined. Contrary to what creationists claim, this would present a selective advantage. Any vision at all is better than no vision, and even slightly better vision makes an organism more likely to succeed reproductively. For example, some simple organisms have a very primitive type of eye – an eyespot, which is only capable of distinguishing between light and darkness. This can be useful for survival. Sudden darkness might mean a shadow had been cast across the organism, possibly meaning that a predator was passing by – a good time to retreat into a burrow. Further adaptations might include turning a flat eyespot into a curved cup, with improved capacity to detect the direction of a light source, and from that point developing a pinhole-camera eye, which allows forming a true image rather than a simple blur. Over time, all the adaptations of the modern vertebrate eye could develop and be refined. Charles Darwin himself foresaw this possibility, and gave a simple story about how a modern eye could have evolved. For more detail, see this article on the Talk.Origins Archive: An Old, Out of Context Quotation.
Similar arguments apply to the development of the wing. Rather than assuming that only a fully formed, fully flight-capable wing would present any selective advantage, it is helpful to look at the possible intermediates. There are two hypotheses currently competing to explain this. One is that wings developed in tree-dwelling organisms originally as a method of gliding from branch to branch; animals such as the flying squirrel may be modern-day examples of this. The other is that it developed in organisms that took off for short distances from a running start; over time, longer and longer hops would develop into gliding and finally true flight. In either case, the intermediate stages leading to a modern wing would still present selective advantage, showing that such a process is plausible.
The creationists’ arguments notwithstanding, there is no obstacle to the step-by-step evolution of complex biological forms and intermediate structures. The argument from personal incredulity, which takes the form “X could not have evolved because I cannot imagine how X could have evolved”, is logically invalid. Finally, regardless of what armchair reasoning tells us, there is only one way to put a hypothesis to the test: go out and look at the fossil record! And what we find, when we do such an investigation, is a wealth of transitional fossils linking many, if not most, of the major branches in the history of life. As with Ambulocetus, the species that creationists claim to be impossible really did exist. In the last resort it is always the scientific evidence that decides the truth or falsity of a theory, and in this case, the evidence stands squarely behind evolution.