Several years later, the fossil Ambulocetus was discovered in present-day Pakistan, along the shores of an ancient waterway called the Tethys Sea. This tactic reliably provoked gales of laughter from his creationist audiences. 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. Evolution has had hundreds of millions of years to test different solutions, and often arrives at results that human beings could never have anticipated. 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. 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. 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. 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.
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.Īnother 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. 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. 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. 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. (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. 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.
The first thing to note is that evolution does not require living things to be suited to only one environment. Like many other creationist objections to evolution, it is heavily oversimplified, and the very details which it overlooks are its stumbling block. However, on detailed examination, it does not hold up. To the layperson unacquainted with how evolution works, this argument may sound superficially convincing. (Intelligent-design advocate Michael Behe bases his entire argument on this idea, though he shrinks it down to the molecular level.) A “half-built” wing or eye would provide no advantage at all, and thus could not be selectively favored for further improvement. 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. 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.
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.
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