Darwin’s theory of evolution by natural selection states that the adaptations of an organism to its environment arise because of the greater chance of survival of those offspring that have a favourable variation caused by random mutation, which they then pass on to their offspring.
What a challenging idea this was! Ever since its introduction to the scientific and popular community in Darwin’s great book of 1859, On the origin of species by means of natural selection, some people have misunderstood and challenged it. How can such perfect, often very complex design possibly arise from random variation? How can an incipient, or incomplete stage of an evolving adaptation be advantageous? Has there really been enough time for so vastly many changes to accumulate? How can a completely new kind of organism gradually evolve from its ancestor, when it must have changed so many of its characters, all of which had to stay tightly integrated with each other throughout? But every one of these and other objections has been successfully countered over the years by sound scientific evidence. The origin of even the most unlikely looking adaptations can be explained by a sequence of feasible intermediate stages of increasing improvement, all within the time shown by fossils to have been available. Here are two examples picked from a great many to illustrate this.
The human eye was often taken as a test for whether natural selection really can produce a complex new adaptation by numerous small steps: what good is half an eye? Zoologists can point to a perfectly possible sequence of stages, starting from no more than a small patch of pigmented surface cells in which photons of light stimulate a chemical reaction that creates an electrical potential.
This could have been followed by gradually shielding the pigment cells in a shallow cup, which would create a sense of the direction the light was coming from. A more restricted opening to the cup would the give the eye a degree of image discrimination, like a pinhole camera. If the liquid content of the cup became closed off and part of it optically denser, it would start to act like a simple lens, focussing the image more sharply at the back of the cup. With just a few more refinements, a virtually complete human eye would exist. Although we cannot know for certain that this is how it evolved, it is not just guesswork. To start with, versions of all these stages exist today in various invertebrate animals. In each case, the light receptor, however simple, is well suited to the life of its modern possessor. A human’s high resolution image would be no more use to a sea anemone fixed to a rock than would its simple light sensitive patch of skin be to a human. Further indirect evidence comes from the embryonic development of the vertebrate eye, which goes through stages generally similar to these. Thirdly, the exciting discovery that several of the same genes are involved in the development of the different kinds of eyes in very different animals, points to how relatively small changes in the genetic control could drive the succession of evolutionary steps.
Mammals are warm-blooded, or more accurately endothermic. Their tissues and organs have a high rate of metabolism, which generates enough heat to keep them fully active, even during the cool of night and in winter. Endothermy is a very complex adaptation because many parts and processes of the body are involved in generating the high metabolism, and in controlling, conserving, and utilising the high body temperature. Compared to reptiles such as lizards and crocodiles, mammals need to find, consume and digest ten times as much food a day and to breathe in and circulate around the body correspondingly more oxygen. They also need an insulating fur coat to prevent too much heat being lost, and the females must be able to keep the developing foetuses in the womb warm, and to nourish the young with milk after birth. We are fortunate to have discovered fossils of several intermediate grades between ancestral reptile-like animals and the earliest mammals which appeared about 200 million years ago. They are called synapsids, and the series below illustrates the gradual evolution of mammalian adaptations, over the course of about 120 million years.
Synapsid evolution 
Synapsid fossil
We see how the limbs evolved faster and more agile movement to improve foraging, while at the same time the teeth and jaws became more effective for chewing. The front part of the rib cage enlarged for enhanced breathing. The senses of hearing and smell, and we can guess also sight, became more acute, and the brain enlarged. All these changes happened hand in hand, showing how even a major new kind of organism such as a mammal, with all its significant new adaptations, can evolve from its ancestor by natural selection acting on its correlated characters.
Further reading and videos:
Your task
Like mammals, birds are highly evolved endotherms. Scientifically they are thought of as miniature, feathered, flying dinosaurs. As with the mammal story, the fossil record of their evolution includes several intermediate grades of adaptation, in this case various dinosaurs. Investigate the evolution of bird characteristics such as bipedalism (walking on the back legs), feathers for insulation, air sacs to aid breathing, and wings for flying; here is a source you might start with.
Write a 500 word essay on what you have found out about how the ability of birds to fly evolved. You may want to include diagrams of your own to help explain your findings.
Dr Tom Kemp, Emeritus Research Fellow in Biology
I spent many years at Oxford University’s famous Natural History Museum, researching fossils of vertebrate animals, and thinking and writing about evolution. At the same time, I was lucky enough to be Biology Tutor at St John’s College, where I grew to love teaching these subjects. Even though I am supposed to be retired now, I still write and teach, and it is a privilege to be involved in helping you come to understand how important evolution is to understanding much about the world we live in.