In this book we will be looking at some of the evolutionary forces that shaped, and are shaping, immune defence systems. However, before doing that it makes sense to look at some of the basic features of the evolutionary process. Perhaps the best place to start is with an essay written by Ernst Mayr, at that time professor of Zoology at Harvard, which was published in Science in 1961 . In this essay Mayr considered the case of a little bird that fluttered around outside his office window all summer and then, as autumn approached, took off and flew south. Mayr pointed out that there are two types of question one might ask about all this. The first is what he called the “how” question. How does the little bird know that the year is nearly over? How does it organise its flight south? Clearly the bird must be able to detect some feature of approaching autumn—perhaps by using some appropriate receptor system to detect the shortening of the day or the gradual fall in average daily temperature. Then it must convert this signal into electrical impulses that can be interpreted by its brain, after which an output signal must be generated, which encourages the little bird to fly off in the correct direction. Finding the answers to the “how” questions would involve a lifetime or two of fascinating research, and the results would be expressed in terms of molecules and of biochemical and neural pathways. These things are familiar to all of us, since molecules and pathways are what most scientists spend most of their working lives dealing with.
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