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9 Lamarckism Evolving: The Evolution of the Educated Guess We hope that readers who have reached this penultimate chapter are by now convinced that DNA is not the be all and end all of heredity. Information is transferred from one generation to the next by many inter-acting inheritance systems. Moreover, contrary to current dogma, the variation on which natural selection acts is not always random in origin or blind to function: new heritable variation can arise in response to the conditions of life. Variation is often targeted, in the sense that it preferentially affects functions or activities that can make organisms better adapted to the environment in which they live. Variation is also constructed, in the sense that, whatever their origin, which variants are inherited and what final form they assume depend on various “filtering” and “editing” processes that occur before and during transmission. Some biologists have great difficulty in accepting this “Lamarckian” aspect of evolution. To them it smacks of teleology, seeming to suggest that variations arise for a purpose. It appears as if the hand of God is being introduced into evolution by the backdoor. But of course there is nothing supernatural or mysterious about what happens—it is simply a consequence of the properties of the various inheritance systems and the way they respond to internal and external influences. We know, however, that we have left something out—something that may make it look as if we are still retaining naive teleology. We have assumed all along that the inheritance systems through which potentially adaptive variations are generated and transmitted already exist. Could it be that in assuming this we have slyly introduced some mysterious intelligence into evolution right at the beginning? If we are to get rid of any suspicion of invoking the hand of God, we need to explain how such intelligent systems came into being in the first place. Before we start looking at the origins of the systems that introduce instructive elements into evolution, we want to highlight an old and well-established piece of evolutionary biology, which is central to a lot of 314 Chapter 9 our arguments. It is that many new adaptations begin as by-products or modifications of characters that were originally selected for very different functions. For example, the vertebrate jaw had its origins in the skeletal elements that supported the gills (respiratory organs) in primitive jawless fish. The front gill support was gradually co-opted for new, feeding-related functions , and became the jaws of later fish. Modification did not stop there: in later vertebrates, changes in the manner of feeding led to altered jaw articulations, and freed up some of the bones at the hind end of the jaw. These were put to a new use—to carry vibrations. They eventually ended up as the three tiny bones in our middle ear. So what started as breathing aids became feeding aids, and the feeding aids later evolved into hearing aids. A structure that was originally selected for one function evolved to have a very different one.1 Often when an existing structure is recruited for a new job, its old function is not lost, so it ends up with several functions. Mammalian hair is a good example: it probably evolved originally for insulation and temperature regulation, and for many mammals this has remained its most important job, but in some it also has a role in courtship displays and camouflage. In addition, some hair now has a sensory function. The “whiskers” on a mammal’s snout are hairs that have evolved to become exquisitely sensitive to touch. We have shown this and other examples of evolutionary modification and diversification of function in figure 9.1, and in the following sections we describe how comparable changes have contributed to the evolution of the various heredity systems. The Origin and Genetics of Interpretive Mutations Much has been written about the origin of life. It was once seen as a metaphysical problem, but it has now acquired scientific status, and scientists working on prebiotic evolution are closing in on plausible and testable theories about how living things came into existence. We are not going to dwell on these because the origin of life is really outside the scope of this book.2 We want to start at the point where, through a complex process of chemical and biochemical evolution in which natural selection probably played a big role, DNA had become the hereditary material. Natural selection continued to...

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Additional Information

ISBN
9780262322676
Related ISBN
9780262525848
MARC Record
OCLC
878130644
Pages
576
Launched on MUSE
2014-04-24
Language
English
Open Access
No
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