Brett Dennen is a fine musician, but listening to his lyrics, one might be tempted to think that because we ‘do [or see] it every day’, then it does not deserve an explanation. Our book has dealt with a variety of everyday phenomena such as ageing, sex, species, a green world, and a blue sea, and we hope that by now our readers will agree that there is a reason why things are this way. Indeed, the fact that these phenomena are so commonplace makes the questions all the more important. The exciting thing is that while considerable progress has been made in each of the areas we address, we still do not have a complete answer to any of the questions we have posed. We use this short concluding chapter to pull together some common threads and to discuss some of the interrelationships between our answers. First and foremost, even the most casual reader will note that there is a close interrelationship between the ecological and evolutionary explanations we have presented. Taking the perspective of evolutionary biology, almost all of the evolutionary explanations we have proposed include an important ecological component. For example, ageing is now widely seen to arise as a consequence of there being relatively weak natural selection late in an organism’s life. Yet the primary reason for this ‘selective shadow’ is that predators and parasites are likely to have killed the organism long before it reaches an advanced stage of maturity. Likewise, one explanation for the evolution of sex is that the variation it generates allows at least some of the offspring to better compete with members of the same species, or to avoid parasitism. In a similar vein, many of the ecological phenomena we have sought to explain have evolutionary origins. For example, tropical areas may have more species because rates of speciation are greater in the tropics, or because rates of extinction are greater at high latitudes, or both. Likewise, plants have evolved secondary compounds to deter herbivory, and the presence of these compounds may go some way towards understanding why the world remains green.
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