Chapter 1: Adaptation by Natural SelectionAll around us are organisms that seem almost perfect for their respective environments. From the woodpecker's beak to the specialized echolocation systems employed by bats, many life forms exhibit a wide variety of characteristics that allow them to effectively exploit their particular surroundings. However, the questions that are not easily answered concern the "how" and the "why" of adaptation. Prior to the middle of the nineteenth century, explanations for why animals and plants are so suited for their environments were based exclusively on the notion of divine creation. For some people, this is still the case. This conclusion combines theological and philosophical arguments with observations in the natural world. For example, William Paley's argument in his Natural Theology (1802) was based on the premise that the intricate machinery of a biological organism implies it had an intelligent, heavenly creator. In contrast to this "watchmaker argument," two young Englishmen—Charles Darwin and Alfred Russell Wallace—proposed a theory that explained the process of adaptation without attributing it to the influence of a higher, spiritual power.
The Theory of Natural SelectionThree Postulates
Darwin's theory is based on three postulates:
Evidence to corroborate Darwin's analysis piled up over the twentieth century, but the theory of natural selection is still attacked on many fronts. One frequently cited counterargument is that even if Darwin's theory is true, the amount of time needed for small variations to cause dramatic changes in organisms exceeds the age of the Earth. In the years between "creation" and "present," the full range of organisms could not have appeared. In the nineteenth century, estimates of the age of the Earth ranged from six thousand to tens of thousands of years, these coming from either biblical referencing or geological guesses. Little did scientists of the period realize that approximately 4.5 billion years were unaccounted for in their calculations! Darwin's critics also point to the improbability of a single-step change that could have produced the integrated harmony and seeming perfection of a complex organ like the human eye, much less a complex organism like a human being. Of course, this statement is true if we assume variation does not pass from parent to offspring. However, if variation is heritable, as Darwin stated, then incremental changes may accumulate over time. Coupled with burgeoning evidence more accurately documenting the antiquity of the Earth, as well as the discovery of larger numbers of "intermediate" species, natural selection demands serious consideration as a mechanism for the diversification of life on Earth.
Darwin's Difficulty Explaining VariationWhen all was said and done, evolution by natural selection seemed to be a plausible explanation of the wide variation and diversity both between and among organisms. However, the theory remained controversial because variation, and its inheritance, could not be adequately explained in terms of the existing body of knowledge. The current of the times favored a blending mode of inheritance, which would effectively deplete a population of organisms of all variation.
Darwin could not provide a counterargument to the question of blending inheritance during his lifetime. At first glance, blending seems to be a good theory. Do not many human children appear to look like both their mother and father? As diagrammed above, though, blending would have grave implications for any evolutionary novelty or variation; descendants would merely become increasingly homogenized with each passing generation. However, as Gregor Mendel would point out, while phenotypic variation may indicate blending of sorts, the inherited material—namely the genes—remains discrete.
Given that one of the fundamental assumptions of natural selection is that variation must exist within populations, how would a blending mode of inheritance ever result in anything other than a population full of "average" organisms? This was a problem that remained unresolved until after the death of Darwin. The rediscovery of the research of a little-known Silesian monk by the name of Gregor Mendel contributed greatly to the explanation of how variation is maintained.