Chapter One: Adaptation by Natural Selection
Theory of Natural Selection
All 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 exploit their particular surroundings effectively. However, the questions that are not easily answered concern the "how" and the "why" of adaptation and the existence of complex adaptations. Before the middle of the 19th century, explanations for why animals and plants are so suited for their environments were based almost exclusively on the notion of divine creation. For some people—especially contemporary proponents of Scientific Creationism and Intelligent Design—this is still the case. These conclusions combine 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.
Darwin's theory is based on three postulates:
- The ability of a population to expand is infinite, but the resources available to sustain that population are finite. This dynamic causes a struggle for existence among individuals as they compete for resources.
- Organisms vary in their physical qualities; these variations allow some members to reproduce more successfully than others.
- These variations are inherited by offspring from their parents.
Evolution of Complex Adaptations
Evidence to corroborate Darwin's analysis piled up over the 20th 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 19th century, estimates of the age of the Earth ranged from 6000 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—in fact they must and do—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 the mechanism for the diversification of life on Earth.
Natural selection in theory and “in the field.”
With advances in both theory and technology, contemporary researchers are putting Darwin’s postulates to the test, so to speak. On the Galapagos Islands, for example, the dynamic couple of Peter and Rosemary Grant (along with a number of their graduate students!) have shown that minute variations in beak depth of finches can make the difference between life and death in extreme environmental conditions, such as droughts or flooding rains. They carefully documented every inch of one of the islands—Daphne Major—and thus were able to measure the exact interactions between finches and their environment. They were also able to show that selection does not proceed in a fixed direction, but often “wobbles” back and forth depending on what is most advantageous for a given environment. Additionally, researchers in Sweden recently built a complex mathematical model of the eye of an aquatic organism. They were able to mimic the possible formation of a complex vertebrate eye from a simple series of light sensitive patches by modeling which adaptations would be the most favorable and what kinds of intermediate forms might be possible. Thus, through these and other field and lab experiments by modern evolutionary biologists, natural selection has both been envisioned theoretically and documented in the real world..
Darwin's Difficulty Explaining Variation
With the close of the 19th century, 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, using present logic, would effectively deplete a population of organisms of all variation (Fig. 1.1).
Figure 1.1. Diagram of the difference between blending and Mendelian inheritance.
Figure Credit: W.W. Norton & Company, Inc.
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. Many human children certainly appear to look like both their mother and their father. Blending, however, would have grave implications for any evolutionary novelty or variation. Descendants would merely become increasingly homogenized with each passing generation.
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. As Mendel would point out, while phenotypic variation may indicate blending of sorts, the inherited material—namely the genes—remains discrete.
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