Darwin's contribution was not in observing change over time, nor in noticing the adaptive design of mechanisms. Rather, Darwin revolution -ized the field of biology by pro osing a theory of the process by which adaptations are created and change takes place over time. He called it the theory of natural selection.
Darwin noticed that species seemed to produce many more of f-spring than could possibly survive and reproduce. He reasoned that changes, or variants, that better enabled an organism to survive and reproduce would lead to more descendants. The descendants, therefore, would inherit the variants that led to their ancestors' survival and reproduction. Through this process, the successful variants were selected and unsuccessful variants ent adaptation to environmental conditions?
weeded out. Natural selection, therefore, results in gradual changes in a species over time, as successful variants increase in frequency and eventually spread throughout the gene pool, replacing the less successful variants. Over time, these successful variants come to characterize the entire species, whereas unsuccessful variants decrease in frequency and vanish from the species.
This process of natural selection, sometimes called survival selection, led Darwin to focus on the events that impede survival, which he called the hostile forces of nature. These hostile forces included food shortages, diseases, parasites, predators, and extremes of weather . Whatever variants helped or ganisms survive these hostile forces of nature would lead to an increased likelihood of successful reproduction. Food preferences for substances rich in fat, sugar , and protein, for example, would help organisms survive food shortages. An immune system teeming with antibodies would help organisms survive diseases and parasites. Fear of snakes and spiders would help them survive these dangers. These mechanisms, resulting from a long and repeated process of natural selection, are called adaptations, inherited solutions to the survival and reproductive problems posed by the hostile forces of nature.
Even after Darwin came up with his theory of natural selection, there remained many mysteries in the or ganic world that puzzled him. He noticed that many mechanisms seemed to fly in the face of survival. The elaborate plumage, large antlers, and other conspicuous features displayed by the males of many species seemed costly in terms of survival. He wondered how the brilliant plumage of peacocks could evolve, and become common, when it posed such an obvious threat to survival, acting as a blatant advertisement to predators. In response to anomalies of this sort, Darwin proposed a second evolutionary theory—the theory of sexual selection.
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