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The process of evolution is generally described as having two parts: random mutation followed by natural selection. But mutations aren't always truly random, either in place or effect. Some of the types of challenges that confront organisms recur often enough to be predictable. Much like variations in anatomical structures, like the shapes of beaks, variations in the probability of particular mutations may affect the survival of a strain of organisms and therefore should be subject to natural selection. Various biochemical quirks in DNA sequence can abet the rise of predictable genetic changes. For example, repeated sequences in genomes can expand or shrink in length, and such variations in length can affect the activity levels of nearby genes or their regulation in response to environmental cues. One can think of genomes encoding one sequence explicitly and a range of other sequences implicitly. This assured genetic diversity can protect the descendants of an individual against pathogens and an erratic environment.
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