Methods of Speciation - 9 Darwin's model for evolution, which stresses the process of natural selection gradually changing populations, is one way for evolution to proceed, and works well for the changes we see within many species through time. The gradualist model of evolution can be difficult to document in nature (although not in laboratory situations), since we observe the current state, not the past or the future. There is also evidence that suggests that gradualism is not always the way that populations change. Many fossils, for example, "suddenly" appear in sediments, and many organisms remain unchanged for thousands or even millions of years. Even Darwin proposed that changes probably occur in relatively short periods of times, perhaps alternating with longer, more stable periods. Genetically, the method for such evolution is a series of incremental mutations, each of which can change the organism bit by bit until the accumulation is significant. As an alternative, some biologists propose that change occurs through non-
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