Beaks

... survived to reproduce which in turn shifted the population from mostly white to mostly black. With the finch example, those birds with smaller beaks survived to reproduce which in turn shifted the population to a ditribution of beak sizes which were smaller. Over many generations, this can ...

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... selective pressure for reduced beak size in the finches living there who depended on those seeds. As a result, the finches evolved to have smaller beaks. Keep in mind, that beak size is represented by a range of sizes. The result of evolution was that that range was shifted to the smaller end. ...

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Say that all the big beaks live, and all the small beaks died out. You could only have variation now from the big beak birds which will evolve in a way that may bring variation, but the spectrum will now be limited. Variation now only ...

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OK so if there is a population on one side of the island with diversifying selection acting with just big beaks and small beaks, intermediates dieing out while on the other side of the island, there is a population with big beaks, small beaks and intermediates and no diversifiing ...

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... the two extreme genetic pools available. If you do not have the two extremes, then the spectrum of variation may die out. Say that all the big beaks live, and all the small beaks died out. You could only have variation now from the big beak birds which will evolve in a way that may bring variation, ...

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