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In 1977, the amount of rainfall plummeted to only 20 mm. The drought was a sudden change in the environment of Daphne Major that the medium ground finches lived on. Due to the lack of rainfall, the portulaca plant that produced small, soft seeds the finches could easily access became obsolete and only the tribulus plants that didn't require as much water survived. The finches with small beaks died off since they didn't have a source of food, leaving only a fraction of finches behind. The finches left behind had larger beaks that could crack into the large, hard tribulus seed. The finches were able to reproduce and their offspring had larger beaks also. This shows that the larger beaked finches were know the most fit to live in the environment. Survival of the fittest, or natural selection, is proven to be what caused caused the length of the medium ground finch beak length to increase.

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"Finch Characteristics in 1976"
Data Sheet
"Finch Characteristics in 1978"
Data Sheet

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"Rainfall By Year"
Data sheet

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"Seed Type Abundance" data sheet

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In 1976 the rainfall amount was 130 mm, in 1977 it was 20 mm, and in 1978 it was 540 mm.

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The amount of portulaca plants were at zero by the end of 1977, and the tribulus plants were at six per square mile.

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The beak depth increased by 0.9 from 1976 to 1978, and beak length increased by 0.9 from 1976 to 1978.

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Natural Selection caused the beak of the medium ground finch population on Daphne Major increase in size from 1976 to 1978.