Darwin's Finches
Darwin observed that beak shape varies among finch species. He postulated that the beak of an ancestral species had adapted over time to equip the finches to acquire different food sources. This illustration shows the beak shapes for four species of ground finch: 1. Geospiza magnirostris (the large ground finch), 2. G. fortis (the medium ground finch), 3. G. parvula (the small tree finch), and 4. Certhidea olivacea (the green-warbler finch).
Source
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"OpenStax College, Discovering How Populations Change. December 6, 2013."
http://cnx.org/content/m45487/latest/
OpenStax CNX
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