The Holy Trinity of Ground Finches

 It is a common though incorrect belief that Darwin's Finches were Charles Darwin's inspiration for his 'Origin of the Species'.  It was actually the Mockingbirds of the Galapagos that begin his train of thought that led him to his theory of natural selection.  Nonetheless Darwin's Finches pose a great identification challenge to the layperson and expert.  Species are very similar like the three below and hybrids occur. Some species do not co-exist on the same island. The species below are the 3 types of Ground Finch that occur in the Galapagos.  They are named not for their body size but for the size of their bill.

 The fellow above is a Large Ground Finch. They are a rather uncommon sight. The bill is massive and great for cracking hard seed cases.  The bill exerts a huge amount of force. This male was poking around a beach on Genovesa.

The Medium Ground Finch above is abundant.  The size of its bill is very variable but not as dainty and pointed as the Small Ground Finch below. Unlike Large Ground Finch the length of the upper mandible is greater than the dept of the bill.  

  

The Small Ground Finch is very common and often feeds on the skin parasites of Galapagos tortoises and land and marine iguanas.  Identification between Small and Medium Ground Finches can be difficult as on some islands beak size is increasing with subsequent generations so Small Ground Finches are becoming Medium Ground Finches. This change is observable in DNA, thus natural selection can be seen both at the molecular level and in the beak changes of these birds.  So nay sayers who say that it is impossible to directly observe natural selection are wrong.