Why Do Some Finches Have Large Beaks While Others Have Small Ones?

Cameroonian finches have always had either a large beak size, or smaller beaks.  This had always been thought to be due to a difference in how hard the seeds they eat, much like the theory of Darwin's finches.  However, birder and biologist Tom Smith, investigated this further by establishing a breeding colony of these finches.  Based on the results, the trait seemed to be due to Mendelian genetics, and the larger beak was the dominant trait, with a 3:1 ratio of large to small beaks.  This was the stepping stone to figuring out which genes affected this trait, and with the help of Bridgett vonHoldt, Princeton biologist, the question was solved.  She found a stretch of DNA with 300,000 base pairs that always seemed to vary between the large and small-beaked finches, with the gene IFG-1 in the center.  This gene is a growth-factor gene. vonHoldt said that this is a gene she has studied in canines as well and that if it is changed how it is expressed, a normal-sized dog could be changed into a dwarf-sized dog.  This being said, not only can he gene affect the whole animal, but a specific trait as well, which is present in the case of the finches and their beak size.  This all depends on where the gene is located on the genome and where it gets expressed, says vonHoldt.
https://www.sciencedaily.com/releases/2018/11/181119064118.htm
https://www.princeton.edu/news/2018/11/19/princeton-geneticist-solves-long-standing-finch-beak-mystery

Evolution of Galapagos Finches

After a drought on Daphne Major Island biologists began to notice a change in which some finches were striving more than others. The lack of rain resulted in a food shortage causing the smaller beaked medium ground finches were living longer than finches with longer brakes. This change sparked Peter and Rosemary Grant's interest to study the evolution of the finches on the island and how they were effected by natural selection.

According to the Grant's, a gene called HMGA2 is the principal factor in beak size of the birds.  In humans this specific gene influences body size and height. Birds with the gene coding for a shorter beak were more likely to survive during the drought. A study done in 2015 showed that the birds have  been evolving to fit their environment since 2 million years ago when their common ancestor arrived on the island. Some of the traits that were studied were a result of natural selection and others were caused from hybridization.

This research is important to the understanding of natural selection. Eventually this information can be used to amply to human studies. By studying animals researchers can get a better understanding of what genes contribute to which traits.