Galapagos Finch Evolution – Dan Lewitt – HHMI (2013)

The Galapagos is home to many iconic species. Each are unique on their own and form a case study in evolutionary biology. Darwin’s finches and mocking birds with their striking beak morphology provided the first clues to Darwin in formulating the mechanism of evolution by natural selection. The Galapagos was the origin of the Origin of the Species.

Finches in the island of Daphne Major have been studied for more than 40 years by Princeton University scientists Peter and Rosemary Grant. Throughout their long-term studies they have encountered some fascinating observations. For instance, in 1975, a cactus finch got a cactus spine stuck in its throat. Instead of singing the normal cha cha cha song, it sang jar jar jar. The sons learned and copied their father’s song. After this comical accident, their great great grandsons were still singing the mutated song. They have also observed arrival of a different species of a finch from a near by island and since then has induced formation of a new species.

Just like Darwin’s finches, many bird species became models of evolution for speciation such as the birds of paradise of Malay Archipelago or Juncos of Central and North America. Variation of the same theme took place in other oceanic volcanic islands including the I’iwi honeycreeper bird of Hawaii. Galapagos mocking birds also fascinated Darwin:

An extensive genetic study conducted on Darwin’s finches living on the island of Daphne Major, has revealed a complicated family tree with a surprising level of interbreeding between species. Perhaps the most striking finding of the study uncovered the genetic basis of the changes in beak size as the Humboldt Current shifts due to El NiƱo Southern Oscillation and the rainfall pattern change drastically. Beak morphology is controlled by a gene encoding a transcription factor called ALX1. Transcription factors serve as master switches. They control activities of multiple genes by binding on the control regions on DNA.

The ALX1 gene provides instructions for making a protein that is a member of the homeobox protein family. Homeobox proteins direct the formation of body structures during early embryonic development. The ALX1 protein is necessary for normal development of the head and face, particularly the formation of the eyes, nose, and mouth. The ALX1 protein controls the activity of genes that regulate cell growth and division (proliferation) and movement (migration), ensuring that cells grow and stop growing at specific times and that they are positioned correctly during development.

Researchers discovered two variants of ALX1. One is associated with pointed beaks seen in the ancestral form. The other is a variant associated with the blunt beaks observed in derived form. The blunt beak version provides a competitive advantage for cracking tough seeds that become abundant under dry conditions. Conversely, pointed beak becomes advantageous during wet times. ALX1 does not control beak size by itself but controls many genes playing role in beak developments in embryo stage.

In Humans ALX1 gene mutations impair its ability to bind to DNA and regulate gene function. As a result, the proliferation and migration of cells during development is not controlled, which can lead to small or missing eyes, openings (clefts) in the nose or mouth, and other severe facial malformations.

Islands provide a convenient setting for studying biological systems in an ecological setting. They are simple enough to understand and at the same time, complex enough to make generalizations about the real world.

Availability of genetic methods for non-model species provided new ways to study organisms in their natural setting. Mind blowing advances in DNA sequencing technology now allows whole genome sequencing of many organisms including bird genomes.

 

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