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Finch character displacement

Larry H. Bernstein, MD, FCAP

LPBI

 

Genetic Study of Darwin’s Finches Catches Evolution in Action

http://www.genengnews.com/gen-news-highlights/genetic-study-of-darwin-s-finches-catches-evolution-in-action/81252647/

http://www.genengnews.com/Media/images/GENHighlight/Apr22_2016_PeterRGrant_MediumGroundFinch2037196235.jpg

The medium ground finch (Geospiza fortis), shown here, diverged in beak size from the large ground finch (Geospiza magnirostris) on Daphne Major Island, Galápagos following a severe drought. Genomic screening of the genomes of medium ground finches revealed that a particular gene, HMGA2, played a large role in the rapid evolution of a smaller overall beak size in the medium ground finch. [Peter R. Grant]

An evolutionary phenomenon first described by Charles Darwin has the support of new and unusually strong supporting evidence. The phenomenon, called character displacement, may occur when species compete for the same food source. The species may evolve different body shapes, such as different beak sizes in the case of finches, diverging from each other until they relieve competitive stress.

Darwin developed the idea of character displacement after observing the finches of the Galápagos Islands. He proposed that changes in the size and form of the beak have enabled different species to utilize different food resources, such as insects, seeds, and nectar from cactus flowers, as well as blood from seabirds.

In a study of character displacement among Darwin’s finches, researchers from Uppsala University and Princeton University have now identified a gene that explains variation in beak size within and among species. The gene contributed to a rapid shift in beak size of the medium ground finch following a severe drought.

The details of the study appeared April 22 in the journal Science, in an article entitled, “A Beak Size Locus in Darwin’s Finches Facilitated Character Displacement during a Drought.” The article describes how the researchers alighted on a gene called HMGA2 after screening the genomes of medium ground finches that survived or died during a drought that occurred between 2004 and 2005. The researchers found that the HMGA2 gene comes in two forms: one is common in finches with small beaks, whereas the other is common in finches with large beaks. The proportion of the two forms in the birds’ genome changed as a result of the better survival of birds with small beaks.

“We used genomic analysis to investigate the genetic basis of a documented character displacement event in Darwin’s finches on Daphne Major in the Galápagos Islands,” wrote the authors. “We discovered a genomic region containing the HMGA2 gene that varies systematically among Darwin’s finch species with different beak sizes. Two haplotypes that diverged early in the radiation were involved in the character displacement event.”

In a previous study from the same team, the ALX1 gene was revealed to control beak shape (pointed or blunt). The HMGA gene that figures in the current study was previously associated with variation in body size in dogs and horses, and it is one of the genes that show the most consistent association with variation in stature in humans, a trait that is affected by hundreds of genes. HMGA2 has also a role in cancer biology as it affects the epithelial–mesenchymal transition (EMT) that is important for metastasis and cancer progression.

“Our data show that beak morphology is affected by many genes, as is the case for most biological traits,” said Sangeet Lamichhaney, the current study’s first author and a doctoral student in the laboratory of Leif Andersson, one of the study’s senior authors and a genomics professor at Uppsala. “However, we are convinced that we now have identified the two loci with the largest individual effects that have shaped the evolution of beak morphology among the Darwin’s finches.”

Andersson collaborated with Princeton researchers Peter Grant, the Class of 1877 Professor of Zoology, Emeritus, and B. Rosemary Grant, a senior research biologist, emeritus, in ecology and evolutionary biology.

“It was an exceptionally strong natural-selection event,” noted Peter Grant, who pointed out that that because Daphne Major is in an entirely natural state, the occurrence was completely unaffected by humans. “Now we have demonstrated that HMGA2 played a critical role in this evolutionary shift and that the natural selection acting on this gene during the drought is one of the highest yet recorded in nature.”

“This research tells us that a complex trait such as beak size can evolve significantly in a short time when the environment is stressful,” Rosemary Grant added. “We know that bacteria can evolve very quickly in the lab, but it is quite unusual to find a strong evolutionary change in a short time in a vertebrate animal.”

 

Linked loci and Galapagos finch size

Observations of parallel evolution in the finches of the Galapagos, including body and beak size, contributed to Darwin’s theories. Lamichhaney et al. carried out whole-genome sequencing of 60 Darwin’s finches. These included small, medium, and large ground finches as well as small, medium, and large tree finches. A genomic region containing the HMGA2 gene correlated strongly with beak size across different species. This locus appears to have played a role in beak diversification throughout the radiation of Darwin’s finches.

Science, this issue p. 470

Ecological character displacement is a process of morphological divergence that reduces competition for limited resources. We used genomic analysis to investigate the genetic basis of a documented character displacement event in Darwin’s finches on Daphne Major in the Galápagos Islands: The medium ground finch diverged from its competitor, the large ground finch, during a severe drought. We discovered a genomic region containing the HMGA2 gene that varies systematically among Darwin’s finch species with different beak sizes. Two haplotypes that diverged early in the radiation were involved in the character displacement event: Genotypes associated with large beak size were at a strong selective disadvantage in medium ground finches (selection coefficient s = 0.59). Thus, a major locus has apparently facilitated a rapid ecological diversification in the adaptive radiation of Darwin’s finches.

 

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