Cross-mating finches exchanged genes and beak shape

The transfer of genes from one population to another, or gene flow, was once thought to be an unlikely result of crossbreeding. However,  recent studies have shown that this type of evolution is much more common than what was expected. 

A study led by Uppsala University is providing the latest evidence of this phenomenon. According to the researchers, gene flow between two distinct species of Darwin’s finches has changed their beak morphology.

Study co-authors Rosemary and Peter Grant of Princeton University observed populations of Darwin’s finches on the small island of Daphne Major for 40 consecutive years. Two distinct species, the common cactus finch and the medium ground finch, occasionally cross-mated. 

The cactus finch has a pointed beak that is specialized to feed on cactus, while the medium ground finch has a blunt beak for feeding on seeds.

“Over the years, we observed occasional hybridization between these two species and noticed a convergence in beak shape. In particular, the beak of the common cactus finch became blunter and more similar to the beak of the medium ground finch,” said the Grants. “We wondered whether this evolutionary change could be explained by gene flow between the two species.”

“We have now addressed this question by sequencing groups of the two species from different time periods and with different beak morphology,” said study co-author Professor Sangeet Lamichhaney. “We provide evidence of a substantial gene flow, in particular from the medium ground finch to the common cactus finch.”

Fan Han of Uppsala University analyzed the data as part of her PhD thesis. Han said the team was surprised to find that the observed gene flow was substantial on most autosomal chromosomes but negligible on the Z chromosome, one of the sex chromosomes. 

“In birds, the sex chromosomes are ZZ in males and ZW in females, in contrast to mammals where males are XY and females are XX,” explained Han.

“This interesting result is in fact in excellent agreement with our field observation from the Galápagos,” said the Grants. “We noticed that most of the hybrids had a common cactus finch father and a medium ground finch mother.” 

“Furthermore, the hybrid females successfully bred with common cactus finch males and thereby transferred genes from the medium ground finch to the common cactus finch population. In contrast, male hybrids were smaller than common cactus finch males and could not compete successfully for high-quality territories and mates.”

Darwin’s finches are imprinted on the song of their fathers. The sons sing a song similar to their father’s song and daughters prefer to mate with males that sing like their fathers. 

In addition, hybrid females receive Z chromosomes from their cactus finch father and W chromosomes from their ground finch mother. The fact that parents of the hybrid contribute equally explains why genes on the Z chromosome cannot flow from the medium ground finch to the cactus finch through the hybrid females while other genes can.

“Our data show that the fitness of the hybrids between the two species is highly dependent on environmental conditions which affect food abundance,” explained study co-author Leif Andersson. “That is, to what extent hybrids, with their combination of gene variants from both species, can successfully compete for food and territory. Therefore, the long-term outcome of the ongoing hybridization between the two species will depend on environmental factors as well as competition.”

“One scenario is that the two species will merge into a single species combining gene variants from the two species, but perhaps a more likely scenario is that they will continue to behave as two species and either continue to exchange genes occasionally or develop reproductive isolation if the hybrids at some point show reduced fitness compared with purebred progeny. The study contributes to our understanding of how biodiversity evolves.”

The study is published in the journal Nature, Ecology, and Evolution. 

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By Chrissy Sexton, Earth.com Staff Writer