Genomics on the Galapagos: Exploring the genetic diversity of Darwin’s Finches and their relatives

Can we use the knowledge on genomic diversity to inform conservation?

The physicist Richard Feynman said: “It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.” This simple statement captures the essence of science: you formulate a hypothesis and you test it with observations or experiments. And if your hypothesis is not explained by the data, it is wrong. Time to move on to the next guess (Of course, things are more complicated in practise, but you get the main idea). Let’s try this approach with some population genomic data:

  1. Genomic diversity will increase with population size.
  2. Genomic diversity will decrease with body size.

The first hypothesis is intuitive: a bigger population has more individuals which probably differ genetically, leading to more genomic diversity. The second hypothesis requires a bit more explanation. Big animals tend to have longer generation times, less offspring and smaller population sizes compared to small animals. Together these factors culminate in lower genomic diversity. A recent study in the journal Molecular Ecology tested these hypotheses using island populations of several bird species. What did they find?

Lesser_Antillean_Bullfinch_RWD6

The Lesser Antillean Bullfinch © Dick Daniels | Carolina Birds

 

17 species

To test these hypotheses, Anna Brüniche‐Olsen and her colleagues used published data from all 15 species of Darwin’s Finches that reside on the Galapagos Islands (and nearby Coco Island) and two Tanager species from Barbados (the Black‐faced Grassquit Tiaris bicolor and the Lesser Antillean Bullfinch Loxigilla noctis). For each island population, they estimated genetic diversity based on heterozygosity. Next, they correlated this estimate with island area and body size.

The results were in line with the hypotheses mentioned above. The researchers write that “We find [a] significant positive correlation between island size and genomic diversity, [and] a significant negative correlation between body size and genomic diversity.”

Green_Warbler_Finch

Green Warbler Finch © Paul McFarling | Darwin Foundation

 

Red List Status

These findings suggest that we can use genomic diversity to assess the conservation status of small populations. So, the researchers compared the estimates of heterozygosity with conservation status on the IUCN Red List (threatened vs. unthreatened). Surprisingly, there was no effect of Red List status on heterozygosity. What an anticlimax!

But wait, there is more. They also tested another measure of genetic diversity: Watterson’s θ. In this case, there was a significant effect: threatened species had significantly lower estimates of this measure compared to unthreatened species. This result can probably be explained by the way Watterson’s θ is calculated. It is the product of the effective population size and the neutral mutation rate. Taking into account effective population size provides a rough (but reliable) estimate of actual population size. So, this statistic might be a useful indicator for species at significant risk of decline. 

wattersons.jpg

A Vegetarian Finch looking away from the significant difference in Watterson’s θ between non-threatened and threatened species. © Mike’s Birds | Flickr

 

References

Brüniche‐Olsen, A., Kellner, K. F., & DeWoody, J. A. (2019). Island area, body size and demographic history shape genomic diversity in Darwin’s finches and related tanagers. Molecular Ecology, 28(22), 4914-4925.

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