Comparing different methods to estimate Ne.
How many individuals are there in a population? This seemingly simple question is one of the most daunting challenges in biology. Numerous methods have been developed to estimate population sizes, including transect counts, camera traps and genetic techniques. In the context of population genetics, researchers often refer to the effective population size (abbreviated as Ne). This concept can be interpreted as the number of breeding individuals in a population. Hence, the effective population size (Ne) is usually smaller than the actual or consensus population size (Nc).
The early population geneticists – Ronald A. Fisher and Sewall Wright – defined the effective population size as “the size of an idealized population that experiences the same rate of genetic drift and inbreeding as the population in focus.” The central role of genetic drift and inbreeding in this definition indicates that estimating Ne is easiest when working with small populations. As the population size increases, the effects of genetic drift and inbreeding become smaller, making it more difficult to accurately determine Ne. But that did not stop Krystyna Nadachowska‐Brzyska, Ludovic Dutoit and their colleagues from trying. They focused on a large island population of Collared Flycatchers (Ficedula albicollis) and recently published their findings in Molecular Ecology.
Genetic Drift
The researchers sequenced the genomes of 85 Collared Flycatchers (45 individuals from 1993 and 40 individuals from 2015). Next, they applied two sets of methods to estimate Ne. The first set of methods takes a temporal approach by analyzing the change of allele frequencies over time. The larger the changes in allele frequencies between generations, the smaller the effective population size. This effect is nicely illustrated by the simulation figures below. As the effective population size increases, you can see less fluctuations in allele frequencies over time.
Playing Darts
The second set of methods relies on linkage disequilibrium (LD). This population genetic concept refers to the non-random association between alleles at different genetic loci. In an infinite population, each genetic locus follows its own independent path. But as the population size decreases, some genetic loci might start to follow similar trajectories and become associated with each other. You could compare this situation to throwing darts (i.e. the alleles) at a special target where each square represents an individual. In a small population, some darts will likely end up in the same square and become associated with each other. In a bigger population, however, the chances of hitting the same square are much smaller and fewer darts will be associated with each other. Hence, the degree of linkage disequilibrium can provide insights into the effective population size.
The Best Method?
In the end, the researchers used three temporal methods and one LD-based method (see table below for the details). All three temporal methods gave similar results, suggesting an effective population size between 4000 and 7000 individuals. The LD-based method, however, was less reliable. It provided much higher estimates (between 20,000 and 35,000 individuals) with a large level of uncertainty (even extending into infinity). These findings show that it is feasible to estimate large effective population sizes (more than 1000 individuals) with genomic data, although LD-based methods should be used with caution.
References
Nadachowska‐Brzyska, K., Dutoit, L., Smeds, L., Kardos, M., Gustafsson, L., & Ellegren, H. (2021). Genomic inference of contemporary effective population size in a large island population of collared flycatchers (Ficedula albicollis). Molecular Ecology, 30(16), 3965-3973.
Featured image: Collared Flycatcher (Ficedula albicollis) © Andrej Chudy | Wikimedia Commons