Interview with the author: Do estimates of contemporary effective population size tell us what we want to know?

For our first interview that goes behind the scenes of one of recent paper, it is our pleasure to introduce Professor Nils Ryman from Stockholm University. Read the associated paper here: https://onlinelibrary.wiley.com/doi/full/10.1111/mec.15027

What led to your interest in this topic / what was the motivation for this study? 
I have since the beginning of my career had a strong interest in conservation genetics, and because the concept of effective population size (Ne) is a key parameter in this field it has long been in my interest to understand this complex and often unintuitive concept. Several years ago my colleagues Ola Hössjer (professor of mathematical statistics) and Linda Laikre (professor of population and conservation genetics) formed an interdisciplinary team aiming at improving the mathematical tools for understanding various types of effective population size in populations under migration, and then applying such tools to practical conservation and molecular ecology questions. This is one of the studies applying the tools to understand what we expect to estimate when assessing Ne for non-isolated populations.

Brown Trout (photo Anastasia Andersson)

What difficulties did you run into along the way? 
Effective population size (Ne) is a difficult concept, and my experience is that you always run into difficulties when working with it. Also, it is difficult to explain, so presenting this study in text that can be understood by molecular ecologists was a challenge. Another difficulty, along the rather long way leading up to the theoretical framework that provides the basis for this study, has been to establish common grounds in our interdisciplinary team. Although we population geneticists and molecular ecologists might think that we are rather strong in theory, we are very far away from the world of a mathematician.

What is the biggest or most surprising finding from this study? 
That the different effective population sizes that quantify different types of genetic change (inbreeding, allele frequency variance, additive genetic variance and linkage disequilibrium in this case) behave so differently in populations that are subject to even small rates of immigration.

Moving forward, what are the next steps for this research? 
We have several planned studies aimed at further improving our understanding of different types of effective sizes in metapopulations. In practical conservation, guidelines and recommendations for such systems are largely lacking. Understanding the effects of subpopulation extinction on metapopulation effective size is one topic. Also, an obvious question is how do we measure inbreeding effective size in populations that are not isolated but parts of metapopulations? This is often the rate of genetic change that we want to quantify, but when we use the most frequently available software we do not measure this rate at all in populations under migration. Further, as we discuss in the paper, there are also good reasons for focusing more on inbreeding itself, rather than the inbreeding effective size.

What would your message be for students about to start their first research projects in this topic? 
Read the classic, theoretical population genetics literature and make sure that you have a good grip on assumptions and definitions! The work by e.g. Crow & Kimura, Wright, Nei, and others are still essential to know and understand. I am actually worried that the theoretical part of population genetics is forgotten in the face of the genomic explosion. I think that would be most unfortunate and I agree with my close friend and colleague Fred Allendorf who is “concerned that current training focuses too much on techniques and too little on understanding the conceptual basis needed to interpret these data” (Molecular Ecology (2017) 26, 420-430).

What have you learned about science over the course of this project? 
When working with Ne, always be prepared for two steps forward and then at least one step back.

Describe the significance of this research for the general scientific community in one sentence.
Currently applied methods for assessing rates of inbreeding and loss of additive genetic variation do not tell us what we want to know – in spatially structured populations they do not measure the actual rates.

Metapopulations (Photo Nihls Ryman)

Describe the significance of this research for your scientific community in one sentence.
Different types of Ne – here the inbreeding, variance, additive genetic variance and linkage disequilibrium effective sizes – behave very differently in populations under migration as compared to completely isolated ones.

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