Volatility in the effective size of a freshwater gastropod population.

Robert T Dillon

doi:10.1002/ece3.3912

Abstract

Despite the utility of gastropod models for the study of evolutionary processes of great generality and importance, their effective population size has rarely been estimated in the field. Here, we report allele frequency variance at three allozyme‐encoding loci monitored over 7 years in a population of the invasive freshwater pulmonate snail Physa acuta (Draparnaud 1805), estimating effective population size with both single‐sample and two‐sample approaches. Estimated N e declined from effectively infinite in 2009 to approximately 40–50 in 2012 and then rose back to infinity in 2015, corresponding to a striking fluctuation in the apparent census size of the population. Such volatility in N e may reflect cryptic population subdivision.

Highlights

Since the introduction of the concept by Wright (1931), effective population size (Ne) has been adopted as a parameter in scores of evolutionary models, adaptive and neutral alike (Crow, 2010)
Two categories of methods to estimate Ne from field data were developed in the early 1980s, a single-s ample approach based on disequilibrium between alleles at unlinked loci and a two-sample approach based on variance in allelic frequencies between generations (Caballero, 1994)
The dramatic fluctuation in Ne seemed to correspond to a fluctuation in apparent census size noticeable in the field, from thousands in the spring of 2009 to a very few in 2012, and back up to thousands

Summary

| INTRODUCTION

Since the introduction of the concept by Wright (1931), effective population size (Ne) has been adopted as a parameter in scores of evolutionary models, adaptive and neutral alike (Crow, 2010). The earliest field estimates were those of Murray (1964) and Greenwood (1974), who applied simple single-sample approaches to shell color polymorphism in an English population of the important land snail model Cepaea nemoralis. Another thirty years would elapse before estimates of effective population size were offered for other land snail populations, those of Arnaud and Laval (2004) using microsatellite markers and a two-sample method, and Ursenbacher, Alvarez, Armbruster, and Baur (2010) using a one-sample approach. The effects of migration on Ne have been studied by Gilbert and Whitlock (2015)

| MATERIALS AND METHODS

| DISCUSSION