Are immigrants outbred and unrelated? Testing standard assumptions in a wild metapopulation

Lisa Dickel, Peter Arcese, Pirmin Nietlisbach, Lukas F. Keller, Henrik Jensen, Jane Reid

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4 Citations (Scopus)
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Immigration into small recipient populations is expected to alleviate inbreeding and increase genetic variation, and hence facilitate population persistence through genetic and/or evolutionary rescue. Such expectations depend on three standard assumptions: that immigrants are outbred, unrelated to existing natives at arrival, and unrelated to each other.
These assumptions are rarely explicitly verified, including in key field systems in evolutionary ecology. Yet, they could be violated due to non-random or repeated immigration from adjacent small populations. We combined molecular genetic marker data for 150-160 microsatellite loci with comprehensive pedigree data to test the three assumptions for a song sparrow (Melospiza melodia) population that is a model system for quantifying effects of inbreeding and immigration in the wild. Immigrants were less homozygous than existing natives on average, with mean homozygosity that closely resembled outbred natives. Immigrants can therefore be considered outbred on the focal population scale. Comparisons of homozygosity of real or hypothetical offspring of immigrant-native, native-native and immigrant-immigrant pairings implied that immigrants were typically unrelated to existing natives and to each other. Indeed, immigrants’ offspring would be even less homozygous than outbred individuals on the focal population scale. The three standard assumptions of population genetic and evolutionary theory were consequently largely validated. Yet, our analyses revealed some deviations that should be accounted for in future analyses of heterosis and inbreeding depression, implying that the three assumptions should be verified in other systems to probe patterns of non-random or repeated dispersal and facilitate precise and unbiased estimation of key evolutionary parameters.
Original languageEnglish
Pages (from-to)5674-5686
Number of pages13
JournalMolecular Ecology
Issue number22
Early online date13 Sept 2021
Publication statusPublished - Nov 2021

Bibliographical note

We thank the Tsawout and Tseycum First Nations Bands for allowing access to Mandarte, everyone who contributed to long-term data collection, Natural Sciences and Engineering Research Council, Canada (NSERC), the Swiss National Science Foundation (recently P400PB-180870), the Research Council of Norway (SFF-III, project 223257) and the Norwegian University of Science and Technology (NTNU) for funding.

Data Availability Statement

The data used in this study are available from the Dryad digital re-pository at (Dickel et al., 2021).

Additional supporting information may be found in the online ver-sion of the article at the publisher’s website.


  • conservation genetics
  • gene flow
  • homozygosity
  • migration
  • pedigree inbreeding
  • relatedness


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