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The Roles of Migration, Drift and Demographic Stochasticity bioRxiv preprint doi: https://doi.org/10.1101/2021.08.05.455207; this version posted August 6, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 1 Genetic load and extinction in peripheral populations: the roles 2 of migration, drift and demographic stochasticity 1;2; 1 1 3 Himani Sachdeva x, Oluwafunmilola Olusanya , Nick Barton 1Institute of Science and Technology Austria, Am Campus 1, Klosterneuburg 3400, Austria. 2Department of Mathematics, University of Vienna, Vienna 1090, Austria. Corresponding author x 4 Abstract 5 We analyse how migration from a large mainland influences genetic load and population 6 numbers on an island, in a scenario where fitness-affecting variants are unconditionally 7 deleterious, and where numbers declines with increasing load. Our analysis shows that 8 migration can have qualitatively different effects, depending on the total mutation target and 9 fitness effects of deleterious variants. In particular, we find that populations exhibit a genetic 10 Allee effect across a wide range of parameter combinations, when variants are partially 11 recessive, cycling between low-load (large-population) and high-load (sink) states. Migration 12 further reduces load in the sink state (by increasing heterozygosity) but increases load in 13 the large-population state (by hindering purging). We identify various critical parameter 14 thresholds at which one or other stable state collapses, and discuss how these thresholds are 15 influenced by the genetic vs. demographic effects of migration. Our analysis is based on 16 a `semi-deterministic' analysis, which accounts for genetic drift but neglects demographic 17 stochasticity. We also compare against simulations which account for both demographic 18 stochasticity and drift. Our results clarify the importance of gene flow as a key determinant 19 of extinction risk in peripheral populations, even in the absence of ecological gradients. 20 Keywords: deleterious variants, genetic load, extinction, migration, demographic stochas- 21 ticity, semi-deterministic approximation 22 Introduction 23 Most outcrossing populations carry a substantial masked mutation load due to recessive variants, 24 which can contribute significantly to inbreeding depression in peripheral isolates or after a 25 bottleneck. The extent to which the accumulation of deleterious mutations due to small numbers 26 exacerbates extinction risk in isolated populations has been a subject of long-standing interest 27 [1, 2, 3, 4]. Theory predicts that moderately deleterious mutations contribute the most to genetic 28 load and extinction in small populations [2, 5]; however, the prevalence of such deleterious 29 variants of mild or moderate effect and their dominance values remain poorly characterised, 30 except for a few model organisms [6, 7]. 31 The relative risks posed by mutation accumulation and demographic stochasticity to a popu- 32 lation depend crucially on its size, with some theory suggesting that these may be comparable 33 for populations in their thousands [1]. Moreover, environmental stochasticity | catastrophic 34 events, as well as fluctuations in growth rates and carrying capacities, may dramatically lower 35 extinction times [8]. Both demographic and environmental fluctuations, in turn, reduce the 36 effective size of a population, making it more prone to fix deleterious alleles; the consequent 1 bioRxiv preprint doi: https://doi.org/10.1101/2021.08.05.455207; this version posted August 6, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 37 reduction in fitness further depresses size, pushing populations into an `extinction vortex', which 38 is often characterised by a complex interaction between the effects of genetic drift, demographic 39 stochasticity and environmental fluctuations [9]. 40 Peripheral populations at the edges of species' ranges may be subject to dispersal from the 41 core to an extent which varies over space and time. Moreover, in populations threatened by 42 habitat loss, ranges may be fragmented and individual sub-populations connected to each other 43 via low and possibly declining levels of migration. This makes it necessary to ask: under what 44 condition, can such extinction vortices be arrested by migration, and what are the genetic and 45 demographic underpinnings of this effect, when it occurs? 46 Migration boosts numbers, mitigating extinction risk due to demographic and environmental 47 stochasticity, or at the very least, allows populations to regenerate after chance extinction. The 48 demographic consequences of migration are especially important in fragmented populations with 49 many small patches [10]: above a critical level of migration, the population may survive as a 50 whole over long timescales even if individual patches frequently go extinct [11, 12]. 51 Migration also influences extinction risk via shifts in the frequencies of fitness-affecting variants: 52 the resultant changes in fitness may decrease or increase population size, thus further boosting or 53 depressing the relative contribution of migration to allele frequency changes within a population, 54 setting in motion a positive feedback which may culminate in extinction (when gene flow is 55 largely maladaptive; e.g., [13, 14]) or population rescue (e.g., if gene flow supplies variation 56 necessary for adaptation to local conditions, or reduces inbreeding load; e.g., [5, 15, 16]). 57 The maladaptive consequences of migration have largely been explored in the context of gene 58 flow between subdivided or extended populations under spatially varying environmental condi- 59 tions: here, gene flow typically hinders local adaptation, especially in peripheral populations, 60 leading to `swamping' and extinction [17]. However, the consequences of gene flow for fitness, 61 and consequently survival, are not always intuitive when the fitness effects of genetic variants 62 are uniformly deleterious (or beneficial) across populations. For example, while gene flow may 63 alleviate inbreeding load by preventing the fixation of deleterious alleles, especially in very small 64 populations, it can also render selection against recessive mutations less effective by increas- 65 ing heterozygosity. A striking consequence is that under a range of conditions, the fitness of 66 metapopulations is maximised at intermediate levels of migration [18]; more generally, fitness 67 due to recessive variants changes non-monotonically with the degree of population structure, 68 for other kinds of structure as well, e.g., due to selfing [19]. 69 A key consideration is whether or not gene flow is symmetric, i.e., whether some sub-populations 70 are merely influenced by the inflow of genes from the rest of the habitat or if all sub-populations 71 influence the genetic composition of the population as a whole [20]. Asymmetric dispersal is 72 common at the geographic peripheries of species' ranges or on islands. Moreover, populations 73 occupying small patches within a larger metapopulation with a wide distribution of patch sizes, 74 or sub-populations with lower-than-average fitness (and consequently, atypically low numbers) 75 may also experience predominantly asymmetric inflow of genes. Asymmetric gene flow allows for 76 allele frequency differences across the range of a population even in the absence of environmental 77 heterogeneity, e.g., when population sizes (and hence the efficacy of selection relative to drift) 78 vary across the habitat. This, in turn, may generate heterosis or outbreeding depression across 79 multiple loci, when individuals from different regions hybridize. 80 The consequences of asymmetric gene flow are typically simpler to analyse from a conceptual 81 point of view, as they allow us to consider a focal population, while taking the state of the rest 82 of the population as ‘fixed’. Such analyses are key to understanding more general scenarios 2 bioRxiv preprint doi: https://doi.org/10.1101/2021.08.05.455207; this version posted August 6, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 83 where genotype frequencies and population sizes across different regions co-evolve. 84 Here, we analyse the eco-evolutionary dynamics of a single island population subject to migra- 85 tion from a larger mainland population in a scenario where selection across the two populations 86 is uniform, i.e., where fitness is affected by a large number of variants that are unconditionally 87 deleterious. We ask: under what conditions can migration from the mainland alleviate in- 88 breeding load, thus preventing `mutational meltdown' and extinction of the island population? 89 Further, how are the effects of migration mediated by the genetic architecture of load, i.e., by 90 the genome-wide mutation target and fitness effects of deleterious variants? A key focus is to 91 understand the coupled evolution of allele frequencies (across multiple loci) and population size: 92 to this end, we consider an explicit model of population growth with logistic regulation, where 93 growth is reduced
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