Genetic variation in natural populations: a modeller's perspective

Thanks to advances in genome sequencing, empirical patterns of within- and between-species genetic variation are readily available. By studying these patterns much has been learned about the evolutionary histories of species. But the causes and consequences of different evolutionary histories are still difficult to tell apart. To this end, comparative analyses of genetic variation under different models are required. This thesis analyses genetic variation under specific models that are relevant for a number of biological species. Firstly, this thesis discusses a method for inferring the population-size history of the population in question using simulated, as well as empirically observed frequency spectra of mutations. The method performs well when applied to simulated data, provided that a large number of mutations is sampled. However the estimation based on empirical data is biased. Secondly, the thesis studies a mainland-island colonisation model. The model allows for different levels of multiple paternity in the population. Multiple paternity promotes genetic variation. This effect is much larger during colonisation than on the long run. Therefore, multiple paternity may facilitate the establishment of species in new areas. Thirdly, this thesis analyses a colonisation model for species that reproduce both sexually and asexually, and have limited dispersal capabilities. Due to limited dispersal capabilities, sexual reproduction may be hindered locally, especially during colonisation. Unless the individuals are highly sexual, a few clones establish the front of the colonisation forming wide clonal colonies. Finally, this thesis analyses a joint effect of migration, selection and random genetic drift during adaptation in subpopulations subject to different environments. When divergent adaptation is driven by mutations, the frequency at which mutations appear, as well as how strongly they are selected for are the decisive parameters for whether or not subpopulations can adapt to their respective environments despite migration and drift. This remains to be analysed further

Reciprocal transplants support a plasticity-first scenario during colonisation of a large hyposaline basin by a marine macro alga

Abstract Background Establishing populations in ecologically marginal habitats may require substantial phenotypic changes that come about through phenotypic plasticity, local adaptation, or both. West-Eberhard’s “plasticity-first” model suggests that plasticity allows for rapid colonisation of a new environment, followed by directional selection that develops local adaptation. Two predictions from this model are that (i) individuals of the original population have high enough plasticity to survive and reproduce in the marginal environment, and (ii) individuals of the marginal population show evidence of local adaptation. Individuals of the macroalga Fucus vesiculosus from the North Sea colonised the hyposaline (≥2–3‰) Baltic Sea less than 8000 years ago. The colonisation involved a switch from fully sexual to facultative asexual recruitment with release of adventitious branches that grow rhizoids and attach to the substratum. To test the predictions from the plasticity-first model we reciprocally transplanted F. vesiculosus from the original population (ambient salinity 24‰) and from the marginal population inside the Baltic Sea (ambient salinity 4‰). We also transplanted individuals of the Baltic endemic sister species F. radicans from 4 to 24‰. We assessed the degree of plasticity and local adaptation in growth and reproductive traits after 6 months by comparing the performance of individuals in 4 and 24‰. Results Branches of all individuals survived the 6 months period in both salinities, but grew better in their native salinity. Baltic Sea individuals more frequently developed asexual traits while North Sea individuals initiated formation of receptacles for sexual reproduction. Conclusions Marine individuals of F. vesiculosus are highly plastic with respect to salinity and North Sea populations can survive the extreme hyposaline conditions of the Baltic Sea without selective mortality. Plasticity alone would thus allow for an initial establishment of this species inside the postglacial Baltic Sea at salinities where reproduction remains functional. Since establishment, the Baltic Sea populations have evolved adaptations to extreme hyposaline waters and have in addition evolved asexual recruitment that, however, tends to impede local adaptation. Overall, our results support the “plasticity-first” model for the initial colonisation of the Baltic Sea by Fucus vesiculosus

Data_Fig2C_3_loss

Data used to generate Fig. 2C, and Fig. 3. For further details, please refer to ReadMeFirst file (available separately)

Data_Fig2A_3_gain

Data used to generate Fig. 2A (circles), and Fig. 3. For further details, please refer to ReadMeFirst file (available separately)

Data from: A universal mechanism generating clusters of differentiated loci during divergence-with-migration

Genome-wide patterns of genetic divergence reveal mechanisms of adaptation under gene flow. Empirical data show that divergence is mostly concentrated in narrow genomic regions. This pattern may arise because differentiated loci protect nearby mutations from gene flow, but recent theory suggests this mechanism is insufficient to explain the emergence of concentrated differentiation during biologically realistic timescales. Critically, earlier theory neglects an inevitable consequence of genetic drift: stochastic loss of local genomic divergence. Here, we demonstrate that the rate of stochastic loss of weak local differentiation increases with recombination distance to a strongly diverged locus and, above a critical recombination distance, local loss is faster than local “gain” of new differentiation. Under high migration and weak selection, this critical recombination distance is much smaller than the total recombination distance of the genomic region under selection. Consequently, divergence between populations increases by net gain of new differentiation within the critical recombination distance, resulting in tightly linked clusters of divergence. The mechanism responsible is the balance between stochastic loss and gain of weak local differentiation, a mechanism acting universally throughout the genome. Our results will help to explain empirical observations and lead to novel predictions regarding changes in genomic architectures during adaptive divergence

Inversions and genomic differentiation after secondary contact: When drift contributes to maintenance, not loss, of differentiation

Due to their effects on reducing recombination, chromosomal inversions may play an important role in speciation by establishing and/or maintaining linked blocks of genes causing reproductive isolation (RI) between populations. This view fits empirical data indicating that inversions typically harbor loci involved in RI. However, previous computer simulations of infinite populations with two to four loci involved in RI implied that, even with gene flux as low as 10–8 per gamete, per generation between alternative arrangements, inversions may not have large, qualitative advantages over collinear regions in maintaining population differentiation after secondary contact. Here, we report that finite population sizes can help counteract the homogenizing consequences of gene flux, especially when several fitness-related loci reside within the inversion. In these cases, the persistence time of differentiation after secondary contact can be similar to when gene flux is absent and notably longer than the persistence time without inversions. Thus, despite gene flux, population differentiation may be maintained for up to 100,000 generations, during which time new incompatibilities and/or local adaptations might accumulate and facilitate progress toward speciation. How often these conditions are met in nature remains to be determined.This study was supported by the European Regional Development Fund (FCOMP-01-0124-FEDER-014272), FCT – Foundation for Science and Technology (PTDC/BIA-EVF/113805/2009), Ministerio de Ciencia e Innovación, Spain (PGC2018-101927-B-I00, MINECO/FEDER, UE), by the Spanish National Institute of Bioinformatics (PT17/0009/0020), and by “Unidad de Excelencia María de Maeztu,” funded by the MINECO (ref: MDM-2014-0370). MR was funded by the Hasselblad Foundation (Grant for Female Scientists), European Research Council and the Swedish Research Councils VR and Formas (Linnaeus grant to the Centre for Marine Evolutionary Biology), and by an additional grant from Formas (to MR; grant number 2019-00882). JLF was funded by support from the National Science Foundation and United States Department of Agriculture NIFA program. RF was funded by FCT (SFRH/BPD/89313/2012) and by the European Union's Horizon 2020 research and innovation program, under the Marie Sklodowska-Curie grant agreement number 706376; and is currently funded by FEDER through the Operational Competitiveness Factors Program COMPETE and by National Funds through the FCT project “Hybrabbid” (PTDC/BIA-EVL/30628/2017 and POCI-01-0145-FEDER-030628). The simulations were performed on resources at Chalmers Centre for Computational Science and Engineering (C3SE), and at National Supercomputer Centre at Linköping University (NSC) provided by the Swedish National Infrastructure for Computing (SNIC), partially funded by the Swedish Research Council through grant agreement no. 2018–05973.Peer reviewe