Three modes of adaptive speciation in spatially structured populations

Adaptive speciation with gene flow via the evolution of assortative mating has classically been studied in one of two different scenarios. First, speciation can occur if frequency-dependent competition in sympatry induces disruptive selection, leading to indirect selection for mating with similar phenotypes. Second, if a subpopulation is locally adapted to a specific environment, there is indirect selection against hybridizing with maladapted immigrants. While both of these mechanisms have been modeled many times, the literature lacks models that allow direct comparisons between them. Here, we incorporate both frequency-dependent competition and local adaptation into a single model, and investigate whether and how they interact in driving speciation. We report two main results. First, we show that, individually, the two mechanisms operate under separate conditions, hardly influencing each other when one of them alone is sufficient to drive speciation. Second, we also find that the two mechanisms can operate together, leading to a third speciation mode, in which speciation is initiated by selection against maladapted migrants, but completed by within-deme competition in a distinct second phase. While this third mode bears some similarity to classical reinforcement, it happens considerably faster, and both newly formed species go on to coexist in sympatry. KEYWRODS: parapatric speciation, adaptive speciation, assortative mating, frequency dependent selection, reinforement, local adaptatio

Integrating local environmental observations and remote sensing to better understand the life cycle of a thermokarst lake in Arctic Alaska

On 29 June 2022, local observers reported the drainage of a 0.5 ha lake near Qikiqtaġruk (Kotzebue), Alaska, that prompted this collaborative study on the life cycle of a thermokarst lake in the Arctic. Prior to its drainage, the lake expanded from 0.13 ha in 1951 to 0.54 ha in 2021 at lateral rates that ranged from 0.25 to 0.35 m/year. During the drainage event, we estimate that 18,500 m3 of water drained from the lake into Kotzebue Sound, forming a 125-m-long thermo-erosional gully that incised 2 to 14 m in ice-rich permafrost. Between 29 June and 18 August 2022, the drainage gully expanded from 1 m to >10 m wide, mobilizing ~8,500 m3 of material through erosion and thaw. By reconstructing a pre-lake disturbance terrain model, we show that thaw subsidence occurs rapidly (0.78 m/year) upon transition from tundra to lake but that over a seventy-year period it slows to 0.12 m/year. The combination of multiple remote sensing tools and local environmental observations provided a rich data set that allowed us to assess rates of lake expansion relative to rates of sub-lake permafrost thaw subsidence as well as hypothesizing about the potential role of beavers in arctic lake drainage

Evolution of Assortative Mating in a Population Expressing Dominance

In this article, we study the influence of dominance on the evolution of assortative mating. We perform a population-genetic analysis of a two-locus two-allele model. We consider a quantitative trait that is under a mixture of frequency-independent stabilizing selection and density- and frequency-dependent selection caused by intraspecific competition for a continuum of resources. The trait is determined by a single (ecological) locus and expresses intermediate dominance. The second (modifier) locus determines the degree of assortative mating, which is expressed in females only. Assortative mating is based on similarities in the quantitative trait (‘magic trait’ model). Analytical conditions for the invasion of assortment modifiers are derived in the limit of weak selection and weak assortment. For the full model, extensive numerical iterations are performed to study the global dynamics. This allows us to gain a better understanding of the interaction of the different selective forces. Remarkably, depending on the size of modifier effects, dominance can have different effects on the evolution of assortment. We show that dominance hinders the evolution of assortment if modifier effects are small, but promotes it if modifier effects are large. These findings differ from those in previous work based on adaptive dynamics

TIPP and the Spectral Expansion Method

this paper we present a variant of SPA which enables the Spectral Expansion solution method (SE) to be used, thus allowing the modelling of processes with infinite state space. Whether SE is applicable to a given problem can be decided before generating a detailed description of the state space. The mapping from the SPA to the SE formalisms can be automated and the technicalities of the solution can be hidden from the user. This approach is illustrated on a small but non-trivial example. 1. Introductio

Effects of genetic architecture on the evolution of assortative mating under frequency-dependent disruptive selection

We consider a model of sympatric speciation due to frequency-dependent competition, in which it was previously assumed that the evolving traits have a very simple genetic architecture. In the present study, we numerically analyze the consequences of relaxing this assumption. First, previous models assumed that assortative mating evolves in infinitesimal steps. Here, we show that the range of parameters for which speciation is possible increases when mutational steps are large. Second, it was assumed that the trait under frequency-dependent selection is determined by a single locus with two alleles and additive effects. As a consequence, the resultant intermediate phenotype is always heterozygous and can never breed true. To relax this assumption, here we add a second locus influencing the trait. We find three new possible evolutionary outcomes: evolution of three reproductively isolated species, a monomorphic equilibrium with only the intermediate phenotype, and a randomly mating population with a steep unimodal distribution of phenotypes. Both extensions of the original model thus increase the likelihood of competitive speciation