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Wittmann, M. J.

Publications and source records attributed to Wittmann, M. J..

17 recordsLinked to original sources
bioRxiv2026

A metabolic model to investigate the evolution of chemodiversity

1. Plants produce a great chemodiversity, which is the diversity of specialized metabolites (SMs). These SMs are produced in complex metabolic pathways and play an important role in inter-species interactions. There are numerous hypotheses about the evolutionary processes which brought about and maintain chemodiversity. Some have been partially tested in lab and field studies. However, some of their assumptions and predictions are better tested by quantitative modeling, and so far no quantitative model has investigated the role of metabolic pathways. 2. To close this gap, we developed an individual-based model for metabolic pathway evolution. It models enzymes creating metabolites with various modifications. Enzymes undergo inheritance and mutation. We used the model to compare the screening and interaction diversity hypotheses. 3. The screening hypothesis predicts promiscuous enzymes, genetic drift, the presence of many non-beneficial metabolites, and high metabolite richness. The interaction diversity hypothesis predicts specialized enzymes, selection, the almost exclusive presence of beneficial metabolites, and situation- dependent metabolite richness. We found that the patterns predicted by the screening hypothesis did not occur, while those predicted by the interaction diversity hypothesis did. 4. This provides reason to favor the interaction diversity hypothesis over the screening hypothesis when connecting empirical results to their evolutionary context

evolutionary biology↗
bioRxiv2026

How does individual trait variation impact the survival of populations with an Allee effect?

The Allee effect is a phenomenon where individual fitness is reduced in small populations, for example because of mate-finding difficulties or increased predation. Allee effects matter in conservation biology because they can drive small populations to extinction. The severity of Allee effects can depend on traits such as mate-search rate and defense against predators. Many natural populations exhibit considerable intraspecific trait variation (ITV) in such traits, but most studies so far assume these traits to be constant. Thus the impact of ITV on populations with Allee effect is largely unknown. Here we create two individual-based stochastic models that simulate a small population experiencing either a mate-finding Allee effect or a predator-driven Allee effect. We analyze how ITV, trait inheritance, and mutation affect the proportion of surviving populations. Under the mate-finding Allee effect, higher ITV hindered population survival and increased Allee thresholds. This can be explained by Jensens inequality and the negative curvature of the mate-finding function. Under the predator-driven Allee effect, ITV effects were weak, but higher mutation standard deviations were beneficial, likely because they provided more substrate for selection to act on. We thus recommend to take into account ITV when dealing with threatened populations with an Allee effect.

ecology↗
bioRxiv2025

Can the marginal male hypothesis explain spatial density variation in pinnipeds?

The marginal male hypothesis is a mechanism proposed to explain gregariousness, for example in pinnipeds. Here, we explore whether this mechanism, combined with density-dependent pup survival, can also account for heterogeneity in density across colonies, as observed for example in Antarctic fur seals (Arctocephalus gazella). We build a discrete-time matrix model inspired by the Antarctic fur seal to investigate how differences in density between two colonies can emerge through mate choice and density-dependent pup mortality. Our model assumes a heritable male colony preference that is coupled to competitive ability, i.e. a local siring advantage. Female colony preference is modelled as an independent trait that is allowed to evolve separately. Pup survival decreases with local density. Based on numerical analysis of the model, we find slight differences in density between the colonies at equilibrium. When siring advantage is high, and the survival penalty for males with siring advantage is low, the colony that is preferred by the high-quality males actually becomes the less dense colony. Our model serves as a proof of concept that such density differences can emerge. We expect that by including additional environmental factors, future models will explain a larger part of the observed variation in density.

ecology↗
bioRxiv2025

Individualised niches in a variable environment - Consequences for environmental change responses

O_LIIntraspecific trait variation (ITV) can be important for population performance in a variable and changing environment because individuals with different traits have different fitness responses. Furthermore, there are three mechanisms via which individuals can interact with their environment to potentially improve fitness: niche conformance, niche construction, and niche choice (NC3). These processes become increasingly important in the presence of environmental change, but there is still no mathematical modelling framework that would unite the effects of ITV and the NC3 mechanisms. C_LIO_LIIn this paper, we build a general model incorporating ITV and two of the NC3 mechanisms (niche conformance and construction, NC2) to investigate how they affect populations in a changing and variable environment via non-linear averaging. We quantify the effects of NC2 and ITV on average individual fitness using an analytical Taylor approximation and a sampling approach. C_LIO_LIOur method allows us to answer the question of what would have happened if individuals in the study system did not have ITV or did not perform NC2 mechanisms. The answer to this question depends on the curvature of the fitness function and can be estimated via the Taylor approximation. C_LIO_LIWe apply the method to two case studies: great tits adjusting their laying date to yearly changes in vegetation green-up, and a host-parasite system in which the parasite changes its environment by immunodepression of the host. In the great tits, we found a slight negative effect of ITV and a slight positive effect of niche conformance on the population fitness. In the host-parasite system, we found ITV to have no effect without niche construction, but with niche construction ITV decreased virulence. Also, niche construction had a strong negative effect on virulence. C_LIO_LIOur extension of non-linear averaging theory, combining intraspecific and environmental variation, niche conformance, and niche construction, allowed us to assess average population performance with those mechanisms at play. However, how well one can estimate such performance depends on the type of data available. This framework can be extended further to niche choice and evolution, therefore including all processes that can change the match between individuals and their environment. C_LI

ecology↗
bioRxiv2024

The role of evolving niche choice in herbivore adaptation to host plants

Individuals living in heterogeneous environments often choose microenvironments that provide benefits to their fitness. Theory predicts that such "niche choice" can promote rapid adaptation to novel environments and help maintain genetic diversity. An open question of large applied importance is how niche choice and niche choice evolution affect the evolution of insecticide resistance in phytophagous insects. We, therefore, developed an individual-based model based on phytophagous insects to examine the evolution of insecticide resistance and host plant choice via oviposition preferences. To find biologically realistic parameter ranges, we performed an empirical literature survey on insecticide resistance in major agricultural pests and also conducted a density-dependent survival experiment using potato beetles. We find that, in comparison to a scenario where individuals randomly oviposit eggs on toxic or non-toxic plants, the evolution of niche choice generally leads to slower evolution of resistance and facilitates the coexistence of different phenotypes. Our simulations also reveal that recombination rate and dominance effects can influence the evolution of both niche choice and resistance. Thus, this study provides new insights into the effects of niche choice on resistance evolution and highlights the need for more studies on the genetic basis of resistance and choice.

evolutionary biology↗
bioRxiv2024

How does plant chemodiversity evolve? Testing five hypotheses in one population genetic model

O_LIPlant chemodiversity, the diversity of plant specialized metabolites, is an important dimension of biodiversity. However, there are so far few quantitative models to test verbal hypotheses on how chemodiversity evolved. Here we develop such a model to test predictions of five hypotheses: the "fluctuating selection hypothesis", the "dominance reversal hypothesis", the interaction diversity hypothesis, the synergy hypothesis, and the screening hypothesis. C_LIO_LIWe build a population genetic model of a plant population attacked by herbivore species whose occurrence fluctuates over time. We study the model using mathematical analysis and individual-based simulations. C_LIO_LIAs predicted by the "dominance reversal hypothesis", chemodiversity can be maintained if alleles conferring a defense metabolite are dominant with respect to the benefits, but recessive with respect to costs. However, even smaller changes in dominance can maintain polymorphism. Moreover, our results underpin and elaborate predictions of the synergy and interaction diversity hypotheses, and, to the extent that our model can address it, the screening hypotheses. By contrast, we found only partial support for the "fluctuating selection hypothesis". C_LIO_LIIn summary, we have developed a flexible model and tested various verbal models for the evolution of chemodiversity. Next, more mechanistic models are needed that explicitly consider the organization of metabolic pathways. C_LI

evolutionary biology↗
bioRxiv2023

Reviving collapsed networks from a single species: the importance of trait variation and network architecture

Mutualistic ecological networks can suddenly transition to undesirable states, due to small changes in environmental conditions. Recovering from such a collapse can be difficult as reversing the original environmental conditions may be infeasible. Additionally, such networks can also exhibit hysteresis, implying that ecological networks may not recover. Here, using a dynamical eco-evolutionary framework, we try to resurrect mutualistic networks from an undesirable low-functional collapse state to a high-functioning state. We found that restoring the original environmental conditions rarely aided in recovering the original network due to the presence of hysteresis. By combining concepts from signal propagation theory and eco-evolutionary dynamical modeling, we show that network resurrection could be readily achieved by perturbing a single species that controls the response of the dynamical networks. We show that during the resurrection of collapsed networks, the historical network architecture, levels of trait variation, and eco-evolutionary dynamics could aid in the revival of the network even in undesirable environmental conditions. Our study argues that focus should be applied to a few species whose dynamics one could steer to resurrect the entire network from a collapsed state.

ecology↗
bioRxiv2023

Multi-scale Effects of Habitat Loss and the Role of Trait Variation

Habitat loss (HL) is a major cause of species extinctions. Although effects of HL beyond the directly impacted area have been previously observed, they are not very well understood, especially in an eco-evolutionary context. To start filling this gap, we study a two-patch deterministic consumer-resource model, with one of the patches experiencing loss of resources. Our model allows foraging and mating within a patch as well as between patches. We then introduce heritable variation in consumer traits to investigate eco-evolutionary dynamics and compare results with constant or no trait variation scenarios. Our results show that HL indeed reduces consumer densities in the neighboring patch, but when the resources are overexploited, HL in one patch can increase the consumer densities in the neighbouring patch. Yet at the landscape scale, the effect of HL on consumer densities is consistently negative. In presence of HL, patch isolation has positive effects on consumer density in the patch experiencing HL and mostly negative effects on the neighbouring patch. The landscape level pattern depends on which of these effects are dominant at the local scale. Evolution always increased resistance of consumers in the affected patch to HL, with varied effects at the landscape level. Finally, we also show a possibility of landscape level consumer extinction due to HL in a local patch when the cross-patch dependence is high, and foraging and mating preferences are coupled. Eco-evolutionary dynamics can rescue consumers from such extinction in some cases if their death rates are sufficiently small. Our findings show that HL at a local scale can affect the neighbouring patch and the landscape as a whole, and that heritable trait variation can provide some resistance against HL. We thus suggest joint consideration of multiple spatial scales and trait variation when assessing and predicting the impacts of HL.

ecology↗
bioRxiv2023

Understanding the phytochemical diversity of plants: Quantification, variation and ecological function

Plants produce a great number of phytochemical compounds mediating a variety of different functions. Recently, phytochemical diversity (chemodiversity), a way which to quantify the complex phenotype formed by sets of phytochemicals, has been suggested to be important for function. However, no study has systematically examined the potential (in)direct functional importance of chemodiversity on a general level, partly due to a lack of an agreement on how to quantify this aspect of the plant phenotype. This paper has four aims: 1) We discuss how chemodiversity (deconstructed into components of richness, evenness and disparity) may quantify different aspects of the phenotype that are ecologically relevant. 2) We systematically review the literature on chemodiversity to examine methodological practices, explore ecological patterns of variability in diversity across different levels of biological organization, and investigate the functional role of this diversity in interactions between plants and other organisms. 3) We provide a framework facilitating decisions on which measure of chemodiversity is best used in different contexts. 4) We outline open questions and avenues for future research in this area. A more thorough understanding of phytochemical diversity will increase our knowledge on the functional role phytochemical compounds, and how they shape ecological interactions between plants and their environment.

ecology↗
bioRxiv2022

Modeling the genetic footprint of fluctuating balancing selection: From the local to the genomic scale

Natural selection not only affects the actual loci under selection but also leaves "footprints" in patterns of genetic variation in linked genetic regions. This offers exciting opportunities for inferring selection and for understanding the processes shaping levels of genetic variation in natural populations. Here we develop analytical approximations based on coalescent theory to characterize the genetic footprint of a complex, but potentially common type of natural selection: balancing selection with seasonally fluctuating allele frequencies. We show that seasonal allele frequency fluctuations can have important (and partly unexpected) consequences for the genetic footprint of balancing selection. As also confirmed by stochastic simulations, fluctuating balancing selection generally leads to an increase in genetic diversity close to the selected site, the effect of balancing selection, but reduces diversity further away from the selected site, which is a consequence of the allele-frequency fluctuations effectively producing recurrent bottlenecks of allelic backgrounds. This negative effect usually outweighs the positive effect when averaging diversity levels across the entire chromosome. Strong fluctuating balancing selection even induces a loss of genetic variation in unlinked regions, e.g. on different chromosomes. If many loci in the genome are simultaneously under fluctuating balancing selection this could lead to substantial genome-wide reductions in genetic diversity. This may be the case, even if allele-frequency fluctuations are so small that individual footprints are hard to detect. Thus, together with genetic drift, selective sweeps and background selection, fluctuating selection could be one of the major forces shaping levels of genetic diversity in natural populations. Article summaryIn some species with multiple generations per year, many loci in the genome experience strong seasonally fluctuating selection, in some cases with stable maintenance of polymorphism. Here we investigate the consequences for levels of genetic diversity at linked neutral sites. Using analytical approximations and stochastic simulations, we find a characteristic local genetic footprint with a diversity peak around the selected site and a diversity valley further away and a substantial reduction in diversity levels chromosome-wide and even genome-wide.

evolutionary biology↗
bioRxiv2021

Modeling minimum viable population size with multiple genetic problems of small populations

An important goal for conservation is to define minimum viable population (MVP) sizes for long-term persistence. Although many MVP size estimates focus on ecological processes, with increasing evidence for the role of genetic problems in population extinction, conservation practitioners have also increasingly started to incorporate inbreeding depression (ID). However, small populations also face other genetic problems such as mutation accumulation (MA) and loss of genetic diversity through genetic drift that are usually factored into population viability assessments only via verbal arguments. Comprehensive quantitative theory on interacting genetic problems is missing. Here we develop eco-evolutionary quantitative models that track both population size and levels of genetic diversity. Our models assume a biallelic multilocus genome whose loci can be under either a single or interacting genetic forces. In addition to mutation-selection-drift balance (for loci facing ID and MA), we include three forms of balancing selection (for loci where variation is lost through genetic drift). We define MVP size as the lowest population size that avoids an eco-evolutionary extinction vortex after a time sufficient for an equilibrium allele frequency distribution to establish. Our results show that MVP size decreases rapidly with increasing mutation rates for populations whose genomes are only under balancing selection, while for genomes under mutation-selection-drift balance, the MVP size increases rapidly. MVP sizes also increase rapidly with increasing number of loci under the same or different selection mechanisms until a point is reached at which even arbitrarily large populations cannot survive anymore. In the case of fixed number of loci under selection, interaction of genetic problems did not necessarily increase MVP sizes. To further enhance our understanding about interaction of genetic problems, there is need for more empirical studies to reveal how different genetic processes interact in the genome.

evolutionary biology↗
bioRxiv2021

Nonlinear averaging in 2D: intraspecific variation in two interacting species and consequences for population dynamics

Interactions between two species, e.g. between a predator species and a prey species, can often be described as the sum of many individual-by-individual interactions whose outcomes depend on the traits of the interacting individuals. These traits often vary substantially among individuals in each species, and individuals do not always interact randomly, e.g. due to plastic responses to a shared environmental factor in a heterogeneous landscape. Here we investigate the impact of intraspecific trait variation (ITV) and such interspecific trait correlations on species interactions via nonlinear averaging. Building on past models that integrate over an interaction kernel to obtain the impacts of ITV, we develop a modeling framework that allows to model arbitrary species interactions, with interspecific trait correlations as novel feature. Based on two key ingredients, a joint trait distribution and a two-dimensional interaction function, the average interaction parameters (e.g. average predation rate) can be quantified numerically, approximated using an insightful Taylor approximation, and compared to cases without ITV. We highlight two applications of our framework. First, we study the quantitative and qualitative effects of ITV and trait correlations in a simple predator-prey model and show that even in the absence of evolution, variation and trait correlations among interacting individuals can make or break the coexistence between species. Second, we use simulated field data for a predator-prey system to show how the impact of ITV on an ecological interaction can be estimated from empirical data. HighlightsO_LIWe model how intraspecific trait variation in two species affects their interaction. C_LIO_LIOur framework allows correlations between the traits of interacting individuals. C_LIO_LITrait variation and correlations can strongly affect ecological interactions. C_LIO_LITrait variation and correlations can make or break coexistence. C_LIO_LIThe effect of intraspecific trait variation can be estimated from data. C_LI

ecology↗
bioRxiv2020

Waiting for love but not forever: modelling the evolution of waiting time to selfing in hermaphrodites

Although mixed mating systems involving both selfing and outcrossing are fairly common in hermaphrodites, it is unclear how they are maintained. In some species, individuals delay self-fertilization while they have not found a mating partner. The waiting time is subject to two opposing selection pressures: waiting helps to avoid inbreeding depression in offspring by increasing the density-dependent probability to encounter a mate, but also increases the risk of dying before reproduction. In some species waiting time can vary between individuals and be heritable. We therefore used an individual-based model to explore how delayed selfing evolves in response to density and density fluctuations. We find that at high density, when individuals meet often, drift drives waiting time; at intermediate densities, strong inbreeding depression causes waiting time to increase; and at low densities, inbreeding depression is purged, and waiting time approaches zero. Positive feedback loops drive the system to either complete selfing or complete outcrossing. Fluctuating density can slow down convergence to these alternative stable states. However, mixed mating, in the sense of either a stable polymorphism in waiting times, or stable intermediate times, was never observed. Thus, additional factors need to be explored to explain the persistence of delayed selfing.

evolutionary biology↗
bioRxiv2020

Climate change and green sea turtle sex ratio - Preventing possible extinction

Climate change poses a threat to species with temperature-dependent sex determination. A recent study on green sea turtles (Chelonia mydas) at the northern Great Barrier Reef (GBR) showed a highly female-skewed sex ratio with almost all juvenile turtles being female. This shortage of males might eventually cause population extinction, unless rapid evolutionary rescue, migration or conservation efforts ensure a sufficient number of males. We built a stochastic individual-based model inspired by C. mydas, but potentially transferrable to other species with TSD. Nest depth, level of shade, and pivotal temperature were evolvable traits. Additionally, we considered the effect of crossbreeding between the northern and southern GBR, nest-site philopatry, and conservation efforts. Among the evolvable traits, nest depth was the most likely to rescue the population in the face of climate change, but even here the more extreme climate-change scenario led to extinction. Surprisingly, nest-site philopatry elevated extinction rates. Conservation efforts to artificially increase nest depth promoted population survival and did not preclude trait evolution. Although extra information is needed to make reliable predictions for the fate of green sea turtles, our results illustrate how evolution can shape the fate of long lived, vulnerable species in the face of climate change. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=162 SRC="FIGDIR/small/032227v1_ufig1.gif" ALT="Figure 1"> View larger version (37K): org.highwire.dtl.DTLVardef@8847e2org.highwire.dtl.DTLVardef@12778f7org.highwire.dtl.DTLVardef@76d1f5org.highwire.dtl.DTLVardef@d26061_HPS_FORMAT_FIGEXP M_FIG C_FIG

evolutionary biology↗
bioRxiv2020

Models for Eco-evolutionary Extinction Vortices and their Detection

The smaller a population is, the faster it looses genetic variation due to genetic drift. Loss of genetic variation can reduce population growth rate, making populations even smaller and more vulnerable to loss of genetic variation, and so on. Ultimately, the population can be driven to extinction by this "eco-evolutionary extinction vortex". So far, extinction vortices due to loss of genetic variation have been mainly described verbally. However, quantitative models are needed to better understand when such vortices arise and to develop methods for detecting them. Here we propose quantitative eco-evolutionary models, both individual-based simulations and analytic approximations, that link loss of genetic variation and population decline. Our models assume stochastic population dynamics and multi-locus genetics with different forms of balancing selection. Using mathematical analysis and simulations, we identify parameter combinations that exhibit strong interactions between population size and genetic variation as populations decline to extinction and match our definition of an eco-evolutionary vortex, i.e. the per-capita population decline rates and per-locus fixation rates increase with decreasing population size and number of polymorphic loci. We further highlight cues and early warning signals that may be useful in identifying populations undergoing an eco-evolutionary extinction vortex.

evolutionary biology↗
bioRxiv2019

Density dependence on multiple spatial scales maintains spatial variation in both abundance and traits

Population density affects fitness through various processes, such as mate finding and competition. The fitness of individuals in a population can in turn affect its density, making population density a key quantity linking ecological and evolutionary processes. Density effects are, however, rarely homogeneous. Different life-history processes can be affected by density over different spatial scales. In birds, for example, competition for food may depend on the number of birds nesting in the direct vicinity, while competition for nesting sites may occur over larger areas. Here we investigate how the effects of local density and of density in nearby patches can jointly affect the emergence of spatial variation in abundance as well as phenotypic diversification. We study a two-patch model that is described by coupled ordinary differential equations. The patches have no intrinsic differences: they both have the same fitness function that describes how an individuals fitness depends on density in its own patch as well as the density in the other patch. We use a phase-space analysis, combined with a mathematical stability analysis to study the long-term behaviour of the system. Our results reveal that the mutual effect that the patches have on each other can lead to the emergence and long-term maintenance of a low and a high density patch. We then add traits and mutations to the model and show that different selection pressures in the high and low density patch can lead to diversification between these patches. Via eco-evolutionary feedbacks, this diversification can in turn lead to changes in the long-term population densities: under some parameter settings, both patches reach the same equilibrium density when mutations are absent, but different equilibrium densities when mutations are allowed. We thus show how, even in the absence of differences between patches, interactions between them can lead to differences in long-term population density, and potentially to trait diversification.

evolutionary biology↗
bioRxiv2017

Segregation lift: A general mechanism for the maintenance of polygenic variation under seasonally fluctuating selection.

Most natural populations are affected by seasonal changes in temperature, rainfall, or resource availability. Seasonally fluctuating selection could potentially make a large contribution to maintaining genetic polymorphism in populations. However, previous theory suggests that the conditions for multi-locus polymorphism are restrictive. Here we explore a more general class of models with multi-locus seasonally fluctuating selection in diploids. In these models, loci first contribute additively to a seasonal score, with a dominance parameter determining the relative contributions of heterozygous and homozygous loci. The seasonal score is then mapped to fitness via a monotonically increasing function, thereby accounting for epistasis. Using mathematical analysis and individual-based simulations, we show that stable polymorphism at many loci is possible if currently favored alleles are sufficiently dominant with respect to the additive seasonal score (but not necessarily with respect to fitness itself). This general mechanism, which we call \"segregation lift\", operates for various genotype-to-fitness maps and includes the previously known mechanism of multiplicative selection with marginal overdominance as a special case. We show that segregation lift may arise naturally in situations with antagonistic pleiotropy and seasonal changes in the relative importance of traits for fitness. Segregation lift is not affected by problems of genetic load and is robust to differences in parameters across loci and seasons. Under segregation lift, loci can exhibit conspicuous seasonal allele-frequency fluctuations, but often fluctuations may also be small and hard to detect. Via segregation lift, seasonally fluctuating selection might contribute substantially to maintaining genetic variation in natural populations.

evolutionary biology↗