Biology subjects

Schneemann, H.

Publications and source records attributed to Schneemann, H..

6 recordsLinked to original sources

bioRxiv2025

Predicting Hybrid Fitness: the Effects of Ploidy and Complex Ancestry

Hybridization between divergent populations places alleles in novel genomic contexts. This can inject adaptive variation - which is useful for breeders and conservationists - or reduce fitness, leading to reproductive isolation. Most theoretical work on hybrids involves haploid or diploid hybrids between two parental lineages, but real-world hybridization is often more complex. We introduce a simple fitness landscape model to predict hybrid fitness with arbitrary ploidy and an arbitrary number of hybridizing lineages. We test our model on published data from maize (Zea mays) and rye (Secale cereale), including hybrids between multiple inbred lines, both as diploids and synthetic tetraploids. Quantitative predictions for the effects of inbreeding, and the strength of progressive heterosis, are well supported. This suggests that the model captures the important properties of dosage and genetic interactions, and may help to unify theories of heterosis and reproductive isolation.

evolutionary biology↗

bioRxiv2025

When will hybridization bring long-term benefits?

Hybridization between distinct populations injects genetic variation, which can bring fitness benefits. However, these benefits often appear as F1 heterosis, and might not persist into later generations; especially since, as emphasized by classical theories, heterosis can be caused in several different ways. Here, we study the long-term outcomes of hybridization, using a model that allows us to tune several properties of the genetic variation, including the strength and architecture of heterosis, thereby unifying the classical theories. Results suggest that long-term outcomes depend mainly on the variance in epistasis, which determines the ruggedness of the fitness landscape, but without affecting the heterosis. Together, results suggest that the study of heterosis may tell us relatively little about the long-term outcomes of hybridization, and that hybridization might bring benefits more often than has been assumed.

evolutionary biology↗

bioRxiv2023

Transient overdominance, coadaptation, and the fixability of heterosis

Many species pairs form F1 hybrids that are fitter than their parents. Such heterosis can arise if the parents carry recessive deleterious mutations; and in this case, the heterosis should be fixable, because selecting out the deleterious mutations yields a high-fitness homozygous hybrid. However, heterosis might not be fixable if caused by overdominance (an intrinisic advantage to heterozygosity) or if the parents contain coadapted gene complexes. These alternatives have been tested with introgression lines, where small regions of genome are scored in the heterospecific background. We develop predictions for introgression line data under a simple model of phenotypic selection, where parents diverge by fixing deleterious mutations via genetic drift. We show that this simple process can generate complex patterns in the data, misleading tests for both overdominance and coadaptation. We also suggest new ways to analyse the data to overcome these difficulties. Reanalyses of published data from Solanum and Gossypium suggest that the model can account for the qualitative patterns observed, though not the extent of apparent overdominance.

evolutionary biology↗

bioRxiv2022

How does the mode of evolutionary divergence affect reproductive isolation?

When divergent populations interbreed, the outcome will be affected by the genomic and phenotypic differences that they have accumulated. In this way, the mode of evolutionary divergence between populations may have predictable consequences for the fitness of their hybrids, and so for the progress of speciation. To investigate these connections, we present a new analysis of hybridization under Fishers geometric model, making few assumptions about the allelic effects that differentiate the hybridizing populations. Results show that the strength and form of postzygotic reproductive isolation (RI) depend on just two properties of the evolutionary changes, which we call the "total amount" and "net effect" of change, and whose difference quantifies the similarity of the changes at different loci, or their tendency to act in the same phenotypic direction. It follows from our results that identical patterns of RI can arise in different ways, since different evolutionary histories can lead to the same total amount and net effect of change. Nevertheless, we show how these estimable quantities do contain some information about the history of divergence, and that - thanks to Haldanes Sieve - the dominance and additive effects contain complementary information. Impact SummaryWhen populations of animals or plants evolve differences in their genomes or traits, the nature of the differences will help to determine whether they can continue to interbreed. For example, the hybrid offspring may be infertile, or unlikely to survive to reproductive age, meaning that the two populations remain distinct from one another even after mating. However, in some cases the hybrids may be more fertile than their parents or have some other reproductive advantage. In this study, we use a mathematical model to relate hybrid fitness to the evolved differences separating the parents. We find that the outcome depends on just two properties of these differences, which capture the "total amount" and the "net effect" of evolutionary change. We then show that different evolutionary divergence scenarios or modes can lead to the exact same hybrid fitness. On the other hand, we can still make some inferences about the history of divergence by observing certain properties of hybrid fitness. Determining the relationship between hybrid fitness and the mode of evolutionary divergence will help to understand how new species form, to plan conservation interventions such as moving individuals between isolated populations to increase their adaptive potential, and to understand how existing species might interact when their habitats overlap, for example due to climate change or other human impacts.

evolutionary biology↗

bioRxiv2019

The Geometry and Genetics of Hybridization

We develop an analytical framework for predicting the fitness of hybrid genotypes, based on Fishers geometric model. We first show that all of the model parameters have a simple geometrical and biological interpretation. Hybrid fitness decomposes into intrinsic effects of hybridity and heterozygosity, and extrinsic measures of the (local) adaptedness of the parental lines; and all of these correspond to distances in a phenotypic space. We also show how these quantities change over the course of divergence, with convergence to a characteristic pattern of intrinsic isolation. Using individual-based simulations, we then show that the predictions apply to a wide range of population genetic regimes, and divergence conditions, including allopatry and parapatry, local adaptation and drift. We next connect our results to the quantitative genetics of line crosses in variable or patchy environments. This relates the geometrical distances to quantities that can be estimated from cross data, and provides a simple interpretation of the "composite effects" in the quantitative genetics partition. Finally, we develop extensions to the model, involving selectively-induced disequilibria, and variable phenotypic dominance. The geometry of fitness landscapes provides a unifying framework for understanding speciation, and wider patterns of hybrid fitness.

evolutionary biology↗

bioRxiv2018

The ecological drivers of variation in global language diversity

Language diversity is distributed unevenly over the globe. Why do some areas have so many different languages and other areas so few? Intriguingly, patterns of language diversity resemble biodiversity patterns, leading to suggestions that similar mechanisms may underlie both linguistic and biological diversification. Here we present the first global analysis of language diversity that identifies the relative importance of two key ecological mechanisms suggested to promote language diversification - isolation and ecological risk - after correcting for spatial autocorrelation and phylogenetic non-independence. We find significant effects of climate on language diversity consistent with the ecological risk hypothesis that areas of high year-round productivity lead to more languages by supporting human cultural groups with smaller distributions. Climate has a much stronger effect on language diversity than landscape features that might contribute to isolation of cultural groups, such as altitudinal variation, river density, or landscape roughness. The association between biodiversity and language diversity appears to be an incidental effect of their covariation with climate, rather than a causal link between the two. While climate and landscape provide strong explanatory signal for variation in language diversity, we identify a number of areas of high unexplained language diversity, with more languages than would be predicted from environmental features alone; notably New Guinea, the Himalayan foothills, West Africa, and Mesoamerica. Additional processes may be at play in generating higher than expected language diversity in these regions.

ecology↗