Keiichi Morita

Predicting ecological outcomes of hybridizing species remains challenging because of complex interactions between genetic and ecological processes. Previous studies have mainly focused on exclusion scenarios driven by genetic mechanisms, whereas few have explored the impact of competitive interactions among parent species and their hybrids on generating multiple coexistence regimes. Here, we develop a population dynamics model that explicitly couples hybridization genetics with population dynamics driven by competitive interactions among two parent species and their hybrids. Our analysis reveals that backcrossing can fundamentally reshape coexistence regimes, generating five alternative outcomes: (i) alternative stable states of coexistence of both parent species and hybrids, (ii) frequency-independent coexistence, (iii) frequency-dependent coexistence, (iv) competitive exclusion of a rarer species, and (v) extinction of all populations. These outcomes arise from the balance between two key processes: regeneration of parental genotypes through hybrid × hybrid mating and competitive exclusion mediated by hybrids. Thus, hybridization can generate multiple stable coexistence regimes even when species would otherwise be excluded. This eco-genetic mechanism provides a unifying theoretical basis for predicting the ecological consequences of hybridization, highlighting the crucial role of competitive interactions in determining coexistence outcomes.

Interspecific pollen transfer (IPT), the pollen movement between plant species via shared pollinators, reduces the reproductive success of pollen‐recipient plants due to hybridization with heterospecific pollen grains. As a result, IPT hinders coexistence of sympatric, co‐flowering species by reducing their reproductive success. IPT likely exerts selective pressure on plant reproductive systems, particularly on sex allocation (resource investment to pollen versus ovules). However, the influence of IPT on the evolutionary dynamics of sex allocation and associated ecological processes remains poorly understood. Here, we explore how the female costs incurred by IPT affect the co‐evolution and coexistence dynamics of two plant species. To address this, we construct a mathematical model capturing interactions between two plant species through pollen transfer and resource competition, and reduced fertilization caused by IPT. Our analysis focuses on a scenario where an invasive species with female‐biased sex allocation enters a habitat occupied by a resident species with evolutionarily stable, equal sex allocation (Fisherian sex allocation). Using adaptive dynamics theory, we demonstrated that, irrespective of IPT strength, natural selection drives both species toward equal sex allocation, consistent with the Fisherian sex allocation theory. We present two key predictions of eco‐evolutionary outcomes. First, when the impacts of IPT are comparable for both species, the resulting eco‐evolutionary dynamics lead to their stable coexistence. In contrast, if IPT from the invasive species is disproportionately stronger than IPT from the resident species, the invasive species' evolutionary shift from female‐biased to equal sex allocation drives the resident species to extinction. Since local mate competition in small populations can favour female‐biased sex allocation, our results suggest that invasive species experiencing reduced local mate competition may drive resident species to extinction. Our findings underscore the intricate role of IPT‐driven sex allocation evolution in shaping coexistence of closely related plant species.

Understanding how biological communities assemble in the presence of rapid evolution is becoming an important topic in ecology. Previous studies demonstrated that community assembly can be affected by two types of eco-evolutionary dynamics: evolution-mediated priority effect (EPE) and ecological character displacement (ECD). In EPE, early-arriving species prevent colonization of late-arriving species via local adaptation (i.e. community monopolization), whereas ECD promotes species coexistence by niche partitioning. Researchers tended to discuss the two processes separately, but it should be possible for those processes to operate in the same system depending on various conditions. Here, we developed a theoretical framework that integrates the two processes by using a simple two-species competition model with eco-evolutionary feedback. We revealed that, when an early-arriving species evolves, the difference in immigration timing between the early-arriving and a late-arriving species can be a key parameter. When the difference is small, ECD occurs because insufficient local adaptation of the early-arriving species allows colonization of the late-arriving species. When the difference is large, however, EPE occurs because niche pre-emption by local adaptation of the early-arriving species prevents colonization of the late-arriving species. Further theoretical and empirical studies will be important to better understand eco-evolutionary community assembly with ECD and EPE.

Interspecific pollen transfer (IPT), the movement of pollen grains between different plant species by sharing pollinators, incurs costs (fitness reduction) for seed production. IPT thereby reduces the reproductive success of co-flowering plants sharing pollinators, thus preventing their coexistence. However, the impact of IPT on the evolutionary dynamics and evolution-mediated ecological dynamics of sex allocation resource investment to pollen versus ovules) is poorly understood. Here, we investigate the consequences of the female costs incurred by IPT for the co-evolution and coexistence of two plants, by using a mathematical model where two plant species interact with each other via resource competition, pollen movements within and between species, and reduced fertilization due to IPT. The ecological situation we consider here is that an invasive species with female-biased sex allocation immigrates into a habitat of a resident species whose sex allocation is evolutionarily maintained at Fisherian sex allocation (FSA). By using adaptive dynamics theory, we found that regardless of the strength of IPT, natural selection favours the equal allocation to pollen grains and ovules (FSA) for both species. If the mutual impact of IPT on two species is similar in magnitude, we find that the eco-evolutionary dynamics can lead to their stable coexistence. In contrast, when only the invasive species negatively impacts the resident species through IPT, the evolution in invasive species from female-biased sex allocation to FSA causes the extinction of the resident species. Given that local mate competition in small populations is expected to result in female-based sex allocation, our finding suggests that if invasive species are relaxed from local mate competition, they may drive the resident species to extinction. Our study highlights the importance and complexity of the evolution of biased sex allocation driven by IPT to understand the coexistence of closely related plant species.