The team found the black cottonwood-balsam poplar stable hybrid lineage after analyzing the genetic makeup of 546 poplar tree cuttings collected along seven transects ranging from Alaska to Wyoming, with collections in British Columbia and Alberta, Canada, in between.
Credit: Penn State. CC BY-NC-ND 4.0
Despite creationist dogma that say otherwise, evolution in a population can occur hybridization, especially in plants, where it may be commonplace. Two related plants hybridize and the resulting offspring acquires additional genes which extend its capabilities, enabling it to survive environmental change or move into new niches, and forming a stable population with new allele frequencies.
And as though to rub salt into creationist wounds, some of these happened hundreds of thousands of years before creationism's legendary 'Creation Week', before which, on the say-so of some ignorant Bronze Age pastoralists who were unaware of anything more than a day or two's walk from the Canaanite Hills, creationists think there was once nothing out of which everything was created by some magic words, just a few thousand years ago.
They also hold the diametrically opposite views simultaneously, that evolution is impossible because the Second Law of Thermodynamics [sic] forbids it, and that evolution occurred as a massively accelerated rate in the last few thousand years, unnoticed by anyone, in which several whole new species arose in a single generation by magic.
Now a team led by Penn State University paleobotanists led by Associate Professor Jill Hamilton, from Penn State’s College of Agricultural Sciences, have shown that a hybrid between black cottonwood, or Populus trichocarpa, and balsam poplar, Populus balsamifera, was able to move out of the wet coastal region to which most black cottonwood trees are restricted, into the arid lands to the east. This movement started about 800,000 years ago.
A research team has proposed that hybridization could be commonplace in plants and gives populations with improved ability to adapt to environmental change* Do you have any examples of this? Hybridization in plants is indeed a fascinating process that can enhance adaptability and lead to improved survival in changing environments. Several examples from scientific research highlight how hybridization contributes to plant resilience and adaptability:The team postulate that such hybridizations occur frequently between sister species, especially in forest conditions, and the result may be new genetic combinations able to respond to environmental change.
These examples illustrate how hybridization can create new plant populations with enhanced adaptability and resilience to environmental changes. By combining genetic material from different species or populations, hybrids can exhibit novel traits that improve their chances of survival and reproduction in diverse and dynamic ecosystems.
- Sunflowers (Helianthus species)
In a study on wild sunflowers, researchers found that hybridization between different species of Helianthus contributed to increased genetic diversity and the emergence of new, more adaptable hybrid species. These hybrids showed improved resistance to various environmental stresses, such as drought and soil salinity. This adaptability can be attributed to the combination of beneficial traits from the parent species, allowing the hybrids to thrive in diverse and changing habitats.- Spartina cordgrass (Spartina anglica)
The invasive cordgrass Spartina anglica is a hybrid of the European native Spartina maritima and the introduced American Spartina alterniflora. This hybrid species has shown remarkable adaptability to different tidal environments and has spread extensively across European coastlines. Its success is partly due to the hybrid vigor (heterosis), which provides enhanced growth rates and increased tolerance to environmental stresses like salinity and waterlogging.- Iris species
Hybridization among different Iris species has resulted in hybrids with enhanced environmental adaptability. For example, hybrids between Iris fulva and Iris hexagona have been shown to possess greater tolerance to varying water levels compared to their parent species. This adaptability allows the hybrids to colonize a wider range of habitats, contributing to their survival and spread.- Raphanobrassica
Raphanobrassica is a hybrid between radish (Raphanus sativus) and cabbage (Brassica oleracea). This hybrid, created through intentional crossbreeding, combines desirable traits from both parent species. For instance, Raphanobrassica exhibits improved resistance to pests and diseases, as well as greater tolerance to environmental stresses like cold and drought. This makes it a promising candidate for cultivation in regions with challenging growing conditions.- Bread Wheat (Triticum aestivum)
Bread wheat is a classic example of a hybrid species that originated from natural hybridization events between different grass species. This hybridization has resulted in a highly adaptable crop that can grow in a wide range of environments. The genetic diversity from its hybrid origins has contributed to wheat's resilience to various biotic and abiotic stresses, making it one of the most important staple crops worldwide.- Hawaiian Silversword Alliance
The Hawaiian Silversword Alliance is a group of plants in the Asteraceae family that have diversified from a common ancestor through hybridization and subsequent adaptive radiation. These plants occupy a wide range of habitats across the Hawaiian Islands, from dry deserts to wet forests. Hybridization has played a crucial role in their ability to adapt to different environmental conditions and evolve into distinct species with specialized ecological niches.
The team have published their findings open access in the journal Molecular Ecology and explain it in a Penn State News release:
Discovery of a hybrid lineage offers clues to how trees adapt to climate change
The discovery of a hybrid population of poplar trees in western Wyoming has provided insight into how natural hybridization informs the evolution of many plant species, according to a team led by Penn State researchers. They also said their discovery suggests that genetic exchange between species may be critical for adaptation to environmental change.
The research — which described a novel lineage of hybrid black cottonwood, or Populus trichocarpa, and balsam poplar, Populus balsamifera, — was recently published in Molecular Ecology. It is just the latest study to suggest that natural hybridization has played an important role in the evolution of many plant species, according to team leader Jill Hamilton, associate professor in Penn State’s College of Agricultural Sciences.
Hybridization between different species is occurring in nature far more frequently than we might have thought — particularly in forest trees. This is not necessarily a bad thing, because it may be a natural mechanism to facilitate adaptation in a changing climate. Studies like this one are critical to begin teasing apart how demographic history, gene flow and interaction across varying genomic ancestries have shaped natural hybrid zones to make better predictions for movement of germplasm and climate-assisted forest management in the future.
Associate Professor Jill A. Hamilton, lead author
Department of Ecosystem Science and Management
Pennsylvania State University, Pennsylvania, USA.
Most black cottonwood tree populations exist in wet coastal regions. However, the trees began appearing in arid environments inland and eastward about 800,000 years ago, noted Constance Bolte, postdoctoral researcher for the Schatz Center who spearheaded latter stages of the research. She hypothesized that this movement was likely facilitated by acquiring genetic variation from hybridizing with the balsam poplar trees that allowed them to survive hot, dry conditions.
The hybrid lineage described in the study have “some very interesting genetic combinations,” Bolte pointed out, enabling the trees to thrive in arid habitats.
Hybridization between sister species occurs frequently in forest trees, said Bolte, adding that this study shows the value in leveraging that history of natural hybridization for forest tree breeding and management.
Those coastal populations have specific adaptations to wet conditions, but climate has been changing, and so their distribution is very limited right now, potentially because that region is drier. And so, those hybrids have been doing better because they have the genetic tools to survive in that drier climate.
Our data indicates that stable lineage formation can result from hybridization. Historically, hybrids have not been considered in conservation efforts, and yet, if these hybrids happen to be fit for survival in arid or other extreme climates, it may be crucial to conserve and manage the natural genetic resources in these populations, especially under rapidly changing climate conditions.
Dr. Constance E. Bolte, first author
Department of Ecosystem Science and Management
Pennsylvania State University, Pennsylvania, USA.
A researcher carries a large cutting of poplar in the Pacific Northwest that later was genetically analyzed in the study. Most black cottonwood tree populations exist in wet coastal regions. However, the trees began appearing in arid environments inland and eastward about 800,000 years ago.Credit: Penn State. CC BY-NC-ND 4.0
The team found the black cottonwood-balsam poplar stable hybrid lineage after analyzing the genetic makeup of 546 poplar tree cuttings collected along seven transects — or narrow swaths of territory arranged from north to south — ranging from Alaska to Wyoming, with collections in British Columbia and Alberta, Canada, in between. Such analysis, according to Hamilton, is only possible using big-data-handing techniques and enormous computing power available at facilities such as the ROAR Collaborative Cluster, available through Penn State’s Institute for Computational and Data Sciences.
Contributing to the research at Penn State were Michelle Zavala-Paez, doctoral student in ecology, and Brianna Sutara, undergraduate student majoring in biology and psychology; and Muhammed Can, faculty of Forestry, Duzce University, Turkey; Matthew Fitzpatrick, professor, University of Maryland Center for Environmental Science; Jason Holliday, professor at Virginia Tech; Stephen Keller, associate professor Department of Plant Biology, University of Vermont; and Tommy Phannareth, graduate student at Virginia Tech.
The U.S. National Science Foundation and the U.S. Department of Agriculture’s National Institute of Food and Agriculture funded this research.
AbstractPredictably, creationists faced with this sort of example of how evolution can occur as a single event - a chance hybridization between two related species to give a new population with a different allele frequency to either of the parent species - will resort to the fallback pseudo definition of species used only by creationists, and intelligently designed to be something that science doesn't recognise as evolution - "But they're still trees" - because to qualify as evolution in their misinformed and childish view, one species needs to turn into something completely different - a tree giving rise to an elephant or a potato ("but it's still a plant!"), because one thing a dedicated creationist must never do is agree that an example of evolution is an example of evolution.
Population demographic changes, alongside landscape, geographic and climate heterogeneity, can influence the timing, stability and extent of introgression where species hybridise. Thus, quantifying interactions across diverged lineages, and the relative contributions of interspecific genetic exchange and selection to divergence at the genome-wide level is needed to better understand the drivers of hybrid zone formation and maintenance. We used seven latitudinally arrayed transects to quantify the contributions of climate, geography and landscape features to broad patterns of genetic structure across the hybrid zone of Populus trichocarpa and P. balsamifera and evaluated the demographic context of hybridisation over time. We found genetic structure differed among the seven transects. While ancestry was structured by climate, landscape features influenced gene flow dynamics. Demographic models indicated a secondary contact event may have influenced contemporary hybrid zone formation with the origin of a putative hybrid lineage that inhabits regions with higher aridity than either of the ancestral groups. Phylogenetic relationships based on chloroplast genomes support the origin of this hybrid lineage inferred from demographic models based on the nuclear data. Our results point towards the importance of climate and landscape patterns in structuring the contact zones between P. trichocarpa and P. balsamifera and emphasise the value whole genome sequencing can have to advancing our understanding of how neutral processes influence divergence across space and time.
1 INTRODUCTION
Understanding the processes influencing the formation and maintenance of species is a central goal of evolutionary biology and is crucial to management and conservation of biodiversity in a rapidly changing environment (Frankham, 2010; Hoffmann et al., 2015; Payseur & Rieseberg, 2016). As such, hybridisation and introgression have long been of interest (Dobzhansky, 1936; Heiser, 1949; Jeffrey, 1916; Stebbins, 1959), and recent advances in population-scale genome sequencing have revolutionised our understanding of the frequency and pervasiveness of hybridisation in nature (Hamilton & Miller, 2016.1; Janes & Hamilton, 2017; Mallet, 2005; VanWallendael et al., 2022). Multiple taxa are known to form natural hybrid zones; however, they are particularly prevalent among forest tree species (Abbott, 2017.1; Suarez-Gonzalez, Hefer, et al., 2018; Suarez-Gonzalez, Lexer, & Cronk, 2018.1; Swenson & Howard, 2004). Climatic oscillations during glacial and interglacial periods have influenced the distributions of many temperate and boreal trees, facilitating opportunities for interspecific gene flow during times of secondary contact (Hamilton et al., 2015.1; Hewitt, 2004.1; Jump & Peñuelas, 2005.1; Soltis et al., 2006; Swenson & Howard, 2004). However, despite widespread observations of hybridisation across forest tree species, gaps remain in our understanding of how demographic, neutral and nonneutral evolutionary processes influence the formation and maintenance of natural hybrid zones across space and time. Teasing apart these processes requires an assessment of the historical, landscape and climatic factors that underlay hybrid zone formation. Ultimately, understanding how population demographic changes, alongside landscape heterogeneity and climate adaptation contribute to the timing, stability and extent of hybridisation will be critical to managing species in a rapidly changing climate. Population demographics, including expansion and contraction of a species' range can have substantial influence on standing genetic variation, impacting the evolutionary trajectory of populations across space and time. Hybrid zones often form when previously isolated lineages come into contact and the impacts of these processes, including genetic drift and gene flow, likely play an important role shaping the genetic structure of a contact zone (Abbott, 2017.1; Barton & Hewitt, 1989). Shifts between isolation and contact can coincide with changes in effective population size (Ne) influencing the direction and extent of genetic exchange, while fine-scale variance at the population or genome-level can further influence elimination or fixation of alleles. In forest trees, genetic exchange often appears to be largely asymmetrical favouring the genomic background of one species due solely or in part to differential dispersal capacity, wind-patterns and unidirectional reproductive incompatibilities (El Mujtar et al., 2017.2; Hamilton et al., 2013a; Lepais et al., 2009; Lexer et al., 2005.2, 2006.1). Given that an influx of genetic variants via interspecific gene flow can increase Ne and generate novel genetic recombinants upon which natural selection acts, characterising genetic structure and the impact genetic exchange may have across space and time is needed to predict evolution. Demographic inference can be particularly useful for characterising processes underlying patterns of genetic exchange that can influence species evolutionary relationships, distributional shifts, or adaptive potential (Bacilieri et al., 1996; Petit et al., 2004.2). Several forest tree hybrid zones have characterised patterns of introgression, including oaks (Cannon & Petit, 2020; Eaton et al., 2015.2; Leroy et al., 2020.1; McVay et al., 2017.3), poplars (Chhatre et al., 2018.2; Christe et al., 2017.4; Lexer et al., 2005.2; Suarez-Gonzalez et al., 2016.2; Suarez-Gonzalez, Hefer, et al., 2018; Suarez-Gonzalez, Lexer, & Cronk, 2018.1) and spruce (Hamilton et al., 2013a, 2013.1b). However, few studies have leveraged whole genome sequencing to tease apart the temporal and spatial dynamics underlying the evolutionary history of contact zones using both biparentally and uniparentally inherited genomes. Viewing distinct evolutionary trajectories associated with biparentally and uniparentally inherited genomes through the lens of demographic change and landscape heterogeneity enables assessment of the role these evolutionary processes may play to the formation and maintenance of natural hybrid zones. Here, we use Populus, an ecologically and economically important model system in forest trees that naturally hybridise with congeners where geographical ranges overlap. Populus trichocarpa, the first tree to have its genome fully sequenced (Tuskan et al., 2006.2), has played an extensive role in our understanding of tree genome biology, comparative genomics and adaptive introgression (e.g. Jansson & Douglas, 2007; Shang et al., 2020.2; Suarez-Gonzalez et al., 2016.2). Natural hybrid zones exist between P. trichocarpa and P. balsamifera, largely associated with geographically and climatically steep transitions from maritime to continental climates west and east of the Rocky Mountains. However, divergence has likely involved a combination of intrinsic and extrinsic factors, including a dynamic history of population size change throughout glacial and interglacial periods, landscape-related barriers to gene flow, and climatically-related responses to selection (Geraldes et al., 2014; Keller et al., 2010.1; Levsen et al., 2012; Slavov et al., 2012.1). We compare whole-genome sequence data for biparentally (nuclear) and uniparentally (chloroplast) inherited genomes to quantify divergence and the evolutionary relationship between species across space and time. This will provide an understanding of the role neutral and non-neutral processes play in the formation of long-lived hybrid zones (Petit et al., 2005.3). Using a sampling design of latitudinally arrayed transects, we sequenced whole genomes for 576 trees to capture parental-types and hybrids across repeated zones of contact between P. trichocarpa and P. balsamifera distributed across their entire range of overlap. With these data, we observed genetic structure within and across each contact zone and asked (1) How has climate, geography and landscape-level barriers influenced genetic structure and (2) What is the history and extent of interspecific gene flow? These data provide new insights into how different evolutionary processes contribute to the formation and persistence of hybrid zones over varying spatial and temporal scales.
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