Photo: Wild Mustard, southwest Montana. © 2020 Delena Norris-Tull
The Role of Hybridization in Biological Invasions
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
Ellstrand and Schierenbeck (2000) point out that it is very challenging to determine what causes invasive success for several reasons: very few species become invasive in new environments; many species that become successful invaders only do so after an extended period of time in the new environment; and many only become invasive after being introduced to the new environment multiple times. They propose an evolutionary model to help explain invasive success. “Hybridization between species or between disparate source populations may serve as a stimulus for the evolution of invasiveness.” They reviewed a number of cases in which, “hybridization preceded the emergence of successful invasive populations.” Species with a history of hybridization may have some genetic benefits. A number of the cases they reviewed had significant time lag (many tree and shrub species had been in the new environment more than 100 years, and it is likely that many herbaceous species had been introduced over decades) and had experienced multiple introductions. These conditions may be necessary in order for previously isolated introduced species to come into contact with other populations, and for hybridization to occur.
Ellstrand and Schierenbeck, 2000, do not believe that this model fits all invasive species, but that it may be a useful mechanism to consider in future research. They point out that many hybridization events do not result in invasiveness. They propose that for many species, invasiveness may not be inherent when the species arrive in the new environment, but that evolutionary pressure can result in hybridization that may facilitate invasiveness.
Ellstrand and Schierenbeck, 2000, point out that the fact that some species are readily controlled by biological agents brought over from their home environment, is an indicator that evolution occurring after introduction is not a factor for those species. But the fact that various invasive species do not respond well to biocontrol, presents the possibility that evolutionary changes occurring within the species within the new environment are an important factor, particularly among plant species. Many native and non-native plant populations that have previously been isolated have been shown to rapidly hybridize when they are no longer isolated. And, due to the ability of numerous plant species to undergo polyploidy (in which the chromosomes of both parents are combined), plants more readily hybridize across species lines, than do animal species, although some animal and pathogen hybrids have also resulted in invasive species. Polyploidy in plant species that have hybridized with related species have created offspring so genetically divergent from either parent species, that the offspring represent a genetically new species.
To examine support for their hypothesis, Ellstrand and Schierenbeck, 2000, examined a large number of cases of plant hybridization, and they found that plant hybrids are more common among perennial herbaceous plants than among annuals or woody species, and their paper provided some of the related research that helps explain this tendency. They presented 25 cases of well documented plant hybridization, including both invasive and non-invasive species, to exemplify their hypothesis. Some were hybrids of divergent populations of the same species, and some were related species. The hybrids that have become successful invasive species tended to have one or more of the following genetic advantages in the invaded environment: the development of woodiness, making the hybrid more challenging to remove from an agricultural environment; greater tolerance to climatic ranges, such as temperature tolerance; the ability to out-compete the parent populations; a novel set of traits not present in the parent populations, that enables the hybrid to move into new niches not occupied by the parents; increased genetic diversity; fixed heterosis (new traits that perform better than in either parent population).
Ellstrand and Schierenbeck, 2000, point out that the following anthropogenic (human-induced) activities “could enhance both the likelihood of hybridization and the likelihood of creating new niches that favor hybrid derivatives.”
Unfortunately, as with much of the research on invasive species, not enough funding has been available to thoroughly assess their hypothesis.
Reference:
Next Sections on research on the success of invasive species:
The Role of Hybridization in Biological Invasions
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
Ellstrand and Schierenbeck (2000) point out that it is very challenging to determine what causes invasive success for several reasons: very few species become invasive in new environments; many species that become successful invaders only do so after an extended period of time in the new environment; and many only become invasive after being introduced to the new environment multiple times. They propose an evolutionary model to help explain invasive success. “Hybridization between species or between disparate source populations may serve as a stimulus for the evolution of invasiveness.” They reviewed a number of cases in which, “hybridization preceded the emergence of successful invasive populations.” Species with a history of hybridization may have some genetic benefits. A number of the cases they reviewed had significant time lag (many tree and shrub species had been in the new environment more than 100 years, and it is likely that many herbaceous species had been introduced over decades) and had experienced multiple introductions. These conditions may be necessary in order for previously isolated introduced species to come into contact with other populations, and for hybridization to occur.
Ellstrand and Schierenbeck, 2000, do not believe that this model fits all invasive species, but that it may be a useful mechanism to consider in future research. They point out that many hybridization events do not result in invasiveness. They propose that for many species, invasiveness may not be inherent when the species arrive in the new environment, but that evolutionary pressure can result in hybridization that may facilitate invasiveness.
Ellstrand and Schierenbeck, 2000, point out that the fact that some species are readily controlled by biological agents brought over from their home environment, is an indicator that evolution occurring after introduction is not a factor for those species. But the fact that various invasive species do not respond well to biocontrol, presents the possibility that evolutionary changes occurring within the species within the new environment are an important factor, particularly among plant species. Many native and non-native plant populations that have previously been isolated have been shown to rapidly hybridize when they are no longer isolated. And, due to the ability of numerous plant species to undergo polyploidy (in which the chromosomes of both parents are combined), plants more readily hybridize across species lines, than do animal species, although some animal and pathogen hybrids have also resulted in invasive species. Polyploidy in plant species that have hybridized with related species have created offspring so genetically divergent from either parent species, that the offspring represent a genetically new species.
To examine support for their hypothesis, Ellstrand and Schierenbeck, 2000, examined a large number of cases of plant hybridization, and they found that plant hybrids are more common among perennial herbaceous plants than among annuals or woody species, and their paper provided some of the related research that helps explain this tendency. They presented 25 cases of well documented plant hybridization, including both invasive and non-invasive species, to exemplify their hypothesis. Some were hybrids of divergent populations of the same species, and some were related species. The hybrids that have become successful invasive species tended to have one or more of the following genetic advantages in the invaded environment: the development of woodiness, making the hybrid more challenging to remove from an agricultural environment; greater tolerance to climatic ranges, such as temperature tolerance; the ability to out-compete the parent populations; a novel set of traits not present in the parent populations, that enables the hybrid to move into new niches not occupied by the parents; increased genetic diversity; fixed heterosis (new traits that perform better than in either parent population).
Ellstrand and Schierenbeck, 2000, point out that the following anthropogenic (human-induced) activities “could enhance both the likelihood of hybridization and the likelihood of creating new niches that favor hybrid derivatives.”
- Bringing together previously isolated populations. Modern, rapid and long-distance transportation, and large-scale agricultural activities accelerate these events.
- Opening new niches, due to ecological disturbance.
Unfortunately, as with much of the research on invasive species, not enough funding has been available to thoroughly assess their hypothesis.
Reference:
- Ellstrand, N.C., & Schierenbeck, K.A. (Jan. 27-29, 2000). Hybridization as a stimulus for the evolution of invasiveness in plants? Paper presented at the National Academy of Sciences, Irvine, CA. Reprinted in Euphytica (2006), 148, 35-46.
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