Numerous new sections on important topics such as the evolutionary impact of human activities, taxonomic challenges, gene flow and distribution, hybridisation, speciation and extinction, conservation and the molecular genetic basis of breeding systems will ensure that this remains a classic text for both undergraduate and graduate students in the field.
Toon meer Toon minder. Recensie s Review of previous edition: ' Briggs and Walters has mentored students for over four decades and is as balanced and relevant today as it was in when the first edition was released. The book has grown with the science it describes, but it remains true to the original aim with thoughtful explanations and clear writing.
Its greatest gift to the readers is the understanding it provides of how scientific knowledge develops through observation, experiment, analysis and the hard work of insight. The combination of the history of the investigation of plant variation and evolution and up-to-date results and interpretation is extremely valuable. It has my highest recommendation. Almost fifty years after the publication of the 1st edition, David Briggs updates this classic with a 4th edition for another generation of biologists.
Significant amounts of both revised and new content integrate advances in molecular and analytical tools that have changed our ability to address basic evolutionary questions and to imagine new ones. This edition continues the tradition of explaining ideas succinctly while generously referencing the primary scientific literature.
The result is the rare kind of book that is accessible to anyone curious, but also serves as an essential reference in the field. The conjunction of these two names is now virtually synonymous with 'plant variation and evolution,' a topic of study that is as deep - and rich - an area of inquiry as any in the biological sciences. Indeed, in this latest edition of a benchmark scientific work,we hear echoes of the voices of critical figures such as Charles Darwin, Gregory Mendel, as well as George Ledyard Stebbins, whose own synthetic Variation and Evolution in Plants written in sets the historical backdrop to this great work of synthesis intended for the twenty-first century reader.
For many people, and I include myself in this, this textbook has been an important part of our growth as a plant geneticist. I was using the second addition as an undergraduate, then the third edition was heavily cited in my thesis for graduate school, now this fourth edition will undoubtedly become core reading for the undergraduates in my courses and my graduate students. Simulation data are available as Supporting Information - Appendix S3.
In this manuscript, we synthesize recent research that examines intraspecific variation in seed dispersal and its implications for plant ecology to evaluate our current understanding and to recommend avenues for future research to fill remaining knowledge gaps. We do not discuss what causes rapid changes in trait variability in plants in any detail, but instead refer interested readers to Johnson et al. Then, we discuss the consequences of intraspecific variation in seed dispersal for local population dynamics, spatial spread, community structure and dynamics, and evolution, and argue that this intraspecific variation in dispersal is not simply adding noise, but altering dispersal processes and patterns.
To conclude, we discuss intraspecific variation in seed dispersal within the context of anthropogenic global change and suggest directions for future research. Intrinsic and extrinsic factors influence the seed dispersal process and variability in these factors contributes to intraspecific variability in dispersal Box 1 , see Schupp, this issue, for a detailed review. Approximately one quarter of trait variability within plant communities exists within species i. We highlight intraspecific variability in four types of traits that are known to underlie intraspecific variability in dispersal: fruit and seed size, fruit and seed crop size, plant height and dispersal-specific structures.
We also briefly introduce several extrinsic factors that can cause intraspecific variation in dispersal.
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Variation in these intrinsic and extrinsic factors potentially has significant consequences for plant demography and community composition through its impacts on number of seeds dispersed, the seedscape in which seeds land and the frequency of long-distance dispersal events. Fruit and seed size are highly variable both within and among individual plants Michaels et al.
For abiotic dispersal, size influences dispersal distance as smaller seeds are typically dispersed further by water e. Delefosse et al.
Skarpaas et al. For endozoochorous and synzoochorous where animals intentionally transport seeds without ingestion dispersers, variation in fruit diameter and seed size can affect how many and which disperser species are able to feed on an individual plant Galetti et al. Individual variation in fruit and seed size and individual variation in the traits of the dispersal agents interact to mediate the realized disperser assemblages of each fruits.
This interaction in intrinsic and extrinsic variability has consequences for seed dispersal Zwolak Individual variation in fecundity has important implications for both long-distance dispersal and number of seeds dispersed, particularly in plants with wind or endozoochorous dispersal Jordano and Schupp ; Norghauer et al. For wind-dispersed plants, highly fecund individuals tend to have longer maximum dispersal distances because increasing the number of seeds released increases the probability of some seeds catching rare updrafts that result in long-distance dispersal Nathan et al.
Similarly, larger crop sizes for endozoochorous dispersal may also increase the probability of rare, long-distance dispersal events by animals e.
Prunus mahaleb trees, Jordano and Schupp The consequences of individual variation in fecundity have not, to our knowledge, been explored for other dispersal modes, but are potentially important with any dispersal system since increasing crop size increases the number of dispersal events and thus the probability of a rare long-distance dispersal event.
Plant height explains much of the variation in dispersal distance across plant species Thomson et al. Together with diaspore terminal velocity and seed abscission, seed release height is a key phenotypic driver explaining individual variation in dispersal distances of abiotically dispersed plants Thiede and Augspurger ; Wender et al. For endozoochorous trees, preferential foraging of frugivores at different canopy heights raises the possibility that differences in height may influence the frugivore assemblage to which fruits are exposed Poulsen et al.
Intraspecific variation in specialized structures that aid in seed dispersal can also cause intraspecific variation in dispersal kernels. In wind-dispersed plants, pappus and wing morphology can affect seed falling velocity Riba et al.
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The quantity of low-density tissues in water-dispersed fruits and seeds can affect buoyancy, which may affect dispersal distances Guja et al. For fleshy-fruited plants, fruits with relatively higher energetic rewards e. For fruit dispersed by epizoochory, there is variation in the presence, size and number of appendages that enable mechanical interlocking with animal fur Gorb and Gorb or variation in the degree of heterocarpy, in which individual plants produce morphologically distinct diaspores Monty et al.
However, the impact of this variation on dispersal has not yet been tested for this dispersal mode. Intraspecific studies are also rare among synzoochorous species e. Smallwood et al. These same traits also mean that they are consumed at higher rates, with less perishable seeds cached more frequently reviewed by Vander Wall ; Lichti et al. For example, the interaction between abiotic dispersal vectors and the landscape structure can cause intraspecific differences in water dispersal due to local flow patterns Van der Stocken et al.
Animal-dispersed plant species are impacted by individual variation among seed dispersers reviewed in Zwolak and these may interact with intrinsic factors, such as fruit and seed size as discussed above. Animal behaviour and the plants surrounding a focal plant can also interact with the local fruiting neighbourhood impacting dispersal probabilities and distances Blendinger et al. Finally, differing impacts of anthropogenic drivers across space also causes within-species variation in dispersal; for example, habitat fragmentation can influence frugivore movement patterns and thus dispersal distances Levey et al.
Intraspecific variation in seed dispersal can affect demography by influencing vital rates i. For example, variation in seed dispersal distance can lead to variation in plant survival and growth as some seeds may escape mortality due to natural enemies Janzen or experience reduced competition from siblings Cheplick Variation in how seeds are dispersed can also lead to variation in survival depending on the time and treatment of seeds passing through the gut for endozoochorous species Traveset et al.
Plant variation and evolution
It is critical to recognize the effect of this variation, as these vital rates determine population growth. In addition, individual variation in dispersal can affect metapopulation processes by impacting the frequency of movement, genotypes and traits of individuals that move between populations Cheptou et al. Overlooking intraspecific variation in dispersal can impact conclusions of local population persistence in several ways, particularly in changing environments. First, individual variation in dispersal may impact projections by population matrix and integral projection models.
In these models of local population dynamics, dispersal is rarely considered explicitly see next section for discussion of population spread , but instead subsumed into the various factors affecting the transitions from seed to seedling, or seedling to sapling. Because single individuals can contribute large portions of new recruits in plant communities Wheelwright ; Minor and Kobe , estimated population-level recruitment may change substantially as individual composition changes e.
In addition, demographic models that do not explicitly consider dispersal are unable to forecast how altered dispersal processes i. Models that more mechanistically consider how dispersal and the deposition environment impact growth and survival can incorporate these processes and project population trajectories under altered dispersal e. Caughlin et al. A few phenomenological and mechanistic models do explicitly address how dispersal influences local population dynamics e. Godinez-Alvarez and Jordano ; Brodie et al. This is an important first step in understanding the importance of dispersal.
Next, researchers should examine how using the mean values related to dispersal e. For example, when estimating dispersal distances based on trait allometries e. Norghauer et al. In particular in small populations, individual variation in dispersal can cause population-level patterns of dispersal to differ significantly from expectations based on mean values Lewis and Pacala Simulations that explicitly include intraspecific variation are not equal to models that use the mean and variability of the population Box 3 , with the largest consequence for populations occupying habitats located far away from sites suitable for establishment.
Overall, there is a need for demographic studies to include dispersal explicitly and to explore how and when intraspecific variation in dispersal affects local population dynamics. Simulation setup: A m-long wrapped transect i. The initial population was restricted to a contiguous 80 m, centred either at 0 yellow , 80 grey or dark grey. Simulation results: In the Uniform environment, the results are the same regardless of the starting location.
All simulations used identical rules for individual growth, fecundity and survival. Seeds were dispersed from individual maternal plants according to Gaussian kernels, with a population-level mean dispersal distance of m. Thus, while the population-level mean dispersal distance was identical in both cases, the intraspecific variation scenario had more plants dispersing seeds to short distances while a few plants are dispersing seeds much further. Seedling establishment probabilities and initial population locations also varied by scenario. While the final population size was unaffected i.
Data are available as Supporting Information-Appendix S4. Explaining historical range expansions and predicting future vegetation migration rates is a fundamental question in global change biology and invasion ecology Clark et al. As traditional Gaussian dispersal kernels required an unrealistically large mean dispersal distance to match historical spread rates i.
In contrast to their Gaussian counterparts, these kernels have higher probabilities of short-distance dispersal events, creating a more peaked distribution, and higher probabilities of rare, long-distance dispersal events, creating a fatter or thicker tail Box 4. These dispersal kernels preserve the mean distance travelled by a seed, but lead to faster rates of either constant or ever-accelerating spatial spread Box 4. The Gaussian thin-tailed , Laplacian leptokurtic and log square root leptokurtic and fat-tailed dispersal kernels plotted on the left-hand side represent the density of seeds dispersed a given distance north or south from its parent.
The spatial distribution of the corresponding populations over a year period are shown to the right. Leptokurtic dispersal kernels, such as the Laplacian and log square root, result in faster spread rates than the Gaussian dispersal kernel. Exponentially bounded dispersal kernels, such as the Gaussian and Laplacian, result in asymptotically constant spread rates, whereas distributions that are not exponentially bounded i.
All dispersal kernels have a mean dispersal distance of 0. These population-level leptokurtic kernels could arise due to intraspecific variation in seed dispersal Box 2 , and have been the focus of numerous studies Neubert and Caswell ; Petrovskii and Morozov ; Bouin et al. For example, Neubert and Caswell studied spatial spread of the Neotropical Calathea ovandensis due to dispersal by four ant species. However, the inclusion or removal of P. Horvitz et al. Using frugivore-stratified spread models, they showed that it was the infrequent but longer-distance dispersal by robins that determined rates of spatial spread.
As illustrated in Box 2 , continuous variation in the diffusion coefficient D i. The magnitude of this increase, however, depends subtly on the nature of the distribution underlying the individual variation.