Professor of Biology
Department of Biology
University of California
Riverside, CA 92521

Office: 209 University Laboratory Building
Phone (951) 827-5820


Ph.D., University of Pennsylvania, 1980

UCR Appointment :
Assistant Professor II


FIBR Guppy Research

            2 May 2014
The following research program was initially funded by a multi-investigator grant from the Frontiers in Integrative Biological Research program at NSF:

            2006-2013   National Science Foundation (DEB-0623632EF).   FIBR: From genes to ecosystems: How do ecological and evolutionary processes interact in nature? 

David Reznick(PI, UC Riverside), Joseph Travis(Co-PI, Fl.State), Cathy Pringle(CP, U.Georgia), Douglas Fraser(CP, Sienna College), Regis Ferriere(CP, U. of Arizona), Michael Kinnison(Senior Personnel,U.of Maine), Alex Flecker(SP, Cornell), Cameron Ghalambor(SP, Colorado State), Jim Gilliam(SP, NC State), Andrew Hendry(SP, McGill U.), Paul Bentzen(SP,Dalhousee), Steve Thomas(SP,U. Nebraska), Don deAngelis(SP, USGS).  $5,128,002.

            It is now funded through the end of 2015 by a multi-investigator grant from the Evolutionary Biology program:

            2013-2015  National Science Foundation (DEB- 1258231): Experimental evolution in natural populations of guppies.  $996,855.  David Reznick (PI, UC Riverside), Joseph Travis (Co-PI, Florida State), Andres Lopez-Sepulcre (Co-PI, L’Ecole Normal Superieure, Paris), John Endler (Co-PI, Deacon University, Australia), Paul Bentzen (Co-PI, Dalhousie University).  REU Supplement.  $30,000, joint NSF-CAPES student exchange program.

            We have also executed a side project associated with this research funded by the National Geographic Society:

            2013-2014  National Geographic Society: Experimental studies of species invasions. $21,600. 

D. Reznick (PI, UC Riverside), Andres Lopez-Sepulcre (L’Ecole Normal Superieure, Paris), Joe Travis (Florida State University)

            Tim Coulson is a collaborator on this new grant and has recentely received a grant from NERC that supports the development of theory and empirical research associated with the themes of the research summarized below.  We have shifted our focus to making inferences about evolution and feedbacks between ecology and evolution from the on-going mark-recapture study in the four experimental streams described below.  New approaches and questions associated with this approach include using methods developed by Dr. Coulson and his collaborators to address why and when evolution happens. One focus will be on the potential role of sexual conflict in moderating the course of evolution.

Background and Description:
            Gordon Orians officially coined the terms “functional ecology” and “evolutionary ecology” as subdisciplines of ecology in 1962(Orians 1962).  He did so at a time when the dominant question in ecology was “what factors determine the abundance and distribution of organisms?” Functional ecology follows Andrewartha and Birch’s earlier argument for how we should come to understand where animals are found and how abundant they are. To understand distribution and abundance, we must understand the physiology of the organism in question, the nature of its physical habitat, and the abundance of the organism in different types of habitats.  The answer to this key question that defined ecology was thus seen as rooted in proximate factors.  Orians proposed “evolutionary ecology” as an alternative perspective that discriminates between proximate and ultimate causes of abundance and distribution.  The ultimate factor is evolution.  If we are to understand the abundance and distribution of an organism today, then we have to see it as being the product of a history of adaptation to its surrounding environment.
            Evolutionary ecology emerged in two different flavors.  The first can be characterized by the title of G.E. Hutchinson’s famous essay: “The ecological theater and the evolutionary play” (1965)(Hutchinson 1965) where he argued that the environment defines a template shaped by all of the organisms that interact with the organism in question.  Evolution shapes an organism to best fit into that template. The associated theory models ecological interactions in a fashion that implicitly treats organisms as if they did not evolve because it treats them as constants. Evolution is seen as something that happened in the past.  The justification for this perspective is that the pace of evolutionary change is assumed to be orders of magnitude slower than the pace of ecological interactions.
            Hutchinson’s perspective was very much the same as that proposed by Darwin in ch. 3 of the Origin of Species, entitled, “The Struggle for Existence”.  Darwin observed that all organisms have the capacity for exponential population growth, but this capacity was almost never realized. We instead most often see species abundances remaining within some relatively narrow range of values that is constrained by biotic interactions.  There was thus a natural connection to be made between density dependent population regulation and evolution by natural selection; it was the ability of some phenotypes to be more successful in surviving and reproducing in the face of these interactions that caused evolution. 
            David Pimentel (1961, 1968)(Pimentel 1961; Pimentel 1968) instead envisioned ecology and evolution as two actors that interact with one another during the course of a play. 

Density influences selection; selection influences genetic make-up; and in turn, genetic make-up influences density. The actions and reactions of the interacting populations in the food chain cycling in this mechanism result in the evolution and regulation of animal populations. (Pimentel 1961)

            Pimentel’s concept of “population regulation and genetic feedback” thus emphasized an ongoing interaction between organisms involved in a diversity of pairwise exploitative interactions (predator-prey, host-parasitoid, host-pathogen, competitors, plant-herbivore).  He envisioned each species involved in such an interaction as evolving in a fashion that would result in the dampening of population cycles and an approach to the relative stability of organismal abundances.  He argued for frequency dependent and density dependent selection as the mechanisms that underlie these interactions and the approach to population stability.  Implicit in his argument is the expectation that evolution can happen on a time frame that is similar to ecological interactions.
            At this juncture, it is fair to ask what difference it makes to adopt Hutchinson’s vs. Pimentel’s perspective?  The answer, as has been shown in some experiments and in theory, is that the outcome of ecological interactions can be radically different if the interacting organisms can evolve during the course of those interactions. 
            Hutchinson’s and Pimentel’s perspectives were each defined by emergent theory and the development of empirical approaches to test theory in the lab and nature.  Hutchinson’s perspective blossomed into what we now refer to as evolutionary ecology and grew to dominate the field.  Pimentel’s perspective languished, but is now re-emerging in the form of new types of theory and associated experiments.  There are important distinctions between the two.   The theory and a small body of empirical evidence associated with Pimentel’s perspective bear out Pimentel’s original claim - if evolution is indeed a part of ongoing ecological interactions, then the outcome of those interactions can be fundamentally different from what is predicted from theory that does not formally incorporate evolution.  If Pimentel is right and if we can succeed in fully integrating ecology and evolution, then there is the promise that we can improve the predictive quality of ecology. 
            My multi-investigator project on the interaction between ecology and evolution in natural populations is designed to evaluate the possible role of such interactions in natural ecosystems.   You can find a detailed description of the project, the personnel involved, and our progress in the form of publications and progress reports at <cnas.ucr.edu/guppy>.  The website includes an extensive video menu that details the methods and rationale for the research.  Here I offer just a brief description of the project.
               Our project represents a partnership of ecosystems ecologists, evolutionary/population biologists and evolutionary theoreticians.  The project is built around the experimental introduction of guppies into four focal streams, short-term factorial experiments in artificial streams and complementary comparative studies done in natural streams.
Background: The Northern Range Mountains of Trinidad offer a natural laboratory for studying evolution in action. The rivers draining these mountains flow over steep gradients punctuated by waterfalls that separate fish communities. Species diversity decreases as waterfalls block the upstream dispersal of some species. The succession of communities is repeated in many, parallel drainages, providing us with natural replicates. Guppies are found in a diversity of habitats throughout this succession of communities. In the downstream localities they occur with a diversity of predators, which prey on adult size classes of guppies (high predation, or HP). Waterfalls often exclude predators but not guppies, so guppies found above waterfalls have greatly reduced risks of predation and increased life expectancy (low predation, or LP). The only other fish found in these localities rarely preys on guppies and, when it does, preys on small, immature size classes(Haskins, Haskins et al. 1961; Endler 1978).
Mark-recapture studies on natural populations revealed that HP guppies experience substantially higher mortality rates than LP guppies (Reznick, Butler M. J. et al. 1996).  Predator-induced mortality represents a likely form of selection for the differences in life history, phenotype and behavior. Life history theory predicts that, in high predation environments natural selection should favor those individuals that begin to reproduce at an earlier age and devote more resources to reproduction.  My first goal was to see if there were differences in the life histories of guppies from HP and LP environments that were consistent with these predictions.
I characterized how guppies adapt to either HP or LP communities, with paired comparisons between guppies from HP and LP environments in different river drainages. HP guppies mature at an earlier age and devote more resources to reproduction, as predicted by life history theory.  In addition, they produce more offspring per litter and produce significantly smaller offspring than LP guppies (Reznick and Endler 1982; Reznick, Rodd et al. 1996).  There is a diversity of other adaptive differences between guppies from HP and LP environments.  HP and LP guppies differ in male coloration (Endler 1978), courtship behavior (Houde 1997), schooling behavior (Seghers 1974; Seghers and Magurran 1995), morphology (Langerhans and DeWitt 2004), swimming performance (Ghalambor, Reznick et al. 2004), and diet (Zandonà, Auer et al. 2011). Laboratory studies confirm that all of the life history differences between HP and LP guppies have a genetic basis(Reznick 1982; Reznick and Bryga 1996). Genetic analyses imply that HP guppies invade guppy free environments then evolve into LP phenotypes and that each river represents an independent replicate of this process(Alexander, Taylor et al. 2006).
Rivers can be treated like giant test tubes, since fish can be introduced into portions of stream bracketed by waterfalls, creating in situ experiments (Endler 1978; Endler 1980).When guppies were transplanted from HP environments below a barrier waterfall to previously guppy-free environments above a waterfall, delayed maturation and reduced reproductive allocation evolved, with some changes happening in four years or less (Reznick and Bryga 1987; Reznick, Bryga et al. 1990; Reznick, Shaw et al. 1997). Other attributes, including male coloration(Endler 1980) and behavior (O'Steen, Cullum et al. 2002) also rapidly evolved. These results argue that the presence or absence of predators imposes intense selection on many features of guppy phenotypes.  They also show that evolution by natural selection happens on a time frame that is similar to ecological interactions.  This very rapid evolution adds some justification for considering Pimentel’s perspective of the relationship between ecology and evolution.
However, differences in predation are confounded with differences in population biology. Guppy HP populations are found at lower population densities, are dominated by small, young fish, and have higher individual growth rates than LP populations. These differences in population structure are most likely attributable to indirect effects of predators, which reduce guppy population densities, and increase food availability to the survivors.  For these and other reasons, it became reasonable to ask whether the way guppies adapted to LP environments might be in part adaptation to their impact on the structure of the environment.
We have also obtained some results that are not consistent with traditional life history theory and suggest that ecological interactions might be important.  One such result was our finding that high predation guppies do not begin senescence at an earlier age and have shorter life spans than low predation guppies, as one might predict based on their early life history(Reznick, Bryant et al. 2004).  HP guppies are younger at maturity, have higher levels of reproductive effort throughout their lives, have lower mortality rates and longer lifespans than low predation guppies when the two are compared in a uniform laboratory environment.  Such results suggest that the HP phenotype is unconditionally superior to the LP phenotype and that the LP phenotype should never evolve, yet it does so predictably when guppies are transplanted from HP to previously guppy free LP localities.  The only models for the evolution of senescence that can be reconciled with such results are ones that include density regulation and/or indirect effects of predation(Abrams 1993; Charlesworth 1994; Williams and Day 2003).

Focal Streams:  We created a new series of introduction experiments in 2008-9. We initiated four replicate introduction experiments in which we transplanted guppies from a single HP locality to four previously guppy-free localities. These experiments recreate the scenario of HP guppies invading and adapting to a predator-free habitat. HP guppies initially invade at low densities, so they have abundant food resources and select the highest quality prey, leading to dramatic population growth. From here, an LP phenotype may evolve as a direct response to reduced mortality and/or as an indirect response to increased population density and reduced resource availability.
We overlay these introductions with an experimental manipulation. The four focal streams consist of two pairs. One member of each pair has an intact forest canopy and one has a thinned canopy.  We thinned the canopy to manipulate resource availability independently of predation; thinning increases light penetration and primary production. We have since confirmed that thinning has had this effect in three ways.  Primary productivity and the biomass of invertebrates is higher under the thinned canopies.  Guppy growth rates, asymptotic body sizes and body condition are also consistently higher under the thinned canopies. If risk of mortality is the sole cause of guppy evolution, then thinning should have no effect. If, instead there is an interaction between mortality risk and resource availability, the course of evolution should differ between intact and thinned canopies and selection should fluctuate with resource availability.  Our first two years of data show evolution of male coloration, morphology and age of maturation and fluctuating selection and trade-offs between viability and fertility selection. 
   Our collaborators have generated a time-series description of the ecosystem in the guppy introduction site and in a control section of stream upstream from where the guppies were introduced, beginning one year before the guppy introduction.  Thus far, we have found that guppies significantly deplete the abundance of invertebrates in the introduction site. 
   Other collaborators have performed a time series, mark-recapture study of the Rivulus populations in the introduction and control sites. Matt Walsh, a recent PhD student, established that guppies cause Rivulus life histories to evolve. His results yield the prediction that they do so via their indirect effect on Rivulus abundance.  In past introduction sites (experiments that are now >25 years old), we found that the presence of guppies was associated with reduced Rivulus abundance and increased individual growth rates.  Matt’s results imply that Rivulus that co-occur with guppies have adapted to increased resource availability.  In our focal stream experiments, we have documented a significant reduction in Rivulus abundance and shift in their size distribution in response to the guppy introduction after only two years.
            Our experiment was designed to generate resources for the future study of the genetics of adaptation.  Guppies were collected as juveniles from a single locality where they live with a diversity of predators (high predation environments).  They were reared to maturity in single sex groups, then mated before being introduced.  All introduced fish were individually marked.  Scales were kept from each of them to provide a source of DNA.  All four populations are intensively censused once a month so that we can follow individual growth and movements.  All new recruits are individually marked and scales are collected. All individuals are photographed every time they are caught. The photographs provide a source of data for characterizing growth rate, shape and male coloration.  We have genotyped all individuals at 12 hypervariable, tetranucleotide microsatellite loci and have been able to reconstruct the pedigrees of four replicate, evolving populations.  We archive around 80% of the DNA from each scale sample.
            Detlef Weigel and Christine Dreyer, of the Max Plank Institute for Evolutionary Biology (Tuebingen) are independently studying the genetics of adaptation in guppies. As part of their work, they are generating a complete genome sequence for guppies from the same population that we used to initiate our experiments. The sequence is being annotated with expressed sequences derived from a diversity of tissues. They are also generating tens of thousands of SNP markers.

Artificial Streams:  We constructed 16 artificial streams alongside a natural stream in Trinidad.  We diverted the flow from a natural spring into a holding tank, then gravity feed the water into the channels. The water then flows into the natural stream that was the original recipient of the spring outflow.  With this arrangement, we can set up replicated, factorial experiments in streams that are a good facsimile of natural streams.  These experiments are all parameterized on what we know to be naturally occurring population densities of guppies. We also monitor guppy growth rate so that we can confirm that the performance of guppies in these streams falls within the range of values that we see in natural streams. In other ways we can confirm that the streams accurately represent nature. 
            We have shown that guppies do indeed have a significant impact on the structure of their ecosystem.  Furthermore, we have found that high and low predation guppies differ in their impact on the ecosystem, largely because of their different diets(Bassar, Marshall et al. 2010).  Guppies from high predation environments prey selectively on high quality invertebrate prey(Zandonà, Auer et al. 2011).  Those from low predation environments instead consume invertebrates, algae and detritus in an unselective fashion.  These results represent the first part of the interaction between ecology and evolution; guppies change their ecosystem, both in these artificial streams and the focal streams.  The second part of the cycle requires proving that the evolution of the low predation phenotype is at least in part driven by the way guppies change the ecosystem.  This second phase defines our current research goals.


Literature Cited

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Zandonà, E., S. Auer, et al. (2011). "Diet quality and prey selectivity correlate with life histories and predation regime in Trinidadian guppies." Functional Ecology in press.



For more information, images, and videos on the Guppy Project, please visit: www.cnas.ucr.edu/guppy