Statement of Research

  ‘The challenge of studying adaptive variation in nature is that one has to know so much about the biology of the organism. Thus, it would seem that the second phase of the Dobzhanskian project, to show that genetic differentiation has occurred by natural selection, seems to evade us.’   Lewontin (1997, p. 353)  
The influential studies of Drosophila carried out by Dobzhansky and his colleagues illustrated the existence of adaptive genetic variation both within and between populations (Dobzhansky and Epling 1944; Dobzhansky and Levene 1948). Similarly, experiments conducted by researchers at the Carnegie Institution demonstrated that plant populations display marked local adaptation (Clausen, Keck, and Hiesey, 1940; Hiesey, Nobs, and Björkman 1971). Subsequent studies in both plants and animals provided abundant proof that the morphological and physiological differences observed between species and populations are often ecological adaptations (Antonovics and Bradshaw 1970; Schemske 1984; Endler 1986; Schluter and McPhail 1992).
 
The goal of my research is to characterize the mechanisms of adaptation. This requires information on both the ecological significance of putative adaptive traits as well as an understanding of their genetic basis. Such comprehensive studies are extremely difficult to accomplish; thus it is perhaps not surprising that our current knowledge of adaptation is inadequate. In a recent review of the literature, Orr and Coyne (1992) found only eight studies that had identified both the adaptive value and the genetic basis of differences between species in nature.
 
A central theme of my work is the link between temporal and spatial variation in ecological conditions and the adaptive differentiation of populations and species. I rely on ecological and genetic approaches to investigate the origin and maintenance of biological diversity. Such complex problems often require interdisciplinary solutions. I have established a number of rewarding collaborations that have greatly expanded the scope of my research program. For example, my work with Dr. H. D. Bradshaw, Jr. has introduced genetic mapping approaches to the field of population biology, demonstrating a powerful new approach to the study of natural populations. In the future, we plan to establish interdisciplinary training programs that will bring together researchers from a variety of fields. My long-term goal is to promote the study of adaptation at all levels, from the gene to the population. This research agenda takes its cue from the pioneering efforts of Clausen, Keck and Hiesey, whose collaborations in the mid-20th century provided a framework for the field of population biology. I have endeavored to follow their lead, bringing new technology to bear on the same, fundamental questions: How do organisms adapt to their environment?
 
The following links take you to brief discussions of my specific research interests and the abstracts from selected papers in each area.
 

 
I. What is the genetic architecture of adaptation?
 
Although the evidence for adaptation is overwhelming, few studies have described the genetic basis of adaptive traits (Orr and Coyne 1992). Fisher’s ‘infinitesimal’ model of evolution proposed that adaptation is due to the fixation of many genes with small individual effects, and is based on the assumption that large effect mutations will have negative pleiotropic effects, and will therefore move a population away from its phenotypic optimum (Fisher 1930). Similarly, Lande (1983) demonstrated that adaptation would usually result from micromutation unless selection is strong and persistent. This micromutationist view of ‘adaptive geometry’ (Barton 1998) has had widespread support, but was challenged recently by Orr (1998) who suggested that mutations of large effect can often be beneficial during the early stages of adaptation as populations move towards their optimum phenotype. With this assumption, he found that the distribution of factors fixed during the approach to a stationary phenotypic optimum increased roughly according to a negative exponential distribution.
 
Molecular techniques for identifying the genetic basis of quantitative traits have only recently been applied to natural systems (Bradshaw et al. 1995; Lin and Ritland 1997; Bradshaw et al. 1998; Schemske and Bradshaw 1999). Yet there are still too few empirical studies of natural populations to resolve the debate over the genetic basis of adaptation, and it is therefore important to identify systems where both the genetic basis and ecological significance of adaptive traits can be identified (Orr and Coyne 1992).

 
I(a) Pollinator-mediated selection and the evolution of reproductive isolation in monkeyflowers.
 
Pollinator-mediated selection on floral traits is widely regarded as a common mechanism of adaptation in plants. In collaboration with H. D. Bradshaw, Jr., I have studied the genetic basis of reproductive isolation due to differences in floral morphology between the bumblebee-pollinated monkeyflower Mimulus lewisii and its hummingbird-pollinated congener M. cardinalis. M. lewisii has pale pink flowers with contrasting yellow nectar guides, a wide corolla with inserted anthers and stigma, a small volume of nectar, and petals thrust forward to provide a landing platform for bees. Mimulus cardinalis has red flowers, a narrow tubular corolla, reflexed petals, a large nectar reward, and exserted anthers and stigma to contact the forehead of hummingbirds. Where the two Mimulus species are sympatric at mid-elevations in the Sierra Nevada of California, hybrids are rarely observed despite that fact that M. lewisii and M. cardinalis are interfertile. The low frequency of hybrids in the zone of sympatry is due primarily to pollinator preference; direct observations show that nearly all pollinator visits to M. lewisii are by bumblebees, and hummingbirds are virtually the only pollinators of M. cardinalis.
 
To elucidate the molecular genetic architecture of reproductive isolation by pollinator-mediated selection required three experimental elements:
1. Develop a comprehensive molecular marker linkage map in a large F2 population derived from interspecific F1 hybrids in order to search for quantitative trait loci (QTLs) governing floral characters likely to be involved in differential pollinator visitation and pollen transfer by bumblebees and hummingbirds.
2. Quantify pollinator visitation to F2 plants in the field at a location where the parental species are sympatric, and map QTLs controlling differential rates of visitation by bumblebees and hummingbirds.
3. Introgress (by molecular marker-assisted recurrent backcrossing) QTLs with major effects on differential rates of pollinator visitation into the "opposite" genetic background; i.e., single QTL alleles from M. cardinalis are introgressed into a M. lewisii background, and vice versa. 
   
 
Bradshaw, H. D., Jr., K. G. Otto, B. E. Frewen, J. K. McKay and D. W. Schemske. 1998. Quantitative trait loci affecting differences in floral morphology between two species of monkeyflower (Mimulus). Genetics 149:367-382.  
 
Schemske, D. W. and H. D. Bradshaw, Jr. 1999. Pollinator preference and the evolution of floral traits in monkeyflowers (Mimulus). Proceedings of the National Academy of Sciences 96:11910-11915.
 

 
II. What is the role of random genetic drift in adaptation?
 
Many of the early architects of the Modern Synthesis maintained that adaptive evolution occurred principally by natural selection within populations (Fisher 1930; Haldane 1932; Mayr 1942). This view was perhaps best exemplified by Fisher, who believed that adaptation resulted from the fixation of many individually favorable mutations, each with small phenotypic effects. In contrast, Sewall Wright suggested that adaptation might often involve the stochastic process of genetic drift in addition to natural selection (Wright 1931).

 
II(a) Fine-Scale Genetic Differentiation in Linanthus: Isolation by Distance or Natural Selection?
 
Wright found support for his theory of "isolation by distance" (Wright 1943), from studies of a flower color polymorphism in the annual plant Linanthus parryae. In collaboration with Dr. Paulette Bierzychudek, I have conducted long-term field studies of several L. parryae populations in the Mojave Desert. We find that flower color in this species is subject to selection that varies in space and time. The temporal pattern of selection is best illustrated by the 11 years of study at Pearblossom site 1, where each color morph had a significant advantage in three years, and in only one of the seven years with appreciable plant densities was there no difference in fitness between morphs. Based upon theoretical work conducted by Michael Turelli, we find that fluctuating selection, coupled with a long-lived seed bank, can explain the maintenance of the polymorphism at this site (Turelli, Schemske and Bierzychudek, in press).
 
The spatial pattern of selection was revealed in field investigations demonstrating that a sharp, local discontinuity for flower color in Linanthus parryae is caused by natural selection, not genetic drift. Allozyme markers sampled across regions with different color frequencies showed no evidence of drift, while reciprocal transplant experiments demonstrated natural selection against the rare morph. Recent findings suggest that white-flowered plants possess a higher tissue concentration of potentially toxic cations such as arsenic and selenium, and that population differentiation for flower color is associated with spatial variation in soil chemistry. Our results refute Wright's conclusion that the flower color polymorphism in Linanthus parryae is an example of isolation by distance, a key component of his shifting balance theory of evolution. 
   
 
Schemske, D. W. and P. Bierzychudek. Evolution of flower color in the desert annual Linanthus parryae: Wright revisited. (Evolution, in review)  
 
Turelli, M., D. W. Schemske and P. Bierzychudek. Stable two-allele polymorphisms maintained by fluctuating fitnesses and seed banks: Protecting the blues in Linanthus parryae. (Evolution, in press)
 

 
III. How does spatial variation in the environment contribute to adaptation and speciation?
 
Dobzhansky (1951) proposed that the evolution of biological diversity is a result of adaptation to habitats that vary in time and space. Central to this view is the idea that traits that confer adaptation in one environment may prove detrimental in another. Thus adaptation to distinct ecological niches often precedes the diversification of populations into different species (Lewontin 1997; Schemske 2000). There is a need for comprehensive field and laboratory studies that identify the full spectrum of reproductive barriers between closely related species, and which estimate the relative importance of each stage to the total reproductive isolation. Studies of the genetic basis of speciation should include all forms of reproductive isolation, including ecological traits that may influence geographic distributions. Where closely-related species possess parapatric distributions, understanding the genetic basis of adaptive traits may also reveal the causes of species borders, a subject of considerable recent interest (Kirkpatrick and Barton 1997).

 
III(a) Environmental variation along an elevation gradient, and the evolution of the niche in monkeyflowers.
 
We have just received NSF funding for a long-term project designed to elucidate the genetic architecture of adaptation to different environments in Mimulus cardinalis and M. lewisii. M. lewisii is primarily a species of high elevation (1400-3300m) seeps and other moist disturbed habitats, whereas M. cardinalis is found at lower elevations (0-1400m) in riparian areas. The goal of our research is to characterizing the genetic basis of traits associated with adaptation to an elevation gradient.
 
The first step of the project is to develop a population of 500 recombinant inbred lines (RILs) derived from an interspecific F2 of Mimulus lewisii and M. cardinalis. From this population, we will construct linkage maps and determine the molecular marker genotypes for the RILs. Next, we will determine the adaptive phenotypes (e.g., germination, growth, survival, and flowering phenology) of the RILs in field experiments located along a 3000m elevation transect through the ranges of the parental species. These experiments are conducted at the Stanford (30m), Mather (1400m), and Timberline (3050m) field stations maintained by the Carnegie Institution, thus providing an opportunity to compare our results to those obtained in the classic ecological genetic studies of these same species by Hiesey, Nobs, and Bjorkman (1971). We are using linkage maps to: 1) count the number of adaptive QTLs, 2) estimate their magnitude of effect and mode of action, and 3) search for QTLs responsible for ‘tradeoffs’ in performance. These studies will allow us to explain the ecological and genetic mechanisms responsible for the natural distribution of the parental species. Comprehensive field and laboratory experiments now in progress by Amy Angert, a graduate student in my lab, are designed to answer the questions: What limits the geographic range of a species? This is a fundamental problem first raised by Ernst Mayr nearly a half century ago, yet we still do not have the answer.

 
III(b) Local adaptation and the genetics of adaptation in Linanthus parviflorus.
 
We are now conducting a QTL study of the genetic basis of local adaptation in Linanthus parviflorus, a California wildflower. Populations of this species display extreme local specialization to distinct soil types, with genetic differences in flowering time, growth form and cation uptake observed over a spatial scale of only 10m. Phenotypic data were collected from a large F2 population, and genotyping using AFLP markers is now underway. Our plan is to follow the protocol outlined above for the investigation of floral adaptation in monkeyflowers. Once the genetic map is created, we will produce near-isogenic lines for different combinations of adaptive QTLs and investigate their individual and combined effects on plant performance in the field. An additional goal of this work is to estimate the role of epistasis and pleiotropy in adaptation by producing near isogenic lines in different genetic backgrounds.
 

 
IV. Why are tropical regions so diverse?
 
Few questions have generated such interest, yet we still struggle to find the answers. With the decline of tropical communities throughout the world, there is an urgent need to catalogue its diversity and to understand its origins. As suggested by Dobzhansky (1950, p. 209): "Since the animals and plants which exist in the world are products of the evolutionary development of living matter, any differences between tropical and temperate organisms must be the outcome of differences in evolutionary patterns". The challenge is to determine how the selective forces experienced by organisms in tropical habitats might differ from those in temperate habitats, and how such differences might cause geographic variation in the opportunity for evolutionary diversification. Throughout my career, I have maintained an active research program in tropical systems.

 
IV(a) The Demographic Consequences of Plant-Animal Interactions in a Neotropical herb: Spatio-Temporal Dynamics
 
In collaboration with Dr. Carol Horvitz, I have investigated how temporal and spatial variation in plant-animal interactions affects the ecology and evolution of the understory herb Calathea ovandensis (Schemske and Horvitz 1984; Schemske and Horvitz 1988; Horvitz and Schemske 1995). The theme to emerge from this work is that animals play a major role in the life history of this species, and that spatial and temporal variation in plant-animal interactions is a major cause of variation in plant population dynamics. In addition, we find that major climatic disturbances such as El Niño events contribute to the variation in biotic interactions. We are now in the process of developing a simulation model that incorporates both stochastic and deterministic elements. Our goal is to assess how temporal-spatial variation in plant-animal interactions contributes to plant metapopulation dynamics. 
   
 
Schemske, D. W. and C. C. Horvitz. 1984. Variation among floral visitors in pollination ability: A precondition for mutualism specialization. Science 225:519-521.  
 
Schemske, D. W. and C. Horvitz. 1988. Plant animal interactions and fruit production in a neotropical herb: a path analysis. Ecology 69:1128- 1137.  
 
Horvitz, C. C. and D. W. Schemske. 1995. Spatiotemporal variation in demographic transitions for a neotropical understory herb: projection matrix analysis. Ecological Monographs 65:155-192.
 
Horvitz, C. C. and D. W. Schemske. Leaf herbivory and neighbourhood competition in a neotropical herb: effects on demographic fates. J. Ecology (in review)
 
 
 
IV(b) Plant-Animal Interactions and Adaptive Evolution in Neotropical Costus
 
With support from the Mellon Foundation and the Organization for Tropical Studies, I am investigating the ecology and evolution of plant-animal interactions in the Neotropical genus Costus. This group of understory herbs displays a number of fascinating mutualisms, including ants that tend extrafloral nectaries and both bee- and hummingbird-pollinated species. A phylogenetic analysis conducted in collaboration with Dick Olmstead and Pat Reeves suggests that hummingbird pollination has arisen at least four times. I have produced F1 and F2 hybrids between two species that differ in their pollination system, and we are now conducting an intensive genetic mapping project. The results thus far provide clear evidence of a major gene for flower color, similar to our finding in the Mimulus system. My objective is to develop Costus as a tropical system that parallels our investigations in Mimulus. In addition to the ecology and evolution of the pollination system in Costus, I have ongoing projects investigating 1) the evolution of an ant-plant mutualism, 2) the evidence for reinforcement, and 3) physiological adaptation along a moisture gradient.
 
We are also investigating the evolution of reinforcement in the Costus system. I had previously found nearly complete crossing barriers between two unrelated, bee-pollinated Costus that share the same pollinator in Panama rain forest (Schemske 1981). This failure of interspecific crosses stands in marked contrast to the ease with which hybrids can be made when parents are either allopatric. Kathleen Kay, a current graduate student in my lab, has recently discovered another instance of apparent reinforcement in two, closely-related, hummingbird-pollinated Costus. She is now conducting a comprehensive crossing study to determine the relationship between interspecific crossability and 1) mode of pollination (bee vs. hummingbird), 2) phylogenetic relationship and 3) geographic range (sympatric or allopatric). 
   
 
Schemske, D. W. 1980. The evolutionary significance of extrafloral nectar production by Costus woodsonii (Zingiberaceae): An experimental analysis of ant protection. Journal of Ecology 68:959-967.  
 
Schemske, D. W. 1981. Floral convergence and pollinator sharing in two bee-pollinated tropical herbs. Ecology 62:946-954.  
 
Schemske, D. W. 1982. Ecological correlates of a neotropical mutualism: Ant assemblages at Costus extrafloral nectaries. Ecology 63:932-941.
 
Schemske, D. W. 1983. Breeding system and habitat effects on fitness components in three neotropical Costus (Zingiberaceae). Evolution 37:523-539.
 
 
 
IV(c) Deceit pollination and floral mimicry in tropical Begonia
 
The tropical genus Begonia is comprised of predominantly monoecious species in which the rewardless female flowers often bear a striking resemblance to the polleniferous male flowers. This observation has led to the hypothesis that pollination in Begonia spp. is an example of intersexual mimicry where female flowers mimic conspecific males and pollination is by deceit. Male and female floral displays are very similar in appearance, with the yellow stigmas of the female flowers strongly resembling the anthers of male flowers. The deceit pollination system in Begonia may depend on a consistent source of naive pollinators. With 2,000 species in the genus Begonia, intersexual mimicry may represent a "key" innovation that contributed to speciation. 
   
 
Schemske, D. W., J. Ågren and J. Le Corff. 1996. Deceit pollination in the monoecious, neotropical herb Begonia oaxacana. pp. 292-318. In D. G. Lloyd and S.C.H. Barrett (eds.), Floral Biology. Chapman and Hall.  
 
LeCorff, J., J. Ågren, and D. W. Schemske. 1998. Floral display, pollinator choice and female reproductive success in two monoecious Begonia species pollinated by deceit. Ecology 79:1610-1619.
 
 
 
IV(d) Biotic Interactions and the Evolution of Tropical Diversity
 
We still do not have an adequate explanation for the evolution of species-rich tropical communities. Most of the current hypotheses propose ways of maintaining diversity, but have no explanation for its origins. I have recently reviewed the hypotheses concerning the evolution of tropical diversity (Schemske in press), and am in the process of developing a new theory that includes the relationship between stochastic processes and biotic interactions. 
   
 
Schemske, D. W. Ecological and evolutionary perspectives on the origins of tropical diversity, In R. Chazdon and T. Whitmore (eds.), Foundations of Tropical Biology: Key papers and commentaries. Univ. of Chicago Press. (in press)
 

 
V. Polyploidy as a mechanism of rapid adaptation and speciation in plants.
 
Polyploidy is common in plants, providing a rapid mechanism of adaptation and speciation. We are interested in the dynamics of polyploid formation and establishment, including the following questions: 1) What are the ecological, physiological and morphological consequences of polyploidy? 2) What are the primary mechanisms and rates of polyploid formation in natural populations?, 3) What are the primary mechanisms of reproductive isolation in polyploids, and how do these change through time? And 4) What are the mating system consequences of polyploidy? These questions have been investigated in research conducted in collaboration with Dr. Brian Husband, a former postdoc in my lab, and a current graduate student, Justin Ramsey. They have done all the work! 
   
 
Ramsey, J. and D. W. Schemske. 1998. Pathways, mechanisms and rates of polyploid formation in flowering plants. Annual Review of Ecology and Systematics 29:477-501.  
 
Husband, B. C. and D. W. Schemske. 2000. Ecological mechanisms of reproductive isolation and coexistence of diploid and tetraploid Chamerion angustifolium. Journal of Ecology 88:689-701.
 

 
VI. The ecology and evolution of plant mating systems.
 
Plant mating systems display considerable variation within and between species and populations, due in large part to spatial variation in the costs and benefits of producing self-pollinated or outcrossed offspring. Since Darwin, plant mating systems have been the subject of intense interest. This is due to the fact that the mating system has a profound influence on the distribution and magnitude of genetic variation, the magnitude and timing of inbreeding depression and the rate of spread of advantageous mutations. My empirical work has investigated the evolution of plant mating systems in an ecological context, particularly in the facultatively selfing annual Impatiens. In collaboration with R. Lande, I have developed theory relating to the coevolution of the mating system and inbreeding depression, leading to the prediction that predominant selfing and predominant outcrossing represent two stable states. 
 
 
Schemske, D. W. 1978. Evolution of reproductive characteristics in Impatiens (Balsaminaceae): The significance of cleistogamy and chasmogamy. Ecology 59:596-613.
 
Schemske, D. W. 1984. Population structure and local selection in Impatiens pallida (Balsaminaceae), a selfing annual. Evolution 38:817-832.
 
Lande, R. and D. W. Schemske. 1985. The evolution of self-fertilization and inbreeding depression in plants. I. Genetic models. Evolution 39:24-40.
 
Schemske, D. W. and R. Lande. 1985. The evolution of self-fertilization and inbreeding depression in plants. II. Empirical observations. Evolution 39:41-52.  
 
Husband, B. C. and D. W. Schemske. 1996. Evolution of the magnitude and timing of inbreeding depression in plants. Evolution 50:54-70.
 

 
VII. Research approaches in plant conservation.
 
The field of population biology provides a range of tools that can be used to evaluate the genetic and demographic status of plant populations. I am interested in the interface between plant population biology and conservation, with the goal of identifying the most effective and cost-efficienct consevation strategies. 
   
 
Schemske, D. W., B. C. Husband, M. H. Ruckelshaus, C. Goodwillie, I. M. Parker and J. G. Bishop. 1994. Evaluating approaches to the conservation of rare and endangered plants. Ecology 75:584-606.
 

 
Literature Cited
 
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Barton, N. H. 1998. The geometry of adaptation. Nature 751-752.
 
Bradshaw, H. D., Jr., K. G. Otto, B. E. Frewen, J. K. McKay and D. W. Schemske. 1998. Quantitative trait loci affecting differences in floral morphology between two species of monkeyflower (Mimulus). Genetics 149:367-382.
 
Bradshaw, H.D., Jr., Wilbert, S., Otto, K.G., and Schemske, D.W. (1995). Quantitative trait loci associated with reproductive isolation in monkeyflowers (Mimulus). Nature 376, 762-765.
 
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Horvitz, C. C. and D. W. Schemske. 1995. Spatiotemporal variation in demographic transitions for a neotropical understory herb: projection matrix analysis. Ecological Monographs 65:155-192.
 
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Schemske, D. W. 1981. Floral convergence and pollinator sharing in two bee- pollinated tropical herbs. Ecology 62:946-954.
 
Schemske, D. W. 1984. Population structure and local selection in Impatiens pallida (Balsaminaceae), a selfing annual. Evolution 38:817-832.
 
Schemske, D.W. & Bradshaw, H.D., Jr. (1999) Pollinator preference and the evolution of floral traits in monkeyflowers (Mimulus). Proceedings of the National Academy of Sciences USA 96(21): 11910-11915.
 
Schemske, D. W. and C. C. Horvitz. 1984. Variation among floral visitors in pollination ability: A precondition for mutualism specialization. Science 225:519-521.
 
Schemske, D. W. and C. Horvitz. 1988. Plant animal interactions and fruit production in a neotropical herb: a path analysis. Ecology 69:1128- 1137.
 
Schemske, D. W. 2000. Understanding the origin of species. Evolution 54:1069-1073.
 
Schemske, D. W. Ecological and evolutionary perspectives on the origins of tropical diversity, In R. Chazdon and T. Whitmore (eds.), Foundations of Tropical Biology: Key papers and commentaries. Univ. of Chicago Press. (in press)
 
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Turelli, M., D. W. Schemske and P. Bierzychudek. Stable two-allele polymorphisms maintained by fluctuating fitnesses and seed banks: Protecting the blues in Linanthus parryae. (Evolution, in press)
 
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