Perennial Life History Research Articles

Dynamic Movement Patterns of Florida Gar within a Fluctuating Hydroscape

Fish with perennial life histories that inhabit ecosystems where water availability fluctuates seasonally must have movement behavior that accommodates this environmental change. Landscape structure is expected to influence magnitude and direction of these movements by affecting the distribution of water in space and time. Predicting the effects of climate change and restoring historical hydrology in the Florida Everglades, a large wetland that undergoes regular contraction in ecosystem size during seasonal droughts, requires information on how fish movement is affected by spatiotemporal patterns of water availability. We used radio telemetry to quantify spatial scale and direction of displacement of Florida Gar (Lepisosteus platyrhinchus) from focal areas of the Everglades that differ in their seasonal hydrology and position relative to drought refuges. Displacement distances increased during both rising and dropping water levels compared to periods of stable water depth, but increased more and in more directed fashion when ecosystem size was contracting. The initial position of fish relative to natural (i.e., creeks and sloughs) and artificial (i.e., canals) refuge habitats directed dry-season movement and influenced displacement distances by attracting individuals from long distances and reducing inclination of nearby fish to disperse. During rising water, displacement distances overlapped between locations near and far from dry-season refuges because of increased displacement from crowded refuge habitats and decreased movement distances at short-hydroperiod marshes. Florida Gar display flexible movement strategies that accommodate the influence of landscape context on hydrology and distance to drought refuges. Modification of the canal-levee network, managed water releases, and climate change in the Florida Everglades will interact with seasonal displacement of this abundant predator in shaping its population dynamics and food web impacts during and after drought disturbances.

Annual and perennial life history strategies of Zostera marina populations under different light regimes

The seagrass Zostera marina is geneti- cally considered to be perennial, but annual popula- tions of this species exist as well and are under severe environmental stress in many geographical locations. We investigated why Z. marina in Jindong Bay (Korea) exhibited a typical perennial life history at a shallow site but an annual life history at a deep site. We hy- pothesized that Z. marina shoots in the deep popula- tion die off after fall due to severe light reductions, and that the re-establishment of this population is completely dependent on sexual reproduction. Life histories of these populations were examined by con- structing life tables using monthly measurements of shoot recruitment and mortality. In the deep popu - lation, new shoot recruitment was only achieved by seedlings, and approximately 95% of the surviving seedlings flowered. No shoots overwintered in the deep population. In contrast, shoot recruitment in the shallow population was primarily achieved by asexual reproduction. High summer water temperatures and subsequent severe underwater light reduction during summer and fall probably led to the high mortality in Z. marina shoots in the deep population, resulting in no shoot survival after fall. The deep population likely exhibits an annual life history strategy to achieve pop- ulation persistence under repetitive and acute unfa- vorable environmental conditions. From an evolution- ary perspective, the life history traits of Z. marina would define this species as an intermediate between annual and perennial.

Evolutionary ecology of resprouting and seeding in fire-prone ecosystems.

There are two broad mechanisms by which plant populations persist under recurrent disturbances: resprouting from surviving tissues, and seedling recruitment. Species can have one of these mechanisms or both. However, a coherent framework explaining the differential evolutionary pressures driving these regeneration mechanisms is lacking. We propose a bottom-up approach in addressing this question that considers the relative survivorship of adults and juveniles in an evolutionary context, based on two assumptions. First, resprouting and seeding can be interpreted by analogy with annual versus perennial life histories; that is, if we consider disturbance cycles to be analogous to annual cycles, then resprouting species are analogous to the perennial life history with iteroparous reproduction, and obligate seeding species that survive disturbances solely through seed banks are analogous to the annual life history with semelparous reproduction. Secondly, changes in the selective regimes differentially modify the survival rates of adults and juveniles and thus the relative costs and benefits of resprouting versus seeding. Our approach provides a framework for understanding temporal and spatial variation in resprouting and seeding under crown-fire regimes. It accounts for patterns of coexistence and environmental changes that contribute to the evolution of seeding from resprouting ancestors.

The evolution of multiqueen breeding in eusocial lineages with permanent physically differentiated castes

The hypothesis that obligate eusociality always evolved from ancestral states of strict lifetime monogamy implies that (1) facultatively eusocial lineages had to abandon multifemale breeding to achieve permanent morphologically differentiated castes, and (2) lineages of obligatorily eusocial insects had to independently re-evolve multifemale breeding when that served the inclusive fitness interests of nursing workers. Multiqueen nesting (eusocial polygyny) is known to be common across the ants, but rare in the corbiculate (eusocial) bees, the vespine wasps and the higher termites, but we show that this difference is mostly due to cases of obligate polygyny being restricted to the ants. This pattern is remarkably similar to the distribution of inquiline social parasites that use stealth rather than aggression to invade host colonies, which also repeatedly evolved in ants only. We explore the lineage-specific selection forces that have allowed or constrained de novo evolution of stable eusocial polygyny in Hamiltonian inclusive fitness terms. We argue that perennial life histories, male survival as stored sperm rather than as lifetime mates, and sib competition are possibly sufficient to explain the general prevalence of secondary polygyny across the obligatorily eusocial insects. We infer that obligate polygyny compromises eusocial ‘soma’ and ‘germ-line’ segregation in ways known to decrease developmental stability in metazoans, and we briefly evaluate the selection forces that reduce queen life span in highly, but probably not facultatively, polygynous species. We conclude that secondary polygyny in its obligate (ant) form resembles cooperative breeding with multiple ‘tragedy of the commons’ aspects, but in a peculiar manner because breeding females are selected to exploit the services of unmated workers rather than each other’s. This breeding system has likely been maintained in ants because it allows modular extensions of colonies in directly adjacent habitat of similar quality without the re-emergence of sexual conflicts or unproductive local competition with kin.

Memory of the vernalized state in plants including the model grass Brachypodium distachyon.

Plant species that have a vernalization requirement exhibit variation in the ability to “remember” winter – i.e., variation in the stability of the vernalized state. Studies in Arabidopsis have demonstrated that molecular memory involves changes in the chromatin state and expression of the flowering repressor FLOWERING LOCUS C, and have revealed that single-gene differences can have large effects on the stability of the vernalized state. In the perennial Arabidopsis relative Arabis alpina, the lack of memory of winter is critical for its perennial life history. Our studies of flowering behavior in the model grass Brachypodium distachyon reveal extensive variation in the vernalization requirement, and studies of a particular Brachypodium accession that has a qualitative requirement for both cold exposure and inductive day length to flower reveal that Brachypodium can exhibit a highly stable vernalized state.

Plant evolutionary ecology: molecular genetics, global warming and invasions, and the novel approaches we are using to study adaptations

Plant evolutionary ecology: molecular genetics, global warming and invasions, and the novel approaches we are using to study adaptations

Vegetation dynamics across a chronosequence of created wetland sites in Virginia, USA

Plant species composition, dominance, richness, and diversity were measured across a 15-year chronosequence of created wetland sites in Virginia, USA. Using an age-class categorization (1–2 years, 3–5 years, 6–10 years, or 11–15 years), all classes had a predominance of herbaceous species with perennial life history strategy, and perennials contributed 68.6 % to the overall dominance measure (importance value; IV) averaged across all sites. There was no significant difference in species richness or diversity among age classes. Analysis of Similarity (ANOSIM) indicated that herbaceous species composition was similar between the youngest and oldest age classes, but not the intermediate classes. For woody shrubs and saplings, planted species were more prevalent in the youngest age classes, and volunteer species predominated in the oldest age classes. These results suggest that perennial herbaceous species are important in early plant development on created wetland sites, and may be influential in observed patterns of species composition over time. In the context of plant development in newly created wetlands, dominance shifts from planted to volunteer woody species suggest that planting early successional species, or species with reproductive strategies attuned to created wetland site management, may favor survivorship and recruitment of other species over time.

Population structure of the primary gene pool of Oryza sativa in Thailand

The gene pool of cultivated Asian rice consists of wild rice (Oryza rufipogon Griff.), cultivated rice (O. sativa L.) and a weedy form (O. sativa f. spontanea). All three components are widespread in Thailand, frequently co-occurring within fields and providing the opportunity for gene flow and introgression. The purpose to this study is to understand the on-going evolutionary processes that affect the gene pool of rice by analysis of microsatellite variation. Results indicate that O. rufipogon, the wild ancestor of rice, has high levels of genetic variation both within and among populations. Moreover, the variation is structured predominantly by annual and perennial life history. High levels of variation are detected among cultivars indicating Thai cultivated rice has a broad genetic base with only a 20 % reduction in diversity from its wild ancestor. The weedy rice populations reveal varying levels of genetic variation, from nearly as high as wild rice to near zero. Weedy rice is genetically structured into 2 groups. Some populations of invasive weedy rice are the result of hybridization and gene flow between local wild rice and local cultivated rice in the regions of co-occurrence. Other populations of weedy rice are genetically nearly identical to the local cultivated rice. The diversity analysis indicates that the rice gene pool in Thailand is a dynamic genetic system. Gene flow is ongoing among its three main components, first between cultivated and wild rice resulting in weedy rice. Weedy rice in turn crosses with both cultivated varieties and wild rice.

Generic phylogeny, historical biogeography and character evolution of the cosmopolitan aquatic plant family Hydrocharitaceae

BackgroundHydrocharitaceae is a fully aquatic monocot family, consists of 18 genera with approximately 120 species. The family includes both fresh and marine aquatics and exhibits great diversity in form and habit including annual and perennial life histories; submersed, partially submersed and floating leaf habits and linear to orbicular leaf shapes. The family has a cosmopolitan distribution and is well represented in the Tertiary fossil record in Europe. At present, the historical biogeography of the family is not well understood and the generic relationships remain controversial. In this study we investigated the phylogeny and biogeography of Hydrocharitaceae by integrating fossils and DNA sequences from eight genes. We also conducted ancestral state reconstruction for three morphological characters.ResultsPhylogenetic analyses produced a phylogeny with most branches strongly supported by bootstrap values greater than 95 and Bayesian posterior probability values of 1.0. Stratiotes is the first diverging lineage with the remaining genera in two clades, one clade consists of Lagarosiphon, Ottelia, Blyxa, Apalanthe, Elodea and Egeria; and the other consists of Hydrocharis-Limnobium, Thalassia, Enhalus, Halophila, Najas, Hydrilla, Vallisneria, Nechamandra and Maidenia. Biogeographic analyses (DIVA, Mesquite) and divergence time estimates (BEAST) resolved the most recent common ancestor of Hydrocharitaceae as being in Asia during the Late Cretaceous and Palaeocene (54.7-72.6 Ma). Dispersals (including long-distance dispersal and migrations through Tethys seaway and land bridges) probably played major roles in the intercontinental distribution of this family. Ancestral state reconstruction suggested that in Hydrocharitaceae evolution of dioecy is bidirectional, viz., from dioecy to hermaphroditism, and from hermaphroditism to dioecy, and that the aerial-submerged leaf habit and short-linear leaf shape are the ancestral states.ConclusionsOur study has shed light on the previously controversial generic phylogeny of Hydrocharitaceae. The study has resolved the historical biogeography of this family and supported dispersal as the most likely explanation for the intercontinental distribution. We have also provided valuable information for understanding the evolution of breeding system and leaf phenotype in aquatic monocots.

Characterization and ecological implication of eelgrass life history strategies near the species’ southern limit in the western North Atlantic

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 444:43-56 (2012) - DOI: https://doi.org/10.3354/meps09428 Characterization and ecological implication of ­eelgrass life history strategies near the species’ southern limit in the western North Atlantic Jessie C. Jarvis1,3,*, Kenneth A. Moore1, W. Judson Kenworthy2 1Virginia Institute of Marine Science, School of Marine Science, College of William and Mary, Gloucester Point, Virginia 23062, USA 2Center for Coastal Fisheries and Habitat Research, NCCOS, NOS, NOAA, 101 Pivers Island Road, Beaufort, North Carolina 28516, USA 3Present address: The Richard Stockton College of New Jersey, Galloway, New Jersey 08205-9441, USA *Email: jessie.jarvis@stockton.edu ABSTRACT: Eelgrass Zostera marina L. populations located near the species southern limit in the western North Atlantic were assessed monthly from July 2007 through November 2008. We identified (1) dominant life history strategies and local environmental conditions in southern Z. marina populations, (2) quantified differences in reproductive phenology between populations and different local environmental conditions, and (3) compared reproductive strategies to established annual and perennial life history paradigms. Observed populations expressed both life history strategies with one Z. marina population completely losing aboveground biomass and reestablishing from seeds (annual model) while another population retained aboveground biomass throughout the year (perennial model). A third life history strategy, characterized here as a mixed-annual population, was also observed after some seedlings were found to reproduce both sexually and asexually during their first year of growth thereby not conforming to any currently established life history paradigm. Development of multiple life history strategies within this region may be in response to stressful summer water temperatures associated with the southern edge of the species’ range. We suggest that neither annual nor perennial life history strategies always provide a superior mechanism for population persistence as perennial populations can be susceptible to multiple consecutive years of stress, and annual populations are unable to fully exploit available resources throughout much of the year. The mixed-annual strategy observed here represents another possible life history model which may provide the mechanism necessary for Z. marina populations to persist during times of environmental transition. KEY WORDS: Zostera marina · Life-history · Annual · Perennial · Seed bank · Biomass Full text in pdf format PreviousNextCite this article as: Jarvis JC, Moore KA, Kenworthy WJ (2012) Characterization and ecological implication of ­eelgrass life history strategies near the species’ southern limit in the western North Atlantic. Mar Ecol Prog Ser 444:43-56. https://doi.org/10.3354/meps09428 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 444. Online publication date: January 10, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Multiple Continental Radiations and Correlates of Diversification in Lupinus (Leguminosae): Testing for Key Innovation with Incomplete Taxon Sampling

Replicate radiations provide powerful comparative systems to address questions about the interplay between opportunity and innovation in driving episodes of diversification and the factors limiting their subsequent progression. However, such systems have been rarely documented at intercontinental scales. Here, we evaluate the hypothesis of multiple radiations in the genus Lupinus (Leguminosae), which exhibits some of the highest known rates of net diversification in plants. Given that incomplete taxon sampling, background extinction, and lineage-specific variation in diversification rates can confound macroevolutionary inferences regarding the timing and mechanisms of cladogenesis, we used Bayesian relaxed clock phylogenetic analyses as well as MEDUSA and BiSSE birth-death likelihood models of diversification, to evaluate the evolutionary patterns of lineage accumulation in Lupinus. We identified 3 significant shifts to increased rates of net diversification (r) relative to background levels in the genus (r = 0.18-0.48 lineages/myr). The primary shift occurred approximately 4.6 Ma (r = 0.48-1.76) in the montane regions of western North America, followed by a secondary shift approximately 2.7 Ma (r = 0.89-3.33) associated with range expansion and diversification of allopatrically distributed sister clades in the Mexican highlands and Andes. We also recovered evidence for a third independent shift approximately 6.5 Ma at the base of a lower elevation eastern South American grassland and campo rupestre clade (r = 0.36-1.33). Bayesian ancestral state reconstructions and BiSSE likelihood analyses of correlated diversification indicated that increased rates of speciation are strongly associated with the derived evolution of perennial life history and invasion of montane ecosystems. Although we currently lack hard evidence for "replicate adaptive radiations" in the sense of convergent morphological and ecological trajectories among species in different clades, these results are consistent with the hypothesis that iteroparity functioned as an adaptive key innovation, providing a mechanism for range expansion and rapid divergence in upper elevation regions across much of the New World.

Why Are Trade-Offs between Flower Size and Number Infrequently Detected? A Test of Three Hypotheses

Although the flower is the unit of sexual reproduction in angiosperms, a plant’s reproductive success is determined by its entire floral display. A trade-off between the size and number of flowers produced is an assumption of models of floral display evolution, but this trade-off is often not observed. We tested three hypotheses for why a trade-off between flower size and number is not observed, using 32 populations of Mimulus guttatus and 83 genera from the California flora. We found support for the hypothesis that high variance in resource acquisition masks a trade-off between flower size and number. In contrast, we did not find support for the hypothesis that trade-offs between current and future reproduction mask negative correlations between flower size and number in lineages with a perennial life history. We also did not find support for the hypothesis that negative flower size-number correlations are less likely to be observed in lineages with lower variation in flower number because the correlatio...

Anomalous capitulum structure and monoecy may confer flexibility in sex allocation and life history evolution in the Ifloga lineage of paper daisies (Compositae: Gnaphalieae)

Evolutionary significance of the Compositae capitulum and variation in its structure is poorly understood, although it may permit flexibility in sexual expression. Optimal sex ratio differs with life-history and reproductive strategy. We explore how the genus Ifloga and related members of southern African Gnaphalieae achieved different sex ratios, and the associations of these ratios with annual and perennial life history. Sex allocation was measured using the male to female ratio (M/F), a novel approximator of the pollen to ovule ratio (P/O). Life-history (annuality/perenniality), capitulum structure, capitular sexual system, and M/F were reconstructed on time-proportional phylogenies. Trait associations were examined using phylogenetically independent contrasts (PICs). Annual taxa have strongly female-biased capitula, as measured by M/F, and either gynomonoecious or monoecious sexual systems, while perennials have equal or male-biased capitula that are hermaphroditic or monoecious. These results are largely supported by PIC analysis. Different sexual systems afford differing flexibility in sex allocation, with hermaphrodites having the least, and monoecious taxa the greatest, range in M/F. Within Ifloga, the anomalous capitulum evolved in an annual, gynomonoecious ancestor, followed by two independent gains of monoecy. Two subsequent gains of perenniality occurred within a monoecious sublineage. Different life histories have divergent sex allocation optima and are strongly associated with different sexual systems in gnaphalioid daisies. An anomalous capitulum structure in Ifloga may have facilitated the evolution of monoecy, which in turn may be linked to the evolution of life-history diversity in the genus.

Understanding the relative roles of disturbance and species interactions in shaping Mississippi River island plant communities

In order to better understand the factors governing community assembly on riverine islands, we conducted a greenhouse experiment that examined the effects of species interactions in combination with flooding that mimicked hydrologic gradients found on Mississippi River islands. Specifically, we examined the effects of inter and intra-specific competition and three disturbance treatments (drained, drought, and flood) on the growth performance of three island-ubiquitous herbaceous plant species (Amaranthus palmeri, Cyperus strigosus. and Xanthium strumarium). We predicted a decrease in interspecific competitive effects in disturbance treatments. Specifically, we expected A palmeri to be most affected by flooding treatment based on its facultative upland status suggesting that it should be a poor tolerator of flooded conditions. X. strumarium and C. strigosus both facultative wetland species in their status, should tolerate flooding, but we predict that C. strigosus should have greater stress tolerance due to its perennial life history. Because we find these species coexisting at the same elevation zones on riverine islands, we also predicted stronger intraspecific competition effects. Our results indicate that disturbance indeed affected competitive interactions, but the effects were species-specific. X. strumarium showed stronger competitive effects when grown with conspecifics while C. strigosus experienced greater competition with heterospecifics under drought treatments. Based on its wetland indicator status and life history characteristics, we expected A palmeri to exhibit a facilitative effect, as it is typically an upland species, but instead it was impacted most by interspecific competition. Our study indicates that factors other than competition, facilitation, and flood tolerance (e.g., priority effects) may be controlling island plant community dynamics, further studies are required.

Indicator species and functional groups as predictors of proximity to ecological thresholds in Mongolian rangelands

We focused on responses to grazing by individual species and functional groups in relation to ecological thresholds in Mongolian rangelands, with repeated measures from the same ecological sites to account for rainfall variability. At all sites, even under rainfall fluctuations, there were robust combinations of indicator species that could be used to forewarn managers to take action to minimize the probability of crossing ecological thresholds. Depending on the landscape condition of each site, the cover of functional groups, which shared traits of perennial life history, grass or forb growth form, linear leaf shape, and alternate leaf attachment, or the cover of functional groups of woody shrubs dramatically decreased before an ecological threshold was crossed. Thus, across all sites, the responses of certain functional groups to grazing appeared to predict the crossing of an ecological threshold. The ecological indicators derived in this study should help to improve land managers’ ability to prevent adverse changes in states before ecological thresholds are reached.

Diversity of plant life cycles is generated by dynamic epigenetic regulation in response to vernalization

I developed a genetic–physiological model to investigate the molecular mechanisms regulating annual and perennial life histories, and theoretically explored local adaptation caused by environment-specific selection. The model integrates signal transmission in the vernalization pathway and physiological process regarding growth and resource accumulation. The transition from vegetative to reproductive growth was modeled as the process of accumulating the flowering signal, which is transported from leaves to meristems. The model predicted four distinct flowering behaviors, monocarpic annual/perennial and polycarpic-yearly or -intermittent flowering, depending on the epigenetic regulation of FLOWERING LOCUS C ( FLC), a transcription factor that acts as a floral repressor. When FLC transcription was not activated after repression, plants always behaved monocarpically, while only a low activation rate of FLC allowed plants to become polycarpal. When mortality was high, rapid repression of FLC was evolutionarily favored, resulting in a summer annual phenotype. As mortality decreased, the evolutionarily favored phenotype shifted from summer to winter annuals, and further to polycarpic phenotypes in which FLC repression occurred slowly. Analysis of local adaptation demonstrated that sensitivity to low temperature increased from northern to southern habitats. These predictions provide important insights into the evolution of diversity in plant life cycles under rapid climate change.

Diversification of Lupinus (Leguminosae) in the western New World: Derived evolution of perennial life history and colonization of montane habitats

Previous phylogenetic studies of Lupinus (Leguminosae) based on nuclear DNA have shown that the western New World taxa form a monophyletic group representing the majority of species in the genus, with evidence for high rates of recent diversification in South America following final uplift of the Andes 2–4 million years ago (Mya). For this study, three regions of rapidly evolving non-coding chloroplast DNA ( trnL intron, trnS– trnG, and trnT– trnL) were examined to estimate the timing and rates of diversification in the western New World, and to infer ancestral states for geographic range, life history, and maximum elevation. The western New World species (5.0–9.3 Mya, 0.6–1.1 spp./My) comprise a basally branching assemblage of annual plants endemic to the lower elevations of western North America, from which two species-rich clades are recently derived: (i) the western North American perennials from the Rocky Mountains, Great Basin, and Pacific Slope (0.7–2.1 Mya, 2.0–5.9 spp./My) and (ii) the predominantly perennial species from the Andes Mountains of South America and highlands of Mexico (0.8–3.4 Mya, 1.4–5.7 spp./My). Bayesian posterior predictive tests for association between life history and maximum elevation demonstrate that perennials are positively correlated with higher elevations. These results are consistent with a series of one or more recent radiations in the western New World, and indicate that rapid diversification of Lupinus coincides with the derived evolution of perennial life history, colonization of montane habitats, and range expansion from North America to South America.

Climate and the evolution of annual/perennial life-histories in Nemesia (Scrophulariaceae)

The Cape Floristic Region and the Succulent Karoo in southwestern Africa are both noted for their plant species richness and high levels of endemism. The southwestern tip of Africa is one of the world's five Mediterranean-type climate regions. The biodiversity in the Cape Floristic Region and Succulent Karoo is thought to be at least partly due to changes to the climate of these regions that have occurred since the middle Miocene. Annual species are usually a significant proportion of local flora in Mediterranean-type climate regions. Previous studies of species radiations in the Cape Floristic Region have concentrated on genera that predominantly contain perennial species. Nemesia (Scrophulariaceae) comprises c. 65 species of annual and perennial herbs and sub-shrubs that are native to southern and tropical Africa. Annuals make up a significant proportion (~75%) of Nemesia species. We have reconstructed a phylogeny of 23 Nemesia species using nucleotide sequences of the ITS, ETS and trnL-spacer regions. Species were grouped into five clades, two composed of annual species, one that contained two annual and one perennial species, one that contained one annual and two perennial species, and one that was predominantly composed of perennial species. Phylogenetic dating of the ITS + ETS based phylogenetic tree using penalised likelihood suggested the genus evolved during the Miocene, and that the majority of extant Nemesia species studied radiated during the Pliocene. Ancestral state reconstruction supports at least three separate origins of the annual habit from plants with a perennial life history. One origin can be traced to the late Miocene while the other two transitions occurred more recently during the Pliocene. The transition from perennial to annual life-histories in Nemesia may have been a response to climate change.

Plant speciation.

Like the formation of animal species, plant speciation is characterized by the evolution of barriers to genetic exchange between previously interbreeding populations. Prezygotic barriers, which impede mating or fertilization between species, typically contribute more to total reproductive isolation in plants than do postzygotic barriers, in which hybrid offspring are selected against. Adaptive divergence in response to ecological factors such as pollinators and habitat commonly drives the evolution of prezygotic barriers, but the evolutionary forces responsible for the development of intrinsic postzygotic barriers are virtually unknown and frequently result in polymorphism of incompatibility factors within species. Polyploid speciation, in which the entire genome is duplicated, is particularly frequent in plants, perhaps because polyploid plants often exhibit ecological differentiation, local dispersal, high fecundity, perennial life history, and self-fertilization or asexual reproduction. Finally, species richness in plants is correlated with many biological and geohistorical factors, most of which increase ecological opportunities.

CLIMATE AND LIFE-HISTORY EVOLUTION IN EVENING PRIMROSES (OENOTHERA, ONAGRACEAE): A PHYLOGENETIC COMPARATIVE ANALYSIS

Evolutionary ecologists have long sought to understand the conditions under which perennial (iteroparous) versus annual (semelparous) plant life histories are favored. We evaluated the idea that aridity and variation in the length of droughts should favor the evolution of an annual life history, both by decreasing adult survival and by increasing the potential for high seedling survival via reduced plant cover. We calculated phylogenetically independent contrasts of climate with respect to life history in a clade of winter-establishing evening primroses (sections Anogra and Kleinia; Oenothera; Onagraceae), which includes seven annuals, 12 perennials, and two variable taxa. Climate variables were quantified from long-term records at weather stations near collection localities. To explicitly account for phylogenetic uncertainty, contrasts were calculated on a random sample of phylogenetic trees from the posterior distribution of a Bayesian analysis of DNA sequence data. Statements of association are based on comparing the per-tree mean contrast, which has a null expectation of zero, to a set of per-tree mean contrasts calculated on the same trees, after randomizing the climate data. As predicted, increased annual aridity, increased annual potential evapotranspiration, and decreased annual precipitation were associated with transitions to the annual habit, but these trends were not significantly different from the null pattern. Transitions to the annual habit were not significantly associated with increases in one measure of aridity in summer nor with increased summer drought, but they were associated with significantly increased maximum summer temperatures. In winter, increased aridity and decreased precipitation were significantly associated with transitions to the annual habit. Changes in life history were not significantly associated with changes in the coefficient of variation of precipitation, either on an annual or seasonal (summer vs. winter) basis. Though we cannot attribute causality on the basis of a correlational, historical study, our results are consistent with the idea that increased heat and drought at certain times of the year favor the evolution of the annual habit. Increased heat in summer may cause adult survival to decline, while increased aridity and decreased precipitation in the season of seedling recruitment (winter) may favor a drought-avoiding, short-lived annual strategy. Not all of the predicted patterns were observed: the capability for drought-induced dormancy may preclude change in habit in response to summer drought in our study group.