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Floral scent is a complex trait that mediates many plant-insect interactions, but our understanding of how floral scent variation evolves, either independently or in concert with other traits, remains limited. Assessing variation in floral scent at multiple levels of biological organization and comparing patterns of variation in scent to variation in other floral traits can contribute to our understanding of how scent variation evolves in nature.We used a greenhouse common garden experiment to investigate variation in floral scent at three scales-within plants, among plants, and among populations-and to determine whether scent, alone or in combination with morphology and rewards, contributes to population differentiation in Oenothera cespitosa subsp. marginata. Its range spans most of the biomes in the western United States, such that variation in both the abiotic and biotic environment could contribute to trait variation.Multiple analytical approaches demonstrated substantial variation among and within populations in compound-specific and total floral scent measures. Overall, populations were differentiated in morphology and reward traits and in scent. Across populations, coupled patterns of variation in linalool, leucine-derived compounds, and hypanthium length are consistent with a long-tongued moth pollination syndrome.The considerable variation in floral scent detected within populations suggests that, similar to other floral traits, variation in floral scent may have a heritable genetic component. Differences in patterns of population differentiation in floral scent and in morphology and rewards indicate that these traits may be shaped by different selective pressures.

Premise of research. Staggered flowering phenologies among co-occurring species have often been interpreted as the product of competition for pollination, but interspecific flowering time differenc...

A current frontier of character displacement research is to determine if displacement occurs via multiple phenotypic pathways and varies across communities with different species compositions. Here, we conducted the first test for context-dependent character displacement in multimodal floral signals by analyzing variation in floral scent in a system that exhibits character displacement in flower size, and that has multiple types of sympatric communities. In a greenhouse common garden experiment, we measured quantitative variation in volatile emission rates of the progeny of two species ofClarkiafrom replicated communities that contain one, two, or fourClarkiaspecies. The first two axes of a constrained correspondence analysis, which explained 24 percent of the total variation in floral scent, separated the species and community types, respectively. Of the 23 compounds that were significantly correlated with these axes, nine showed patterns consistent with character displacement. Two compounds produced primarily byC. unguiculataand two compounds produced primarily byC. cylindricawere emitted in higher amounts in sympatry. Character displacement in some volatiles varied across sympatric communities and occurred in parallel with displacement in flower size, demonstrating that this evolutionary process can be context-dependent and may occur through multiple pathways.

Do plants with multiple pollinators evolve unique trait combinations or intermediate phenotypes compared to plants with one pollinator? Using experimental evolution, Schiestl et al. (2018) found that plants pollinated by bumblebees and hoverflies evolved trait values not observed in plants pollinated by one taxon, which provides evidence for the existence of a unique generalized pollination phenotype.

Although the evolution and diversification of flowers is often attributed to pollinator-mediated selection, interactions between co-occurring plant species can alter patterns of selection mediated by pollinators and other agents. The extent to which both floral density and congeneric species richness affect patterns of net and pollinator-mediated selection on multiple co-occurring species in a community is unknown and is likely to depend on whether co-occurring plants experience competition or facilitation for reproduction. We conducted an observational study of selection on four species of Clarkia (Onagraceae) and tested for pollinator-mediated selection on two Clarkia species in communities differing in congeneric species richness and local floral density. When selection varied with community context, selection was generally stronger in communities with fewer species, where local conspecific floral density was higher, and where local heterospecific floral density was lower. These patterns suggest that intraspecific competition at high densities and interspecific competition at low densities may affect the evolution of floral traits. However, selection on floral traits was not pollinator mediated in Clarkia cylindrica or Clarkia xantiana, despite variation in pollinator visitation and the extent of pollen limitation across communities for C. cylindrica. As such, interactions between co-occurring species may alter patterns of selection mediated by abiotic agents of selection.

Premise of research. Many mechanisms have evolved that increase the amount of pollen exported and the number of plants that receive this pollen. One such mechanism is pollen presentation, where plants regulate the amount of pollen available to a pollinator during a single visit. Presenting either a constant proportion or an increasing proportion of available pollen should maximize male fitness in most pollinator environments. But for constant or increasing pollen presentation schedules to evolve, variation in these schedules must have a heritable component.Methodology. We measured pollen presentation schedules of Lobelia siphilitica by simulating a sequence of 16 bee visits to flowers produced by progeny of 10 open-pollinated maternal families. A component of the pollen presentation schedule (the amount of pollen remaining in the flower at the end of the visitation sequence) was measured for plants from all of the families. The complete pollen presentation schedule was measured for plants from four of the...

There is a great need to understand how and why biodiversity, which we define as the variety of organisms found in a given place, changes over time. Current estimates suggest strikingly slow change in traditional measures of biodiversity. These estimates seem to contradict rapid shifts in the abundance of individual species and have led to a rethinking of the mechanisms shaping biodiversity. Conceptual models emphasize the role of competition among species or, more recently, selection on species identity (i.e. selection that favors some species at the expense of others). However, it is difficult to quantify how these mechanisms contribute to biodiversity change. To illustrate this point we present cases where strong competition or selection on species identity leads to no biodiversity change. In view of this disconnect we develop a new approach to studying biodiversity change using the Price equation. We show that biodiversity change responds to selection on species’ rarity, rather than to either competition or selection on species identity. We then show how this insight can be used to quantify the effects of the mechanisms previously thought to influence biodiversity: 1) selection, 2) (ecological) drift, 3) immigration and 4) speciation. Our results suggest the connection between species’ fates and their rarity is fundamental to understanding biodiversity change.

Floral traits are hypothesized to evolve primarily in response to selection by pollinators. However, selection can also be mediated by other environmental factors. To understand the relative importance of pollinator-mediated selection and its variation among trait and pollinator types, we analyzed directional selection gradients on floral traits from experiments that manipulated the environment to identify agents of selection. Pollinator-mediated selection was stronger than selection by other biotic factors (e.g., herbivores), but similar in strength to selection by abiotic factors (e.g., soil water), providing partial support for the hypothesis that floral traits evolve primarily in response to pollinators. Pollinator-mediated selection was stronger on pollination efficiency traits than on other trait types, as expected if efficiency traits affect fitness via interactions with pollinators, but other trait types also affect fitness via other environmental factors. In addition to varying among trait types, pollinator-mediated selection varied among pollinator taxa: selection was stronger when bees, long-tongued flies, or birds were the primary visitors than when the primary visitors were Lepidoptera or multiple animal taxa. Finally, reducing pollinator access to flowers had a relatively small effect on selection on floral traits, suggesting that anthropogenic declines in pollinator populations would initially have modest effects on floral evolution.

Premise of research. Gynodioecy, a dimorphic sexual system where individual plants are either female or hermaphroditic, has been documented in ≪1% of plant species. This rarity suggests that gynodioecy can evolve and persist only under a restrictive set of conditions. One approach to determining these conditions is to identify the phenotypic traits and ecological factors that are associated with gynodioecy, as such traits or factors may facilitate the evolution and persistence of the sexual system.Methodology. We assembled an angiosperm-wide database of gynodioecious species and used this database to test whether gynodioecy was associated with two phenotypic traits/ecological factors: an herbaceous growth form and a temperate geographic distribution. Species-level analyses were used to confirm that gynodioecious species are predominately herbaceous and temperate. Family-level analyses were then used to test whether the presence of herbaceous and temperate species in a lineage was associated with the prese...

Despite long-standing interest in the evolutionary ecology of plants that share pollinators, few studies have explored how these interactions may affect communities during both community assembly (ecological sorting) and through ongoing, in situ evolution (character displacement), and how the effects of these interactions may change with community context. To determine if communities display patterns consistent with ecological sorting, we assessed the frequency of co-occurrence of four species of Clarkia in the southern Sierra foothills (Kern County, CA, USA). To investigate potential character displacement, we measured pollination-related traits on plants grown in a greenhouse common garden from seed collected in communities with one, two or four Clarkia species. Among the four species of Clarkia in this region, the two species that are often found in multi-species communities also co-occur with one another more frequently than expected under a null model. This pattern is consistent with ecological sorting, although further investigation is needed to determine the role of pollinators in shaping community assembly. Patterns of trait variation in a common garden suggest that these two species have diverged in floral traits and converged in flowering time where they co-occur, which is consistent with character displacement. Trait variation across community types also suggests that the process and outcome of character displacement may vary with community context. Because community context appears to affect both the direction and magnitude of character displacement, change in more species-rich communities may not be predictable from patterns of change in simpler communities.

Plant evolutionary ecology is a discipline with deep roots. Pioneers early in the 20th century used reciprocal transplants and common gardens to examine the genetic basis of plant phenotypes and patterns of adaptation [1–3]. Today the discipline has grown to incorporate 21st century molecular and statistical methods, resulting in a wide-ranging and complex array of studies that address the evolutionary ecology of natural plant populations. In a slim book, Approaches to Plant Evolutionary Ecology, G.P.