Selection also favors larger flowers in coastal perennial habitats, further supporting this hypothesis . Flowering time and plant size appear to be widely correlated among populations of M. guttatus. In a survey of annual populations, Kooyers et al. found that plant height and stem diameter are positively correlated with flowering time, suggesting that these traits have evolved together as part of a drought-escape strategy. Similarly,in an F2 mapping population between annual and coastal perennial genotypes, flowering time and stolon production map to several shared QTL, suggesting that vegetative growth through stolons is pleiotropically linked to later flowering . These broad patterns are reflected in this study and influence the strength and pattern of selection. In both annual ecotypes, later flowering is correlated with larger flowering stems across both years. Given that direct selection consistently favored larger flowering stems, this correlation either increased selection for later flowering or weakened selection for earlier flowering. Further, inland perennials experienced selection for later flowering through increased rosette production in 2012, supporting a link between delayed flowering and vegetative growth. Thus, in a perennial habitat, selection may generally favor later flowering either through fecundity selection for larger flowering stems or selection through rosette production. This is in contrast to annual habitats, black plastic plant pots wholesale which impose strong fecundity selection for early flowering before the onset of drought .
The strength and direction of phenotypic selection is highly variable over time , providing one potential explanation for the paradox of strong selection within populations and evolutionary stasis. Given the apparent rarity of stabilizing selection in wild populations, shifting patterns of directional selection may be evidence of a population “wobbling” around a fitness peak . Alternatively, selection may not be detected at all if the breadth of a fitness peak exceeds the phenotypic variance, so that the fitness landscape appears flat. In the native montane perennial ecotype, I only detected selection on flowering time and marginal selection for smaller flowering stems in 2012. Interestingly, selection on flowering time was weak and changed directions between years. Additional temporal replication could test whether this native montane perennial ecotype is near a fitness peak for later flowering. Throughout the Sierra Nevada, 2013 was a severe drought year due to low snowpack in the previous winter. Across all ecotypes, fecundity and survival was reduced in 2013 and LTREs confirm that 2013 contributed negatively to overall population growth. The earlier onset of drought-induced mortality in 2013 increased the strength of selection for larger flowers and flowering stems and caused selection to favor earlier flowering in robust annuals. In inland perennials, the death of all above-ground biomass due to drought imposed an annual life-cycle, changing the pattern of selection from traits that maximized rosette production in 2012 to traits that maximized fecundity in 2013. This resulted in a more “annual-like” pattern of selection for larger flowers and flowering stems. Although the time-scale of this study is limited, this suggests that dry years can exert selection for traits that maximize first-year fecundity even within a perennial habitat, supporting a potential path for the evolution of an annual life history strategy from a perennial ancestor.
By estimating integrated selection, this study highlights the complexity of fitness landscapes once multiple fitness components are considered over several years. Recent reviews have identified the use of single fitness components and lack of temporal replication as major limitations toward a general understanding of selection in natural populations . The few studies that have examined integrated selection have found that the strength and pattern of selection varies among fitness components and years, often altering qualitative conclusions . This study further supports the limited utility of selection estimates based on single fitness components, particularly for iteroparous taxa. For example, the relative importance of individual fitness components in inland perennials changed dramatically over two years. This growing appreciation of the complex interactions among fitness components and traits has motivated the development of methods to decompose the effects of specific environmental conditions and fitness components on long-term selection . Clearly, this is a promising approach for linking selection to specific environments and forecasting evolutionary responses to environmental change. Although previous empirical estimates of integrated selection have relied on long-term demographic data sets , this study demonstrates that integrated selection can be estimated even within an experimental framework or over a relatively short time period . By collecting data on multiple fitness components and estimating a few additional demographic parameters, researchers stand to gain a much richer understanding of natural selection.Species encounter divergent selective environments throughout their range and local adaptation to these environments is the primary mechanism driving speciation .
Adaptive divergence can generate reproductive isolation directly by decreasing the fitness of immigrant or hybrid genotypes and indirectly by shaping traits related to mate choice or reproductive compatibility. Yet, ecological speciation is not a necessary consequence of adaptive divergence, and speciation is best viewed as a continuum of increasingly strong and irreversible reproductive isolation in sexually reproducing species . Understanding the factors that promote or constrain progress towards speciation requires documenting the order in which specific reproductive barriers evolve as well as the mechanisms driving their evolution . Comparative approaches have found strong support for increasing genetic isolation with greater ecological divergence . Yet, ascribing causality with molecular signatures of gene flow is difficult because reduced gene flow may in fact promote adaptive divergence, rather than vice versa . A stronger test is to quantify how reproductive barriers themselves evolve with adaptive divergence . A few studies have implemented this comparative approach by regressing reproductive barriers onto adaptive divergence among taxon-pairs. For example, mating isolation increases with divergence in color pattern in cichlids while intrinsic post-zygotic isolation increases with body size divergence in fish . Across a broad swath of taxa, reproductive isolation is positively correlated with ecological divergence between taxon-pairs . However, to my knowledge, no study has quantified the strength of multiple reproductive barriers among population pairs that vary in their degree of adaptive divergence within a species . Thus, it is unknown how multiple barriers evolve with adaptive divergence within a system or whether the relative importance of different barriers changes as population pairs become more ecologically divergent. In a recent symposium, these questions were identified as “perhaps the most important contribution” towards advancing the field of ecological speciation . Comparing the evolution of multiple reproductive barriers along the speciation continuum is necessary to understand how total reproductive isolation evolves during adaptive divergence. Ecological reproductive barriers may be offset by other trait interactions that act as reproductive enhancers , particularly in the early stages of divergence . In Colias butterflies, for example, males from high-elevation populations prefer the lighter wing patterns of low-elevation females over the more thermally efficient dark patterns of high elevation females . Selection against immigrants and hybrids have been predicted to be the earliest and strongest reproductive barriers to evolve during adaptive divergence, whereas other ecological or intrinsic barriers are predicted to evolve later or more sporadically . Although most speciation studies find strong selection against immigrants , the evidence for ecological selection against hybrids is mixed, with many closely related taxa exhibiting heterosis . Further, black plastic plant pots bulk intrinsic post-zygotic barriers can evolve as a direct by-product of divergent selection and could be important in the early stages of divergence .
Finally, the relative importance of multiple barriers will likely vary throughout a species range, depending on the spatial distribution, opportunities for gene flow, strength of selection, and adaptive potential for a given population. Quantification of these barriers among closely-related population pairs that span a range of adaptive divergence is necessary to test these predictions . To clarify the evolution of reproductive barriers along a speciation continuum, I compared five reproductive barriers among 10 population pairs spanning a range of life history divergence in common monkeyflower, Mimulus guttatus DC . Western North American monkeyflowers are a model system for dissecting the evolution of ecological reproductive barriers .Speciation in this group is characterized by a budding process, in which small ecologically divergent populations give rise to new species , and this ecogeographic divergence is an important component of reproductive isolation . The M. guttatus species complex is particularly diverse, comprising annual and perennial taxa that occupy a wide range of edaphic, coastal, and montane habitats . Yet despite this extensive adaptive divergence, genomic variation in M. guttatus is partitioned geographically rather than among habitats or life history strategies, reflecting recent divergence and on-going gene flow . Thus, ecotypic divergence within M. guttatus provides an ideal system for testing the evolution of early reproductive barriers in an actively diverging complex. I used a common garden experiment to mimic the dispersal of immigrant genotypes from ten populations in central California into a focal population and quantified the ecological and intrinsic barriers to gene flow that would operate in sympatry. I used a regression approach to test whether the strength of individual barriers and total isolation were related to adaptive phenotypic divergence in life history traits. Finally, I compared the contributions of immigrant and hybrid fitness, flowering and pollinator isolation, and intrinsic crossing barriers to total reproductive isolation for each population and ecotype. Taken together, this work provides a comprehensive evaluation of the reproductive barriers operating along a continuum of adaptive divergence for this species.Mimulus guttatus is an herbaceous annual or short-lived perennial plant that grows in mesic habitats in western North America. Life history divergence in this system is driven by variation in the length of the growth season. Annual populations occur in seasonally-drying seeps or meadows, whereas perennial populations occur in habitats with year-round moisture such as lakes, streams, and coastal seeps. Annual and perennial genotypes differ in several life history traits that are linked to a chromosomal inversion region that occurs throughout the species range . In general, perennials flower later at a larger size, produce larger flowers and stems, and reproduce vegetatively through clonal rosettes . Yet within each of these broad life history strategies, populations exhibit extensive adaptive variation in morphological, phenological, and life history traits. Among annual populations, clinal variation in the length of the growth season is correlated with variation in drought escape traits including flowering time and size at first flower . Among perennial populations, coastal, inland, and montane habitats are associated with distinct ecotypes. Coastal perennials have evolved tolerance to salt spray , whereas montane perennials invest extensively in below-ground rhizomes and produce few flowers .Despite this extensive adaptive divergence, taxa within the M. guttatus species complex are at least partially interfertile and taxonomists have divided this complex into as few as 4 and as many as 20 species in different treatments . Although the most recent treatment by Nesom splits M. guttatus into multiple morphological species, recent genomic evidence has challenged this view . Thus, to be consistent with genomic evidence as well as previous work in this complex, I treat life history strategies in this study as ecotypes within M. guttatus sensu lato .Ecological speciation is often defined as the by-product of reciprocal local adaptation to divergent selective environments, and fitness-related reproductive barriers are an integral component of the ecological speciation framework . These barriers are predicted to evolve early in the speciation continuum and to be highly correlated with adaptive divergence, whereas other ecological or intrinsic barriers are predicted to evolve later or more sporadically . However, reviews of the local adaptation literature have found that reciprocal fitness trade-offs are relatively rare . Immigrant genotypes may enjoy a fitness advantage due to maternal effects , sexual selection , inbreeding depression , or local adaptation of pathogens or antagonists . Although these factors would swamp adaptive divergence if gene flow is high , divergent selective environments are often at least partially isolated geographically . Whether strong ecological isolation can evolve in the absence of reciprocal fitness trade-offs, particularly early in the divergence process, is an intriguing question. Indeed, ecology may play a much more general role in the speciation process than those conditions usually considered within the ecological speciation framework . In this study, I found little evidence for immigrant inviability relative to the focal population. The montane perennial ecotype produces few flowers and relies on overwinter survival and vegetative growth. Inland perennial populations had high overwinter survival and growth while also producing more flowers, resulting in a net fitness advantage.