Conditional dispersal, clines, and the evolution of dispersiveness

Conditional dispersal, in which an individual’s decision over whether to disperse is a response to environmental conditions, features prominently in studies of dispersal evolution. Using models of clines, I examine how one widely discussed cost of dispersal, namely, that dispersal impedes local adaptation, changes with conditional dispersal and what this implies for dispersal evolution. I examine the consequences for dispersal evolution of the responsiveness of dispersal to the environment, the accuracy of any proximal cues that individuals rely upon to assess habitat quality, and whether dispersal responds to fitness itself or only to some fitness components (juvenile survivorship). All of the conditional dispersal behaviors that I consider weaken the indirect cost of dispersal inhibiting local adaptation. However, if individuals rely on imprecise cues to assess habitat quality and base dispersal decisions on juvenile survivorship, then conditional dispersal can incur additional costs by exacerbating overcrowding. Conditional dispersal initially leads to steeper clines in traits under direct selection, but when dispersiveness can itself evolve, conditional dispersal allows sigmoidal clines to persist long after those obtained with unconditional movement would become stepped. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The evolution of conspecific gamete precedence and its effect on reinforcement

Conspecific gamete precedence, the usage of conspecific sperm by a female that mates with both a conspecific and a heterospecific male, has been found in many taxa. We construct a population genetic model to examine the evolution of conspecific gamete precedence and its coevolution with premating isolation in the process of reinforcement. Our findings suggest that conspecific gamete precedence can evolve via a process very similar to reinforcement. We explore the nature of the selection against hybridization necessary to drive this evolution. Moreover, our results confirm the prediction of Marshall et al. (Trends Ecol. Evol. 2002;17:558-563) that conspecific gamete precedence will inhibit the evolution of reinforcement between two species. We further find that reinforcement will inhibit the evolution of conspecific gamete precedence. Both reinforcement and conspecific gamete precedence increase reproductive isolation and contribute to the process of speciation. We discuss factors that may affect which of these phenomena are likely to become predominant between incipient species.     

Males and females contribute differently to the evolution of habitat segregation driven by hybridization

Costly heterospecific mating interactions, such as hybridization, select for prezygotic reproductive isolation. One of the potential traits responding to the selection arising from maladaptive hybridization is habitat preference, whose divergence results in interspecific habitat segregation. Theoretical studies have so far assumed that habitat preference is a sexually shared trait. However, male and female habitat preferences can experience different selection pressures. Here, by combining analytical and simulation approaches, we theoretically examine the evolution of sex-specific habitat preferences. Habitat segregation can have demographic consequences, potentially generating eco-evolutionary dynamics. We thus explicitly consider demography in the simulation model. We also vary the degrees of species discrimination to examine how mate choice influences the evolution of habitat preferences. Results show that both sexes can reduce hybridisation risk by settling in the habitats where abundant conspecific mates reside. However, when females can discriminate species, excess conspecific male aggregation intensifies male–male competition for mating opportunities, posing an obstacle to conspecific aggregation. Meanwhile, conspecific female aggregation attracts conspecific males, by offering the mating opportunity. Therefore, under effective species discrimination, females play a leading role in initiating habitat use divergence. Simulations typically result in either the coexistence with established habitat segregation or the extinction of one of the species. The former result is especially likely when the species differ to some extent in habitat preferences upon secondary contact. Our results disentangle the selection pressures acting on male and female habitat preferences, deepening our understanding of the evolutionary process of habitat segregation due to hybridization.     

Male versus female mate choice: sexual selection and the evolution of species recognition via reinforcement

Male mate choice, expressed through courtship preferences, sometime occurs even under the mating system of polygyny, when the operational sex ratio is skewed toward males. The conditions under which male mate choice may be expected during polygyny are not well established. Servedio and Lande (2006, Evolution 60:674-685), assuming strict polygyny where all females have equal mating success, show that when having a preference does not increase the amount of energy that a male can put into courtship, male preferences for "arbitrary" female ornaments should not be expected to evolve; direct selection acts against them because they place males that carry them into situations in which there is high competition for mates. Here I explore in detail two situations under which logic dictates that this effect may be overcome or reversed. First I determine the contributions that direct and indirect selection place on male versus female preferences for traits that increase viability, using notation that allows the exact expression of these measures of selection. I find that direct selection against male preferences still predominates in the male mate choice model, causing less evolution by male than female preferences under these conditions. Second I address whether male mate choice is likely to evolve as a mechanism of premating isolation leading to species recognition, driven by the process of reinforcement. Reinforcement is compared under male and female mate choice, using a variety of models analyzed by both analytical techniques assuming weak selection and numerical techniques under broader selective conditions. I demonstrate that although under many conditions stronger premating isolation evolves under female mate choice, reinforcement may indeed occur via male mate choice alone.     

A test of alternative hypotheses for the evolution of reproductive isolation between spadefoot toads: support for the reinforcement hypothesis

Abstract How do species that interbreed become reproductively isolated? If hybrids are less fit than parental types, natural selection should promote reproductive isolation by favoring the evolution of premating mechanisms that prevent hybridization (a process termed reinforcement). Although reinforcement should generate a decline in hybridization over time, countervailing forces of gene flow and recombination are thought to preclude natural selection from enhancing and finalizing reproductive isolation. Here, I present recent estimates of hybridization frequency between two species of spadefoot toad, Spea multiplicata and S. bombifrons. I compare these recent measures of hybrid frequency with previously published estimates and show that hybridization between these species has declined precipitously over the past 27 years. Although previous studies suggest that reinforcement possibly accounts for this decline in hybrids over time, three alternative hypotheses also can explain the observed decrease in hybridization. First, if one of the two interacting species becomes rare, opportunities for and incidence of hybridization may decrease. Second, if one of the two interacting species is initially rare, hybridization may be initially common if the rare species has difficulty locating conspecific mates. Third, if hybrids are produced only in particular environments, hybrid frequency may decline if habitat changes result in loss of those environments that promote hybrid formation. I found no support for these three alternative explanations of the decline in hybrids. Instead, reinforcement appears to best account for the evolution of enhanced reproductive isolation between these species. Moreover, the finding that hybridization declined precipitously in only 27 years suggests that many systems that have undergone reinforcement may be overlooked because reproductive isolation between the interacting populations or species may already be complete.     

Evidence of reinforcement of premating isolation between two species of the genus Ochthebius (Coleoptera: Hydraenidae)

The increase in premating reproductive isolation between recently diverged and potentially interbreeding taxa resulting from selection against hybridization (reinforcement) is one of the most contentious issues in evolutionary biology. After many years of debate, its plausibility under various conditions has been shown by theoretical studies and some cases have been documented. At present, interest is arising about the frequency and importance of reinforcement in nature. Ochthebius quadricollis and Ochthebius sp. A are two hydraenid beetles inhabiting marine rock pools in the Mediterranean basin. By molecular analysis of a contact zone between the two species along the Italian Tyrrhenian coast, full reproductive isolation between the two species was evidenced. However, the finding of introgressed specimens at some diagnostic loci suggested that gene flow occurred in the past but then ceased. In this article, by analyzing species composition of mating couples collected in sympatric localities, we show the existence of strong assortative mating between the two species in nature. In laboratory multiple-choice mating trials, sympatric populations showed greater assortative mating than allopatric populations. Reinforcement is suggested as the most parsimonious hypothesis to explain the evolution of discriminative mate recognition systems occurring among O. quadricollis and Ochthebius sp. A under sympatric, but not allopatric, populations.     

A likelihood framework for inferring the evolution of geographic range on phylogenetic trees

Abstract At a time when historical biogeography appears to be again expanding its scope after a period of focusing primarily on discerning area relationships using cladograms, new inference methods are needed to bring more kinds of data to bear on questions about the geographic history of lineages. Here we describe a likelihood framework for inferring the evolution of geographic range on phylogenies that models lineage dispersal and local extinction in a set of discrete areas as stochastic events in continuous time. Unlike existing methods for estimating ancestral areas, such as dispersal‐vicariance analysis, this approach incorporates information on the timing of both lineage divergences and the availability of connections between areas (dispersal routes). Monte Carlo methods are used to estimate branch‐specific transition probabilities for geographic ranges, enabling the likelihood of the data (observed species distributions) to be evaluated for a given phylogeny and parameterized paleogeographic model. We demonstrate how the method can be used to address two biogeographic questions: What were the ancestral geographic ranges on a phylogenetic tree? How were those ancestral ranges affected by speciation and inherited by the daughter lineages at cladogenesis events? For illustration we use hypothetical examples and an analysis of a Northern Hemisphere plant clade (Cercis), comparing and contrasting inferences to those obtained from dispersal‐vicariance analysis. Although the particular model we implement is somewhat simplistic, the framework itself is flexible and could readily be modified to incorporate additional sources of information and also be extended to address other aspects of historical biogeography.     

东南亚生物多样性热点地区的形成与演化

东南亚地处热带,生物多样性极为丰富,可分为4个热点地区:印度-缅甸区的中南半岛、巽他区(含马来半岛、婆罗洲、苏门答腊岛)、菲律宾区(菲律宾群岛)、华莱士区(苏拉威西岛、爪哇岛、马鲁古群岛、小巽他群岛等)。中南半岛在泥盆纪便已是欧亚大陆的一部分,在印度板块撞击欧亚大陆之后受挤压而出;巽他区来自于冈瓦纳古陆和澳洲古陆;菲律宾群岛部分来自于劳亚古陆的碎片向南漂移,部分来自于太平洋西南岛弧的向北迁移;华莱士区则是劳亚古陆碎片、太平洋西南岛弧以及澳洲古陆北侧碎片的组合。巽他区地处赤道,常年温湿;菲律宾区、华莱士区、中南半岛则都受到不同程度的季风气候决定的干湿季变动。地质历史和季风气候影响程度的不同,奠定了东南亚4个生物多样性热点地区的雏形。华莱士区保存有大量的早期被子植物原始类群如睡莲目(Nymphaeales)和木兰藤目(Austrobaileyales),是现代被子植物起源地和冰期避难所之一。巽他区(婆罗洲)和中南半岛是亚洲热带植物的现代分布中心和"进化前沿",是整个东南亚地区重要的种源;而华莱士区的爪哇岛和小巽他群岛主要是物种迁入和中转的种库。这样的物种形成历史与迁移格局,塑造了东南亚4个...     

The relationship between postmating reproductive isolation and reinforcement in Phlox

The process of speciation involves the accumulation of reproductive isolation (RI) between diverging lineages. Selection can favor increased RI via the process of reinforcement, whereby costs to hybridization impose selection for increased prezygotic RI. Reinforcement results in phenotypic divergence within at least one taxon, as a result of costly hybridization between sympatric taxa. The strength of selection driving reinforcement is determined by the cost of hybridization and the frequency of hybridization. We investigated the cost of hybridization by quantifying postmating RI barriers among Phlox species that comprise one of the best‐studied cases of reinforcement. We determined if the strength of RI differs among lineages that have and have not undergone reinforcement, how much variability there is within species in RI, and whether RI is associated with phylogenetic relatedness. We found high RI for the species that underwent phenotypic divergence due to reinforcement; however, RI was also high between other species pairs. We found extensive variability in RI among individuals within species, and no evidence that the strength of RI was associated with phylogenetic relatedness. We suggest that phenotypic divergence due to reinforcement is associated with the frequency of hybridization and introgression, and not the cost of hybridization in this clade.     

Postmating-prezygotic isolation is not an important source of selection for reinforcement within and between species in Drosophila pseudoobscura and D. persimilis

Abstract Most work on adaptive speciation to date has focused on the role of low hybrid fitness as the force driving reinforcement (the evolution of premating isolation after secondary contact that reduces the likelihood of matings between populations). However, recent theoretical work has shown that postmating, prezygotic incompatibilities may also be important in driving premating isolation. We quantified premating, postmating-prezygotic, and early postzygotic fitness effects in crosses among three populations: Drosophila persimilis, D. pseudoobscura USA (sympatric to D. persimilis ), and D. pseudoobscura Bogotá (allopatric to D. persimilis ). Interspecific matings were more likely to fail when they involved the sympatric populations than when they involved the allopatric populations, consistent with reinforcement. We also found that failure rate in sympatric mating trials depended on whether D. persimilis females were paired with D. pseudoobscura males or the reverse. This asymmetry most likely indicates differences in discrimination against heterospecific males by females. By measuring egg laying rate, fertilization success and hatching success, we also compared components of postmating-prezygotic and early postzygotic isolation. Postmating-prezygotic fitness costs were small and not distinguishable between hetero- and conspecific crosses. Early postzygotic fitness effects due to hatching success differences were also small in between-population crosses. There was, however, a postzygotic fitness effect that may have resulted from an X-linked allele found in one of the two strains of D. pseudoobscura USA. We conclude that the postmating-prezygotic fitness costs we measured probably did not drive premating isolation in these species. Premating isolation is most likely driven in sympatric populations by previously known hybrid male sterility.     

Sexual selection and the evolution of costly female preferences: spatial effects

Abstract.— Models of Fisher's runaway process show that if there is a cost to female preference, no preference or male trait exaggeration will evolve. Surprisingly, this is true no matter how small the cost, which reveals that these models of Fisher's process are structurally unstable (Bulmer 1989). Here a model of Fisher's runaway process is presented to demonstrate that costly female preference evolves very easily when space is explicitly included in the model. The only requirement is that the optimal male phenotype changes across the species' range. The model shows that the spatial average of the female preference and male trait reach an evolutionary equilibrium that is identical to those of nonspatial models, but that the preference and male trait can deviate greatly from these averages at any point in space. For example, if random mating results in the lowest cost to females, then at equilibrium the spatial average preference will be zero. Nevertheless, there will be some locations at which females prefer males with larger ornaments and others where they prefer males with smaller ornaments. Results also show that the structural instability of nonspatial models of Fisher's process is less of a problem in spatial models. In particular, many of the main qualitative features of cost-free spatial models of Fisher's process remain valid even when there are small costs of female preference. Finally, the model shows that abrupt changes in the optimal male phenotype across space can result in an amplification of this pattern when preference has a small cost, but it can also result in a pattern similar to reproductive character displacement. Which of these occurs depends on the magnitude of the cost of female preference. This suggests that some patterns of reproductive character displacement in nature might be explained simply by sexual selection rather than by hybrid dysgenesis and reinforcement.     

Clinal variation in morphology and assortative mating in the soldier beetle, Chauliognathus pennsylvanicus (Goleoptera: Cantharidae)

In north Georgia populations of the soldier beetle, Chauliognathus pennsylvanicus , the length of the elytral spot varies clinally. At the southern end of a 200 km cline the distribution of spot length is unimodal with longer spot lengths predominating while at the northern end of the cline the distribution is bimodal but with shorter spot lengths being more frequent. North of the cline only short elytral spot lengths are observed, while the converse is true south of the cline. The strength of assortative mating on the basis of elytral spot length increases from south to north along the cline, resulting in complete pre-mating isolation between short and long spot length morphs at the north end of the cline. Laboratory mate choice tests indicate that assortative mating in the field is not the result of differential timing of activity or microhabitat choice but rather that it represents a real behavioural preference. Individuals from monomorphic populations on either side of the cline do not mate assortatively in the laboratory, indicating that reproductive isolation has evolved on the cline.     

Breeding system evolution influenced the geographic expansion and diversification of the core Corvoidea (Aves: Passeriformes)

Birds vary greatly in their life‐history strategies, including their breeding systems, which range from brood parasitism to a system with multiple nonbreeding helpers at the nest. By far the most common arrangement, however, is where both parents participate in raising the young. The traits associated with parental care have been suggested to affect dispersal propensity and lineage diversification, but to date tests of this potential relationship at broad temporal and spatial scales have been limited. Here, using data from a globally distributed group of corvoid birds in concordance with state‐dependent speciation and extinction models, we suggest that pair breeding is associated with elevated speciation rates. Estimates of transition between breeding systems imply that cooperative lineages frequently evolve biparental care, whereas pair breeders rarely become cooperative. We further highlight that these groups have differences in their spatial distributions, with pair breeders overrepresented on islands, and cooperative breeders mainly found on continents. Finally, we find that speciation rates appear to be significantly higher on islands compared to continents. These results imply that the transition from cooperative breeding to pair breeding was likely a significant contributing factor facilitating dispersal across tropical archipelagos, and subsequent world‐wide phylogenetic expansion among the core Corvoidea.     

Inference of selection and stochastic effects in the house mouse hybrid zone

We explored the transition of 13 X-linked markers across two separate portions of the house mouse hybrid zone, asking whether such a comparison can distinguish the effects of selection from random factors. A heuristic search in the likelihood landscape revealed more complex likelihood profiles for data sampled in two-dimensional (2D) space relative to data sampled along a linear transect. Randomized resampling of localities analyzed for individual loci showed that deletion of sites away from the zone center can decrease cline width estimates whereas deletion of sites close to the center can significantly increase the width estimates. Deleting localities for all loci resulted in wider clines if the number of samples from the center was limited. The results suggest that, given the great variation in width estimates resulting from inclusion/exclusion of sampling sites, the geographic sampling design is important in hybrid zone studies and that our inferences should take into account measures of uncertainty such as support intervals. The comparison of the two transects indicates cline widths are narrower for loci in the central part of the X chromosome, suggesting selection is stronger in this region and genetic incompatibilities may have at least partly common architecture in the house mouse hybrid zone.     

Limits to the evolution of assortative mating by female choice under restricted gene flow

The evolution of assortative mating is a key component of the process of speciation with gene flow. Several recent theoretical studies have pointed out, however, that sexual selection which can result from assortative mating may cause it to plateau at an intermediate level; this is primarily owing to search costs of individuals with extreme phenotypes and to assortative preferences developed by individuals with intermediate phenotypes. I explore the limitations of assortative mating further by analysing a simple model in which these factors have been removed. Specifically, I use a haploid two-population model to ask whether the existence of assortative mating is sufficient to drive the further evolution of assortative mating. I find that a weakening in the effective strength of sexual selection with strong assortment leads to the existence of both a peak level of trait differentiation and the evolution of an intermediate level of assortative mating that will cause that peak. This result is robust to the inclusion of local adaptation and different genetic architecture of the trait. The results imply the existence of fundamental limits to the evolution of assortment via sexual selection in this situation, with which other factors, such as search costs, may interact.     

Dispersal, disturbance and the contrasting biogeographies of New Zealand's diadromous and non-diadromous fish species

Aim To examine the relationship between diadromy and dispersal ability in New Zealand’s freshwater fish fauna, and how this affects the current environmental and geographic distributions of both diadromous and non‐diadromous species. Location New Zealand. Methods Capture data for 15 diadromous and 15 non‐diadromous fish species from 13,369 sites throughout New Zealand were analysed to establish features of their geographic ranges. Statistical models were used to determine the main environmental correlates of species’ distributions, and to establish the environmental conditions preferred by each species. Environmental predictors, chosen for their functional relevance, were derived from an extensive GIS database describing New Zealand’s river and stream network. Results In terms of geography, most diadromous species occur in a scattered fashion throughout extensive geographic ranges, and occupy large numbers of catchments of widely varying size. By contrast, most non‐diadromous species show relatively high levels of occupancy of smaller geographic ranges, and most are restricted to a few large catchments, particularly in the eastern South Island. In terms of environment, there is marked separation of diadromous from non‐diadromous species, with diadromous species generally caught most frequently in low‐gradient coastal rivers and streams with warm, maritime climates. With a few notable exceptions, most diadromous species have lower occurrence in river segments that are located above obstacles to upstream migration. Non‐diadromous species are usually caught in inland rivers and streams with cool, strongly seasonal climates, typified by a low frequency of high‐intensity rainfall events. Main conclusions We interpret the contrasting biogeographies of New Zealand’s diadromous and non‐diadromous species as reflecting interaction between their marked differences in dispersal ability and a landscape that is subject to recurrent, often large‐scale, natural disturbance. While both groups are likely to be equally susceptible to local, disturbance‐driven extinction, the much greater dispersal ability of diadromous species has allowed them to persist over wide geographic ranges. By contrast, the distributions of most non‐diadromous species are concentrated in a few large catchments, mostly in regions where less intense natural disturbance regimes are likely to have favoured their survival.     

Differential patterns of introgression across the X chromosome in a hybrid zone between two species of house mice

A complete understanding of the speciation process requires the identification of genomic regions and genes that confer reproductive barriers between species. Empirical and theoretical research has revealed two important patterns in the evolution of reproductive isolation in animals: isolation typically arises as a result of disrupted epistatic interactions between multiple loci and these disruptions map disproportionately to the X chromosome. These patterns suggest that a targeted examination of natural gene flow between closely related species at X-linked markers with known positions would provide insight into the genetic basis of speciation. We take advantage of the existence of genomic data and a well-documented European zone of hybridization between two species of house mice, Mus domesticus and M. musculus, to conduct such a survey. We evaluate patterns of introgression across the hybrid zone for 13 diagnostic X-linked loci with known chromosomal positions using a maximum likelihood model. Interlocus comparisons clearly identify one locus with reduced introgression across the center of the hybrid zone, pinpointing a candidate region for reproductive isolation. Results also reveal one locus with high frequencies of M. domesticus alleles in populations on the M. musculus side of the zone, suggesting the possibility that positive selection may act to drive the spread of alleles from one species on to the genomic background of the other species. Finally, cline width and cline center are strongly positively correlated across the X chromosome, indicating that gene flow of the X chromosome may be asymmetrical. This study highlights the utility of natural populations of hybrids for mapping speciation genes and suggests that the middle of the X chromosome may be important for reproductive isolation between species of house mice.     

Social constraints limit dispersal and settlement decisions in a group-living bird species

Dispersal is a fundamental process affecting the genetic structureof populations, speciation, and extinction. Nevertheless, ourunderstanding of the evolution of dispersal is limited by ourpaucity of knowledge on dispersal decisions at the individuallevel. We investigated the effect of interactions between residentsand juvenile dispersers on individual dispersal and settlementdecisions in Siberian jays (Perisoreus infaustus). In this group-livingbird species, some offspring remain on the parental territoryfor up to 3 years (retained juveniles) whereas other offspringdisperse within 2 months of fledging (dispersers). We foundthat retained juveniles constrained settlement decisions ofdispersers by aggressively chasing dispersers off their territory,resulting in dispersers continuing to disperse and settlingin groups without retained juveniles. Experimental removal ofmale breeders during the dispersal period also demonstratedthat dispersers were unable to settle in high-quality breedingopenings, which were instead filled by older nonbreeding residents.Rather, dispersers immigrated into groups without retained offspringwhere they became subordinate group members, queuing for a breedingopening. Also, they preferably settled in groups with shortqueues where no same-sex juveniles were present. Dispersal didnot inflict a cost to dispersers through increased mortality.However, the presence of immigrants was costly for breedersbecause it increased the rate of conflicts during the breedingseason which negatively affected nestling condition. These resultsdemonstrate that resident individuals constrain both dispersaland settlement decisions of dispersers. Social interactionsbetween residents and dispersers can thus be a key factor tounderstand the evolution of dispersal.