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To authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2745.2010.01715.x Byline: Donald A. Levin (*) Keywords: adaptation; ecological divergence; marginal habitats; niche shifts; phenotypic plasticity; reproductive ecology; reproductive isolation; self-compatibility; speciation Abstract: Summary 1. Pollen-mediated gene flow from ecological core populations may hamper the ability of populations to invade new, closely adjacent habitats. The potential for the invasion of a new habitat is enhanced when selection against immigrants is augmented by the elevation of self-fertilization. This shift imparts a measure of reproductive isolation to the invader. 2. Whereas genetically mediated changes in selfing often accompany niche shifts in plants, the elevation of self-fertilization by plastic responses to novel, stressful, environments has not been explored. Here I will show that invaders of novel habitats may have elevated levels of self-fertility mediated by alterations in floral form and self-compatibility. 3. Heightened selfing facilitates the evolution of local adaptation by increasing the additive genetic variance for tolerance to the new habitat. Heightened selfing also facilitates local adaptation by increasing reproductive assurance. 4. Elevated levels of selfing based on developmental responses are immediate, as opposed to that obtained by multigenerational selection. 5. Synthesis. The likelihood of a niche shift will be substantially enhanced if the invader is protected from the swamping effect of gene flow from a nearby core population. This protection may be afforded by an environment-induced elevation of self-fertility. The likelihood of a niche shift also is enhanced by an increase in reproductive assurance which may be vital to the survival of an invading population whose reproductive success is density-dependent. Article History: Received 17 March 2010; accepted 20 July 2010 Handling Editor: Peter Klinkhamer Article note: (*) Correspondence author. E-mail: dlevin@uts.cc.utexas.edu

To authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2745.2009.01499.x Byline: Donald A. Levin, Christopher D. Kelley, Sahotra Sarkar (*) Keywords: Allee effects; cellular automata; founder effects; individual-based models; S-alleles; self-incompatibility; viability model Abstract: Summary Founder populations of plants are usually small and may become extinct due to many factors including inbreeding and low pollen transfer besides stochastic factors. This positive relationship between population size and individual fitness is taken as evidence of an Allee effect. Very few studies have quantitatively compared the influence of self-incompatibility systems and S-allele counts on the magnitude of observed Allee effects for different population sizes and seed and pollen dispersal distances. This paper uses a stochastic, spatially explicit, individual-based cellular automaton model to simulate the dynamics of newly established plant populations with sporophytic or gametophytic self-incompatibility (SI) alleles and compares these to a null (Mendelian) population. Parameters varied in model simulation are the S-allele count, the initial population size, and the pollen and seed dispersal distances. These parameters were studied over a wide range to assess their relative impact on population viability compared to the null case in order to model the effect of self-incompatibility on population fitness. The results indicate that Allee effects are enhanced and growth rates are slowed in newly established populations of species with S-alleles relative to Mendelian populations, leading to enhanced extinction risk. They also show the importance of the relationship between pollen and seed dispersal ranges on the fitness of the population. Populations with sporophytic self-incompatibility were more extinction-prone than those with gametophytic SI, and the latter more so than Mendelian populations when all other factors were held constant. Synthesis. Overall, the simulation results indicate that Allee effects in small populations are enhanced in plants with S-alleles. Even in relatively large populations, S-allele count controls the strength of observed Allee effects. A high S-allele count is able to compensate for lower initial population size. Pollen and seed dispersal ranges had a similarly strong effect on the viability of populations with S-alleles. These effects are stronger in populations with sporophytic self-incompatibility than in those with gametophytic SI. Simulation results also indicate that S-alleles were not lost to genetic drift over time, in contrast with some earlier studies of S-allele systems displaying Allee effects. Article History: Received 11 October2008; accepted 16 February 2009Handling Editor: Peter Klinkhamer Article note: (*) Correspondence author. E-mail: sarkar@mail.utexas.edu

An attempt is made to determine whether supernumerary B chromosomes are nonrandomly distributed among major angiosperm lineages and among lineages within families, as well as the identity of lineages with unusually high B chromosome frequencies. The B-chromosome frequency and genome size across families are compared to help explain some of the variation observed in the prevalence of B chromosomes across taxa.

The world is awash in biological diversity. Yet even the least discerning observer will notice that the diversity of life is not a random sampling of all possible biological characteristics. [...]

The fitness effects due to initial flowering date in Phlox drummondii were determined for three populations in central Texas (USA) over 3 yr (1990-1992). Mean fitness (seed set) always decreased with the later initiation of flowering. The likelihood of a plant fruiting differed with flowering date in five of the six instances (population by year combinations). Though plants that initiated flowering later tended to have spent more time in the vegetative stage and tended to die later in the year than did earlier flowering plants, this was not sufficient to overcome the reproductive penalties of flowering late. Plants that initiated flowering later in the season spent less time in the adult phase and were smaller. The mean number of flowers, fruits, and seeds per flowering plant always decreased with later flowering. Fruit set was negatively correlated with flowering date in four of the six population by year combinations. Nonparametric fitness functions were used to summarize predicted fitness among different initial flowering dates for each population on a yearly basis. Predicted mean fitness always declined nonlinearly with later flowering; the earliest flowering plants always had the highest predicted fitness. These fitness functions describe directional selection for the early initiation of flowering. Key words: flowering date; directional selection; nonlinear fitness functions; Phlox drummondii; Polemoniaceae.

Genetic and phenotypic correlations in plants are discussed relative to a botanical test of the conjecture of J.M. Cheverud that phenotypic correlations have high probability of being fair estimates of genetic counterparts in many situations. A survey of the literature of 40 years for agriculture and evolution has been carried out to find the degree to which phenotypic correlations agree with genetic counterparts in plants, and it was found that traits from the same suite of characters were more correlated phenotypically and genetically than those from different suites.

1 The fitness consequences of emergence date in Phlox drummondii were determined for three populations in central Texas (USA). We experimentally manipulated the emergence time of seedlings to extend the distribution of emergents across time. 2 In all years (1989-92) significant differences in fitness were found between populations and between emergence dates. Fitness decreased with later seedling emergence in five of six population-by-year combinations tested. 3 The frequency of juvenile mortality also differed with emergence date in five cases. 4 Later emerging plants flowered sooner than would be expected based on the range of emergence dates, but this was not sufficient to overcome the penalty of late emergence on flower and fruit production. Total flowers per flowering plant always decreased with later emergence, and total fruits and seed-set were negatively correlated with emergence date for two of the three populations. 5 Fitness functions were used to describe the fitness relationship between emergence dates. In all cases individual fitness declined non-linearly at later emergence dates, and in many cases the earliest emergents had the highest expected fitness. The variance in fitness differed among years and sites, and ranged from 0.043 to 0.766 seeds [individual.sup.-2]. 6 The degree to which days to seedling emergence was heritable in two further native populations of P. drummondii was measured by conducting divergent artificial selection for two generations. The estimates of realized heritability (0.081 and 0.15) could not be distinguished from zero. 7 Phlox drummondii therefore appears to have little genetic capacity to respond to natural or artificial selection on the timing of emergence. Keywords: emergence date, heritability, natural selection, non-linear fitness functions, Phlox drummondii

'The evolutionary pathway from obligate outcrossing based on self-incompatibility to predominant self-fertilization probably has been followed by more different flowering plant lineages than any other' (Stebbins 1970). In spite of [...]

Conspicuous variation in corolla color is a striking feature of some populations in a multitude of plant species. Variation in both space and time has been documented in Justicia simplex [...]

A study of the reproductive biology of outliers and of rare plants reveals that they produce lesser seeds per flower than plants within local populations but are capable of interbreeding over long distances. Outliers from large gatherings of interbreeding plants are the major sources of extrinsic pollen and seeds, disturbing the spacing of local populations forming the nuclei for new populations. Outliers serve as interpopulation links for pathogens and herbivores.

To determine if the evolution of fitness traits in the annual plant, Phlox drummondii, is constrained by lack of genetic variation, we calculated the heritabidlity and genetic correlation of 16 traits in a field population. Full- and half-sib families of seeds were generated in the greenhouse and planted back into the study population. Of 855 seeds that germinated, 609 survived to produce fruit. For each plant we measured several aspects of plant size and three components of female fecundity: total number of fruits produced, number of seeds per fruit, and mass of individual seeds. Heritability of traits ranged from 0.00 to 0.15. Several traits showed significant levels of additive genetic variance, but we found no evidence of additive genetic variance in components of female fecundity and no evidence of negative genetic correlation between fitness traits. These results suggest that evolution in this population would be constrained by lack of heritable variation in fitness traits.

The numbers are grim: Some 2,000 species of Pacific Island birds (about 15 percent of the world total) have gone extinct since human colonization. Roughly 20 of the 297 known [...]

A study was conducted to investigate that the abundance of polyploidy might be the result of a simple ratcheting process that does not require evolutionary advantages due to the biological properties of organisms. It was shown by using a simple mathematical model that average ploidal level within a plant lineage could continually increase to the levels observed, even if there are ecological or physiological disadvantages to higher ploidy.

Research has been conducted on chloroplast capture which is responsible for inconsistencies between gene trees. The authors have used the genetic model developed for the determination of capture condition occurrence, and they have concluded that these conditions are general.

Inbreeding depression was measured in terms of the relative survivorship of self versus cross seed in 14 Phlox drummondii cultivars and 10 populations of P. cuspidata. The cultivars are predominantly outcrossing; p. cuspidata is predominantly self-fertilizing. The relative survivorship of self versus cross seed averages 0.84 in the cultivars and 0.99 in P. cuspidata. In wild P. drummondii, the average is 0.83. The mean number of lethal equivalents per zygote averages 0.69 in the cultivars and 0.05 in P.cuspidata, versus 0.79 in the wild P. drummondii. There is a significant negative correlation between the amount of automatic self-seed produced by a cultivar and the number of lethal equivalents it carries (r = -0.60). Phlox cuspidata has a lower level of cross-seed abortion than P. drummondii, which probably is not due to a difference in the incidence of lethal genes in their populations.

A controlled pollination study, involving a diallel crossing scheme among 15 Phlox drummondii individuals grown under either control, low water or low nutrient conditions, was performed. The results of this research demonstrate that, although plant size and flower number are responsive to the treatments, pollen germination percentage, the survival probability of developing seeds and the weight of mature seeds are buffered against water and nutrient deprivation. However, the identities of male and female parents did have significant effects on pollen germination and seed survival, and female identity had a significant effect on seed weight.

Variation of 20 quantitative characters was examined within and among 10 populations of the predominantly outcrossing Phlox drummondii and 4 populations of the predominantly selfing P. cuspidata grown in a greenhouse. Multivariate analysis of variance, considering all characters simultaneously, indicated that there were significant differences among populations in both species while analysis of individual characters demonstrated that there were significant population differences for 19 characters in P. drummondii and 13 characters in P. cuspidata. On average, 16% of the total phenotypic variation in P. drummondii occurred among populations compared to less than 4% of the toal variation in P. cuspidata. In addition, P. drummondii exhibited significant differences among families within populations more frequently than P. cuspidata. Most observed variation in both species occurred within families where environmental to have higher heritabilities than P. cuspidata for most characters even when assumptions about breeding systems were relaxed. Thus, the outbreeding species exhibited larger genetic differences among populations and among families within populations than the selfing species in the greenhouse environment. These data suggest that P. drummondii has the greater evolutionary potential of the two species and are consistent with the hypothesis that differences in population structure result from differences in the breeding systems of the two species.

MICHAEL L. ARNOLD. Oxford University Press, New York, 1997. xiv, 215 pp., illus. $60 or 45 £, ISBN 0-19-509974-5; paper, $29.95 or 22.50 £, ISBN 0-19-509775-3. Oxford Series in Ecology [...]