Hypothesis Of Divergence Research Articles

Cardiolipin Is Not Required for Bax-mediated Cytochrome c Release from Yeast Mitochondria

Cardiolipin (CL) is an inner mitochondrial membrane phospholipid that contributes to optimal mitochondrial function and is gaining widespread attention in studies of mitochondria-mediated apoptosis. Divergent hypotheses describing the role of CL in cytochrome c release and apoptosis have evolved. We addressed this controversy directly by comparing the spontaneous- and Bax-mediated cytochrome c release from mitochondria isolated from two strains of Saccharomyces cerevisiae: one lacking CL-synthase and therefore CL (DeltaCRD1) and the other, its corresponding wild type (WT). We demonstrated by liquid chromatography-mass spectrometry that the main yeast CL species [(16:1)2(18:1)2] differs in fatty acid composition from mammalian CL [(18:2)4], and we verified the absence of the yeast CL species in the DeltaCRD1 strain. We also demonstrated that the mitochondrial association of Bax and the resulting cytochrome c release is not dependent on the CL content of the yeast mitochondrial membranes. Bax inserted equally into both WT and DeltaCRD1 mitochondrial membranes under conditions that lead to the release of cytochrome c from both strains of yeast mitochondria. Furthermore, using models of synthetic liposomes and isolated yeast mitochondria, we found that cytochrome c was bound more "loosely" to the CL-deficient systems compared with when CL is present. These data challenge recent studies implicating that CL is required for Bax-mediated pore formation leading to the release of proteins from the mitochondrial intermembrane space. In contrast, they support our recently proposed two-step mechanism of cytochrome c release, which suggests that CL is required for binding cytochrome c to the inner mitochondrial membrane.

RESPONSE: Re: Melanocytic Nevi, Solar Keratoses, and Divergent Pathways to Cutaneous Melanoma

We note with interest the findings of Carli and Palli, which replicate our observations (1) that patients with melanomas of the head and neck were older and were statistically significantly less likely to have numerous nevi than patients with truncal melanoma. Patients in this Italian study were not originally selected with the specific intention of testing the divergent pathway hypothesis; nevertheless, particular aspects of the study design serve to strengthen our belief that the observed effects are real.

Gradual evolution of male genitalia in a sibling species complex of millipedes (Diplopoda : Spirobolida : Rhinocricidae : Anadenobolus)

Jamaican millipedes in the Anadenobolus species complex provide an unusual case study of arthropods having undergone speciation in the absence of conspicuous divergence of male genitalia. Using landmark-based morphometrics, we examined shape deformation of the male anterior copulatory device in three genetically divergent yet morphologically cryptic species. A multivariate analysis of variance and relative warp analysis of nonuniform components show that although male genitalic shape is statistically different among species, many specimens are 'misplaced' in morphological space, perhaps consistent with a condition analogous to incomplete lineage sorting. A simulation of neutral nuclear gene coalescence suggests that such incomplete sorting is expected, given the depth of mtDNA divergences observed across species. The pronounced contrast between deep molecular v. incomplete genitalic divergence is at odds with the paradigm of selection-driven rapid change in male copulatory structure during arthropod speciation. Alternatively, we suggest that male genitalic divergence is evolving neutrally or in concert with other components of the genome (pleiotropy). Although we recognise the empirical validity of rapid genitalic divergence via sexual selection or sexual conflict, such models must be empirically tested using multiple lines of evidence. Accepting the rapid and divergent hypothesis without such multiple evidence scrutiny may result in a gross underestimation of evolutionary diversity and, subsequently, the misinterpretation of processes shaping genitalic change.

Complexity reduction for "large image" processing

We present a method for sampling feature vectors in large (e.g., 2000 /spl times/ 5000 /spl times/ 16 bit) images that finds subsets of pixel locations which represent c "regions" in the image. Samples are accepted by the chi-square (/spl chi//sup 2/) or divergence hypothesis test. A framework that captures the idea of efficient extension of image processing algorithms from the samples to the rest of the population is given. Computationally expensive (in time and/or space) image operators (e.g., neural networks (NNs) or clustering models) are trained on the sample, and then extended noniteratively to the rest of the population. We illustrate the general method using fuzzy c-means (FCM) clustering to segment Indian satellite images. On average, the new method can achieve about 99% accuracy (relative to running the literal algorithm) using roughly 24% of the image for training. This amounts to an average savings of 76% in CPU time. We also compare our method to its closest relative in the group of schemes used to accelerate FCM: our method averages a speedup of about 4.2, whereas the multistage random sampling approach achieves an average acceleration of 1.63.

Life on a limb: ecology of the tree agama (Acanthocercus a. atricollis) in southern Africa

AbstractOne hundred and sixty‐four museum specimens of the tree agama Acanthocercus a. atricollis were measured and dissected to examine sexual size dimorphism, reproduction and diet. Foraging mode and behaviour were also quantified in a wild population to obtain a broader picture of their foraging ecology and to test the hypothesis that tree agamas are ambush foragers. Males and females did not differ significantly in snout–vent length (SVL) or tail length; however, mature males had larger heads than females of the same body size. The smallest female showing sexual maturity was 96 mm SVL and the smallest male was 82 mm SVL. Mean clutch size was 11.3 and was positively correlated with female body size. Reproduction was seasonal and male and female reproductive cycles were synchronous. Testicular volume was greatest during August–September and females contained enlarged follicles during August–December and showed no evidence of multiple clutching. Tree agamas fed on a broad spectrum of arthropods (10 orders), including millipedes, which other lizard taxa have been reported to avoid. Gut contents were dominated numerically by ants (92%), followed by beetles (4%). Volumetrically, orthopterans (26.8%) were most important, followed by beetles (26.3%) and ants (17.9%). Compared to adults, juvenile diet by volume was dominated by ants and consisted of fewer large prey items (e.g. beetles and orthopterans). Seasonal effects in both prey diversity and volume were evident. Tree agamas are classic ambush foragers. They spent only 4% of their time moving and made few movements per minute (mean = 0.4). When stationary, adult tree agamas positioned themselves on tree trunks (34.7%), on lateral branches (41.8%) and occasionally, on the ground (23.4%). No evidence of trophic partitioning (intraspecific niche divergence hypothesis) was found and field observations revealed that males defend territories and engage in combat. This supports the idea that selection may be favouring larger head size in males, as an outcome of male contest competition.

MORPHOLOGICAL PLASTICITY AND DIVERGENCE IN LIFE-HISTORY TRAITS BETWEEN TWO CLOSELY RELATED FRESHWATER SNAILS, LYMNAEA OVATA AND LYMNAEA PEREGRA

To reveal whether there are genetic differences between two closely related freshwater snails, Lymnaea peregra and L. ovata, which differ in habitat preferences, we studied life-history and morphological variation under laboratory conditions. The status of these taxa as biological species remains under debate, since it is not known to what extent their characteristic shell forms are genetically determined. For each species, we used individuals from two separate origins in experiments to test whether lifehistory variation is more pronounced between populations of different species than between populations of the same species, as would be expected if genetic divergence rather than phenotypic plasticity explains the variation. In addition, we compared shell morphology of wild-caught individuals to individuals maintained in the laboratory for two generations. Our results indicate that when these species are grown under the same conditions, the life-history traits (juvenile growth rate and reproductive schedule) do not converge, supporting the genetic divergence hypothesis. However, the shell of both L. ovata and L. peregra showed considerable morphological plasticity in response to laboratory conditions and converged to a similar form in just two laboratory generations.

Maximum-likelihood divergence date estimates based on rRNA gene sequences suggest two scenarios of Trypanosoma cruzi intraspecific evolution.

The phylogenetic relationships of Trypanosoma cruzi strains were inferred using maximum-likelihood from complete 18S rDNA sequences and D7-24Salpha rDNA regions from 20 representative strains of T. cruzi. For this we sequenced the 18S rDNA of 14 strains and the D7-24Salpha rDNA of four strains and aligned them to previously published sequences. Phylogenies inferred from these data sets identified four groups, named Riboclades 1, 2, 3, and 4, and a basal dichotomy that separated Riboclade 1 from Riboclades 2, 3, and 4. Substitution models and other parameters were optimized by hierarchical likelihood tests, and our analysis of the 18S rDNA molecular clock by the likelihood ratio test suggests that a taxa subset encompassing all 2,150 positions in the alignment supports rate constancy among lineages. The present analysis supports the notion that divergence dates of T. cruzi Riboclades can be estimated from 18S rDNA sequences and therefore, we present alternative evolutionary scenarios based on two different views of T. cruzi intraspecific divergence. The first assumes a faster evolutionary rate, which suggests that the divergence between T. cruzi I and II and the extant strains occurred in the Tertiary period (37-18 MYA). The other, which supports the hypothesis that the divergence between T. cruzi I and II occurred in the Cretaceous period (144-65 MYA) and the divergence of the extant strains occurred in the Tertiary period of the Cenozoic era (65-1.8 MYA), is consistent with our previously proposed hypothesis of divergence by geographical isolation and mammalian host coevolution.

Neolecta: A Fungal Dinosaur? Evidence from [beta]-Tubulin Amino Acid Sequences

Were the first ascomycetes yeast-like (unicellular) or filamentous with complex multicellular growth? Molecular studies have indicated that the earliest lineages of ascomycetes were mostly yeast-like and without complex, multicellular fruit bodies. The genus Neolecta stands out as an exception. Neolecta spp. have filamentous growth and discomycete-type of fruit bodies, but constitute one of the basal lineages both in rRNA and RPB2 gene trees. In this paper, we have used 10 previously unpublished and 30 GenBank β-tubulin genes to test the phylogenetic position of Neolecta spp. We compared the phylogenetic information from the amino acid sequences (485 characters) with the phylogenetic information content of 33 characters from intron gains and losses. Due to gene duplications, two paralogous versions of β-tubulin genes occurred in four species included in our analysis. Although phylogenetic interpretation of β-tubulin gene trees was complicated by a history of gene duplications, intron gains and losses, and by unequal rates of amino acid substitution, Neolecta species never formed a monophyletic group with any of the sequences from the filamentous ascomycetes in the Pezizomycotina (euascomycetes) in parsimony or distance analyses. Like the RPB2 and rRNA genes, the β-tubulin genes support the hypotheses of an early divergence of Neolecta from superficially similar filamentous ascomycetes. Neolecta could become a key taxon particularly in comparative studies between the fungal model organisms in the mainly unicellular (=yeast-like) taxa Saccharomyces and Schizosaccharomyces and the filamentous fruit-body forming taxa Neurospora and Aspergillus.

Neolecta—a fungal dinosaur? Evidence from β-tubulin amino acid sequences

Were the first ascomycetes yeast-like (unicellular) or filamentous with complex multicellular growth? Molecular studies have indicated that the earliest lineages of ascomycetes were mostly yeast-like and without complex, multicellular fruit bodies. The genus Neolecta stands out as an exception. Neolecta spp. have filamentous growth and discomycete-type of fruit bodies, but constitute one of the basal lineages both in rRNA and RPB2 gene trees. In this paper, we have used 10 previously unpublished and 30 GenBank β-tubulin genes to test the phylogenetic position of Neolecta spp. We compared the phylogenetic information from the amino acid sequences (485 characters) with the phylogenetic information content of 33 characters from intron gains and losses. Due to gene duplications, two paralogous versions of β-tubulin genes occurred in four species included in our analysis. Although phylogenetic interpretation of β-tubulin gene trees was complicated by a history of gene duplications, intron gains and losses, and by unequal rates of amino acid substitution, Neolecta species never formed a monophyletic group with any of the sequences from the filamentous ascomycetes in the Pezizomycotina (euascomycetes) in parsimony or distance analyses. Like the RPB2 and rRNA genes, the β-tubulin genes support the hypotheses of an early divergence of Neolecta from superficially similar filamentous ascomycetes. Neolecta could become a key taxon particularly in comparative studies between the fungal model organisms in the mainly unicellular (=yeast-like) taxa Saccharomyces and Schizosaccharomyces and the filamentous fruit-body forming taxa Neurospora and Aspergillus.

Bilingual signed and spoken language acquisition from birth: implications for the mechanisms underlying early bilingual language acquisition

Divergent hypotheses exist concerning the types of knowledge underlying early bilingualism, with some portraying a troubled course marred by language delays and confusion, and others portraying one that is largely unremarkable. We studied the extraordinary case of bilingual acquisition across two modalities to examine these hypotheses. Three children acquiring Langues des Signes Québécoise and French, and three children acquiring French and English (ages at onset approximately 1;0, 2;6 and 3;6 per group) were videotaped regularly over one year while we empirically manipulated novel and familiar speakers of each child's two languages. The results revealed that both groups achieved their early linguistic milestones in each of their languages at the same time (and similarly to monolinguals), produced a substantial number of semantically corresponding words in each of their two languages from their very first words or signs (translation equivalents), and demonstrated sensitivity to the interlocutor's language by altering their language choices. Children did mix their languages to varying degrees, and some persisted in using a language that was not the primary language of the addressee, but the propensity to do both was directly related to their parents' mixing rates, in combination with their own developing language preference. The signing-speaking bilinguals did exploit the modality possibilities, and they did simultaneously mix their signs and speech, but in semantically principled and highly constrained ways. It is concluded that the capacity to differentiate between two languages is well in place prior to first words, and it is hypothesized that this capacity may result from biological mechanisms that permit the discovery of early phonological representations. Reasons why paradoxical views of bilingual acquisition have persisted are also offered.

Differential selection after duplication in mammalian developmental genes.

Gene duplication provides the opportunity for subsequent refinement of distinct functions of the duplicated copies. Either through changes in coding sequence or changes in regulatory regions, duplicate copies appear to obtain new or tissue-specific functions. If this divergence were driven by natural selection, we would expect duplicated copies to have differentiated patterns of substitutions. We tested this hypothesis using genes that duplicated before the human/mouse split and whose orthologous relations were clear. The null hypothesis is that the number of amino acid changes between humans and mice was distributed similarly across different paralogs. We used a method modified from Tang and Lewontin to detect heterogeneity in the amino acid substitution pattern between those different paralogs. Our results show that many of the paralogous gene pairs appear to be under differential selection in the human/mouse comparison. The properties that led to diversification appear to have arisen before the split of the human and mouse lineages. Further study of the diverged genes revealed insights regarding the patterns of amino acid substitution that resulted in differences in function and/or expression of these genes. This approach has utility in the study of newly identified members of gene families in genomewide data mining and for contrasting the merits of alternative hypotheses for the evolutionary divergence of function of duplicated genes.

Testing models of diversification in mice in the Abrothrix olivaceus/xanthorhinus complex in Chile and Argentina.

Samples of the forest-dwelling mouse Abrothrix olivaceus and the steppe-dwelling A. xanthorhinus across a transect between 45 and 47 degrees S in southern Chile were analysed using the mitochondrial cytochrome b (cyt b) sequence, substantially adding to the data presented previously for these taxa from Argentina and Chile. The level of variation in the cyt b sequence throughout the entire olivaceus/xanthorhinus complex is comparable to that seen within a single species in many South American sigmodontine rodents, consistent with a previous conclusion that both taxa are sub-species of A. olivaceus. Haplotypes of xanthorhinus have not yet achieved reciprocal monophyly relative to those of olivaceus. We evaluate competing hypotheses for the morphological divergence of xanthorhinus and olivaceus by allopatry in Pleistocene refuges versus postglacial diversification across ecological gradients. Two contrasting patterns are predicted for plots of the distribution of pairwise genetic differences, depending on whether the taxa diverged in allopatric refuges or through selection across a gradient. Examples of both modes of diversification are found in this complex.

Molecular Evidence of an African Phiomorpha–South American Caviomorpha Clade and Support for Hystricognathi Based on the Complete Mitochondrial Genome of the Cane Rat (Thryonomys swinderianus)

The complete mitochondrial genome of an African cane rat, Thryonomys swinderianus (Rodentia, Hystricognathi), was included in a phylogenetic analysis along with 4 rodents, 14 additional eutherians, and 3 noneutherian outgroups. Monophyly of the suborder Hystricognathi, represented by the cane rat and the South American guinea pig, Cavia porcellus, was strongly supported by maximum-parsimony, neighbor-joining, and maximum-likelihood methods. The molecular-based estimate of the divergence time of Old and New World Hystricognathi (approximately 85 million years before present, MYBP) is consistent with an hypothesis of vicariance divergence due to the rifting of the African and South American continents 86–100 MYBP. Monophyly of Rodentia or the superordinal clade Glires (Rodentia and Lagomorpha) were not supported.

Refugial isolation versus ecological gradients. Testing alternative mechanisms of evolutionary divergence in four rainforest vertebrates.

Hypotheses for divergence and speciation in rainforests generally fall into two categories: those emphasizing the role of geographic isolation and those emphasizing the role of divergent selection along gradients. While a majority of studies have attempted to infer mechanisms based on the pattern of species richness and congruence of geographic boundaries, relatively few have tried to simultaneously test alternative hypotheses for diversification. Here we discuss four examples, taken from our work on diversification of tropical rainforest vertebrates, in which we examine patterns of genetic and morphological variation within and between biogeographic regions to address two alternative hypotheses. By estimating morphological divergence between geographically contiguous and isolated populations under similar and different ecological conditions, we attempt to evaluate the relative roles of geographic isolation and natural selection in population divergence. Results suggest that natural selection, even in the presence of appreciable gene flow, can result in morphological divergence that is greater than that found between populations isolated for millions of years and, in some cases, even greater than that found between congeneric, but distinct, species. The relatively small phenotypic divergence that occurs among long-term geographic isolates in similar habitats suggests that morphological divergence via drift may be negligible and/or that selection is acting to produce similar phenotypes in populations occupying similar habitats. Our results demonstrate that significant phenotypic divergence: (1) is not necessarily coupled with divergence in neutral molecular markers; and (2) can occur without geographic isolation in the presence of gene flow.

Evidence of adaptive divergence in plasticity: density- and site-dependent selection on shade-avoidance responses in Impatiens capensis.

We investigated the conditions under which plastic responses to density are adaptive in natural populations of Impatiens capensis and determined whether plasticity has evolved differently in different selective environments. Previous studies showed that a population that evolved in a sunny site exhibited greater plasticity in response to density than did a population that evolved in a woodland site. Using replicate inbred lines in a reciprocal transplant that included a density manipulation, we asked whether such population differentiation was consistent with the hypothesis of adaptive divergence. We hypothesized that plasticity would be more strongly favored in the sunny site than in the woodland site; consequently, we predicted that selection would be more strongly density dependent in the sunny site, favoring the phenotype that was expressed at each density. Selection on internode length and flowering date was consistent with the hypothesis of adaptive divergence in plasticity. Few costs or benefits of plasticity were detected independently from the expressed phenotype, so plasticity was selected primarily through selection on the phenotype. Correlations between phenotypes and their plasticity varied with the environment and would cause indirect selection on plasticity to be environment dependent. We showed that an appropriate plastic response even to a rare environment can greatly increase genotypic fitness when that environment is favorable. Selection on the measured characters contributed to local adaptation and fully accounted for fitness differences between populations in all treatments except the woodland site at natural density.

U.S. and Canadian Industrial Relations: Convergent or Divergent?

Two models, the divergence and the convergence models, address comparisons of Canadian and American industrial relations. Most specialists support the divergence hypothesis. It asserts that the two countries' systems have produced major transnational differences in industrial relations outputs. The convergence model reassessed the assumptions, data, and conclusions of the divergent model and concluded that the two countries' systems produced very similar, although not identical, industrial relations outputs.

EVIDENCE OF ADAPTIVE DIVERGENCE IN PLASTICITY: DENSITY- AND SITE-DEPENDENT SELECTION ON SHADE-AVOIDANCE RESPONSES IN IMPATIENS CAPENSIS

We investigated the conditions under which plastic responses to density are adaptive in natural populations of Impatiens capensis and determined whether plasticity has evolved differently in different selective environments. Previous studies showed that a population that evolved in a sunny site exhibited greater plasticity in response to density than did a population that evolved in a woodland site. Using replicate inbred lines in a reciprocal transplant that included a density manipulation, we asked whether such population differentiation was consistent with the hypothesis of adaptive divergence. We hypothesized that plasticity would be more strongly favored in the sunny site than in the woodland site; consequently, we predicted that selection would be more strongly density dependent in the sunny site, favoring the phenotype that was expressed at each density. Selection on internode length and flowering date was consistent with the hypothesis of adaptive divergence in plasticity. Few costs or benefits of plasticity were detected independently from the expressed phenotype, so plasticity was selected primarily through selection on the phenotype. Correlations between phenotypes and their plasticity varied with the environment and would cause indirect selection on plasticity to be environment dependent. We showed that an appropriate plastic response even to a rare environment can greatly increase genotypic fitness when that environment is favorable. Selection on the measured characters contributed to local adaptation and fully accounted for fitness differences between populations in all treatments except the woodland site at natural density.Editor: A. Caballero

A TEST OF THE GLACIAL REFUGIUM HYPOTHESIS USING PATTERNS OF MITOCHONDRIAL AND NUCLEAR DNA SEQUENCE VARIATION IN ROCK PTARMIGAN (LAGOPUS MUTUS).

The glacial refugium hypothesis (GRH) proposes that glaciers promoted differentiation and generation of intraspecific diversity by isolating populations in ice-free refugia. We tested three predictions of this hypothesis for the evolutionary divergence of rock ptarmigan (Lagopus mutus) during the Wisconsin glaciation of the late Pleistocene. To do this, we examined subspecies distributions, population genetic structure, and phylogenetic relationships in 26 populations across North America and the Bering Sea region. First, we analyzed sequence variation in the mitochondrial control region, in a nuclear intron (Gapdh), and in an internal transcribed spacer (ITS1). Control region sequences of 154 rock ptarmigan revealed strong population and phylogeographic structure. Variation in intron sequences of 114 rock ptarmigan also revealed significant population structure compatible with results for the control region. Rock ptarmigan were invariant for ITS1. Second, we show that five known Nearctic refugia and an Icelandic refugium are concordant with the current distribution of morphologically distinct subspecies; five of these six refugia are geographically concordant with the distribution of closely related control region haplotypes. Third, our estimates of the time since phylogenetic lineages diverged predated the last glacial maximum for all but two lineages. In addition, all lines of evidence suggest that two unknown refugia in the Bering Sea region supported rock ptarmigan during the Wisconsin glaciation. Overall, our results are most consistent with the hypothesis that isolated populations of rock ptarmigan diverged in multiple refugia during the Wisconsin and that geographic variation reflects patterns of recolonization of the Nearctic after the ice receded. The GRH may therefore offer the most plausible explanation for similar biogeographic patterns in a variety of Nearctic vertebrates.

Sexual dimorphism of head size in Gallotia galloti: testing the niche divergence hypothesis by functional analyses

1. Two often cited hypotheses explaining sexual head size dimorphism in lizards are: sexual selection acting on structures important in intrasexual competition, and reduction of intersexual competition through food niche separation. 2. In this study some implicit assumptions of the latter hypothesis were tested, namely that an increase in gape distance and bite force should accompany the observed increase in head size. These assumptions are tested by recording bite forces, in vivo, for lizards of the species Gallotia galloti. In this species, male lizards have significantly larger heads than female conspecifics of similar snout–vent length. 3. Additionally, the average force needed to crush several potential prey species was determined experimentally and compared with the bite force data. This comparison clearly illustrates that animals of both sexes can bite much harder than required for most insect food items, which does not support the niche divergence hypothesis. The apparent ‘excess’ bite force in both sexes might be related to the partially herbivorous diet of the animals. 4. To unravel the origin of differences between sexes in bite capacity, the crushing phase of biting was modelled. The results of this model show different strategies in allocation of muscle tissue between both sexes. The origin of this difference is discussed and a possible evolutionary pathway of the development of the sexual dimorphism in the species is provided.

Partner Preferences as a Function of Gender, Age, Political Orientation and Level of Education

An important current debate concerns the originof gender differences in partner preferences. Thesedifferences have been explained both in terms ofevolutionary theory and in terms of social role theory. The present study determines the relativestrengths of both perspectives by investigating, apartfrom gender, the influence of three other importantfactors on partner preferences and for which the two approaches offer divergent hypotheses: age,political orientation and level of education of therespondent. About 95% of the participants were WhiteDutch citizens, the rest were Dutch with one or twoparents from a different ethnic background.Participants were requested to write down the mostimportant characteristics of a potential partner(open-ended format), followed by an instruction toindicate the importance of 39 pre-selected characteristics. Resultsshow that men and women have highly similar preferencesfor characteristics in a potential partner. In addition,it is demonstrated that on crucial characteristics from an evolutionary perspective (i.e.,physical attractiveness and status) significantinteractions between age, political orientation, levelof education and/or gender of the respondents emerge.Most results offer support for a social role theoryof human mate selection. It is concluded that becausegender on its own merely explains a small proportion ofthe total variance in human mate selection, it is important to include other factors, not onlyin order to facilitate our understanding of the fullcomplexity of partner preferences, but also in order tomake theoretical progress in this area.