Assortative mating and offspring well-being: theory and empirical findings from a native Amazonian society in Bolivia

Mate choice matters for inclusive fitness, household economic efficiency, assimilation, stratification, and economic inequalities in society. In positive assortative mating, people pair with someone who resembles them along a trait, whereas in negative assortative mating, people pair with someone who differs from them along a trait. In industrial nations, people tend to follow positive assortative mating for fundamental demographic dimensions (e.g., age, schooling) and might practice negative assortative mating for economic outcomes (e.g., earnings). Research on assortative mating has focused on industrial nations, generally compared only one trait between couples, and paid scant attention to the effects of assortative mating for offspring well-being. If assortative mating enhances inclusive fitness, it might also enhance offspring well-being. Drawing on data from a farming–foraging society in the Bolivian Amazon (Tsimane') that practices preferential cross-cousin marriage, we (a) identify six parental traits (age, knowledge, wealth, schooling, height, and smiles) for which Tsimane' might practice assortative mating and (b) test the hypothesis that assortative mating enhances offspring well-being. Proxies for offspring well-being include height and school attainment. Tsimane' resemble people of industrial nations in practicing mostly positive assortative mating. Pairwise, mother–father and Pearson correlations of age, schooling, and earnings among Tsimane' resemble correlations of industrial nations. Correlation coefficients for the six parental traits were far higher than correlations that might happened just by chance. We found weak support for the hypothesis that assortative mating improves offspring well-being.     

Sexual size dimorphism and assortative mating for morphological traits in Passer domesticus

In this study, assortative mating for different morphological traits was studied in a captive population of house sparrows (Passer domesticus). Males were larger than females. Assortative mating was found for tail length, wing length and general body size. Males with larger badge size mated with females with longer tails. The strongest assortative mating occurred for tail length (r=0.77), and this assortative mating remained significant after controlling for wing length, mass and tarsus length, suggesting that it was not an artefact of assortative mating for body size. The possibility of sexual selection for tail length in the house sparrow is discussed.     

Adaptive speciation when assortative mating is based on female preference for male marker traits

Adaptive speciation occurs when frequency-dependent ecological interactions generate conditions of disruptive selection to which lineage splitting is an adaptive response. Under such selective conditions, evolution of assortative mating mechanisms enables the break-up of the ancestral lineage into diverging and reproductively isolated descendent species. Extending previous studies, I investigate models of adaptive speciation due to the evolution of indirect assortative mating that is based on three different mating traits: the degree of assortativity, a female preference trait and a male marker trait. For speciation to occur, linkage disequilibria between different mating traits, e.g. between female preference and male marker traits, as well as between mating traits and the ecological trait, must evolve. This can lead to novel speciation scenarios, e.g. when reproductive isolation is generated by a splitting in the degree of assortativeness, with one of the emerging lineages mating assortatively, and the other one disassortatively. I investigate the effects of variation in various model parameters on the likelihood of speciation, as well as robustness of speciation to introducing costs of assortative mating. Even though in the models presented speciation requires the genetic potential for strong assortment as well as rather restrictive ecological conditions, the results show that adaptive speciation due to the evolution of assortative mating when mate choice is based on separate female preference and male marker traits is a theoretically plausible evolutionary scenario.     

Sexual dimorphism and adaptive speciation: two sides of the same ecological coin

Models of adaptive speciation are typically concerned with demonstrating that it is possible for ecologically driven disruptive selection to lead to the evolution of assortative mating and hence speciation. However, disruptive selection could also lead to other forms of evolutionary diversification, including ecological sexual dimorphisms. Using a model of frequency-dependent intraspecific competition, we show analytically that adaptive speciation and dimorphism require identical ecological conditions. Numerical simulations of individual-based models show that a single ecological model can produce either evolutionary outcome, depending on the genetic independence of male and female traits and the potential strength of assortative mating. Speciation is inhibited when the genetic basis of male and female ecological traits allows the sexes to diverge substantially. This is because sexual dimorphism, which can evolve quickly, can eliminate the frequency-dependent disruptive selection that would have provided the impetus for speciation. Conversely, populations with strong assortative mating based on ecological traits are less likely to evolve a sexual dimorphism because females cannot simultaneously prefer males more similar to themselves while still allowing the males to diverge. This conflict between speciation and dimorphism can be circumvented in two ways. First, we find a novel form of speciation via negative assortative mating, leading to two dimorphic daughter species. Second, if assortative mating is based on a neutral marker trait, trophic dimorphism and speciation by positive assortative mating can occur simultaneously. We conclude that while adaptive speciation and ecological sexual dimorphism may occur simultaneously, allowing for sexual dimorphism restricts the likelihood of adaptive speciation. Thus, it is important to recognize that disruptive selection due to frequency-dependent interactions can lead to more than one form of adaptive splitting.     

No evidence for size‐assortative mating in the wild despite mutual mate choice in sex‐role‐reversed pipefishes

Size‐assortative mating is a nonrandom association of body size between members of mating pairs and is expected to be common in species with mutual preferences for body size. In this study, we investigated whether there is direct evidence for size‐assortative mating in two species of pipefishes, Syngnathus floridae and S. typhle, that share the characteristics of male pregnancy, sex‐role reversal, and a polygynandrous mating system. We take advantage of microsatellite‐based “genetic‐capture” techniques to match wild‐caught females with female genotypes reconstructed from broods of pregnant males and use these data to explore patterns of size‐assortative mating in these species. We also develop a simulation model to explore how positive, negative, and antagonistic preferences of each sex for body size affect size‐assortative mating. Contrary to expectations, we were unable to find any evidence of size‐assortative mating in either species at different geographic locations or at different sampling times. Furthermore, two traits that potentially confer a fitness advantage in terms of reproductive success, female mating order and number of eggs transferred per female, do not affect pairing patterns in the wild. Results from model simulations demonstrate that strong mating preferences are unlikely to explain the observed patterns of mating in the studied populations. Our study shows that individual mating preferences, as ascertained by laboratory‐based mating trials, can be decoupled from realized patterns of mating in the wild, and therefore, field studies are also necessary to determine actual patterns of mate choice in nature. We conclude that this disconnect between preferences and assortative mating is likely due to ecological constraints and multiple mating that may limit mate choice in natural populations.     

Mate selection in man: Evidence,theory, and outcome

Abstract

This review presents a comprehensive survey of the literature on mate selection and non‐random mating in man. The topics discussed include: (1) genetic aspects of non‐random mating for complex traits; (2) evidence on resemblance between spouses on a large variety of traits such as intelligence, personality, physical characteristics, and sociocultural traits; (3) a critical review of sociological and psychological theories offered to account for assortative mating, and (4) implications of assortative mating for marital satisfaction. It is suggested that the factors leading to choice of marriage partners need to be studied from the point of view of multivariate profiles rather than single traits. Such studies will require sophisticated methodologies of research design and data analysis.     

Effect of genetic heterogeneity and assortative mating on linkage analysis: a simulation study.

Linkage studies of complex genetic traits raise questions about the effects of genetic heterogeneity and assortative mating on linkage analysis. To further understand these problems, I have simulated and analyzed family data for a complex genetic disease in which disease phenotype is determined by two unlinked disease loci. Two models were studied, a two-locus threshold model and a two-locus heterogeneity model. Information was generated for a marker locus linked to one of the disease-defining loci. Random-mating and assortative-mating samples were generated. Linkage analysis was then carried out by use of standard methods, under the assumptions of a single-locus disease trait and a random-mating population. Results were compared with those from analysis of a single-locus homogeneous trait in samples with the same levels of assortative mating as those considered for the two-locus traits. The results show that (1) introduction of assortative mating does not, in itself, markedly affect the estimate of the recombination fraction; (2) the power of the analysis, reflected in the LOD scores, is somewhat lower with assortative rather than random mating. Loss of power is greater with increasing levels of assortative mating; and (3) for a heterogeneous genetic disease, regardless of mating type, heterogeneity analysis permits more accurate estimate of the recombination fraction but may be of limited use in distinguishing which families belong to each homogeneous subset. These simulations also confirmed earlier observations that linkage to a disease "locus" can be detected even if the disease is incorrectly defined as a single-locus (homogeneous) trait, although the estimated recombination fraction will be significantly greater than the true recombination fraction between the linked disease-defining locus and the marker locus.     

Sexual selection and assortative mating: an experimental test

Mate choice and mate competition can both influence the evolution of sexual isolation between populations. Assortative mating may arise if traits and preferences diverge in step, and, alternatively, mate competition may counteract mating preferences and decrease assortative mating. Here, we examine potential assortative mating between populations of Drosophila pseudoobscura that have experimentally evolved under either increased (‘polyandry’) or decreased (‘monogamy’) sexual selection intensity for 100 generations. These populations have evolved differences in numerous traits, including a male signal and female preference traits. We use a two males: one female design, allowing both mate choice and competition to influence mating outcomes, to test for assortative mating between our populations. Mating latency shows subtle effects of male and female interactions, with females from the monogamous populations appearing reluctant to mate with males from the polyandrous populations. However, males from the polyandrous populations have a significantly higher probability of mating regardless of the female's population. Our results suggest that if populations differ in the intensity of sexual selection, effects on mate competition may overcome mate choice.     

The Genetic Correlation between Height and IQ: Shared Genes or Assortative Mating?

Traits that are attractive to the opposite sex are often positively correlated when scaled such that scores increase with attractiveness, and this correlation typically has a genetic component. Such traits can be genetically correlated due to genes that affect both traits (“pleiotropy”) and/or because assortative mating causes statistical correlations to develop between selected alleles across the traits (“gametic phase disequilibrium”). In this study, we modeled the covariation between monozygotic and dizygotic twins, their siblings, and their parents (total N = 7,905) to elucidate the nature of the correlation between two potentially sexually selected traits in humans: height and IQ. Unlike previous designs used to investigate the nature of the height–IQ correlation, the present design accounts for the effects of assortative mating and provides much less biased estimates of additive genetic, non-additive genetic, and shared environmental influences. Both traits were highly heritable, although there was greater evidence for non-additive genetic effects in males. After accounting for assortative mating, the correlation between height and IQ was found to be almost entirely genetic in nature. Model fits indicate that both pleiotropy and assortative mating contribute significantly and about equally to this genetic correlation.     

Sampling scale can cause bias in positive assortative mating estimates: evidence from two intertidal snails

Assortative mating in the wild is commonly estimated by correlating between traits in mating pairs (e.g. the size of males and females). Unfortunately, such an approach may suffer from considerable sampling bias when the distribution of different expressions of a trait in the wild is nonrandom (e.g. when segregation of different size classes of individuals occurs in different microhabitats or areas). Consequently, any observed trait correlation in the wild can be an artefact of pooling heterogeneous samples of mating pairs from different microhabitats or areas rather than true nonrandom matings. This bias in estimating trait correlations as a result of sampling scale is termed the scale‐of‐choice effect (SCE). In the present study, we use two intertidal littorinid species from Hong Kong to show how the SCE can bias size‐assortative mating estimates from mating pairs captured in the wild, empirically demonstrating the influence of this effect on measures of positive assortative mating. This finding cautions that studies overlooking the SCE may have misinterpreted the magnitude and the cause of assortative mating, and we provide a new analytical approach for protecting against this potential bias in future studies.     

Variation in human mate choice: simultaneously investigating heritability, parental influence, sexual imprinting, and assortative mating

Human mate choice is central to individuals' lives and to the evolution of the species, but the basis of variation in mate choice is not well understood. Here we looked at a large community-based sample of twins and their partners and parents ([Formula: see text] individuals) to test for genetic and family environmental influences on mate choice, while controlling for and not controlling for the effects of assortative mating. Key traits were analyzed, including height, body mass index, age, education, income, personality, social attitudes, and religiosity. This revealed near-zero genetic influences on male and female mate choice over all traits and no significant genetic influences on mate choice for any specific trait. A significant family environmental influence was found for the age and income of females' mate choices, possibly reflecting parental influence over mating decisions. We also tested for evidence of sexual imprinting, where individuals acquire mate-choice criteria during development by using their opposite-sex parent as the template of a desirable mate; there was no such effect for any trait. The main discernible pattern of mate choice was assortative mating; we found that partner similarity was due to initial choice rather than convergence and also at least in part to phenotypic matching.     

Rules for assortative mating in relation to selection for linear merit functions

Summary This study examined how assortative mating (without selection) based on linear combinations of two traits could be used to change genetic parameters so as to increase efficiency of selection. The efficiency of the Smith-Hazel index for improvement of multiple traits is a function of phenotypic and genetic variances and covariances, and of the relative economic values of the traits involved. Assortative mating is known to change genetic variances and covariances. Recursive formulae were derived to obtain these variances and covariances after t generations of assortative mating on linear combinations (mating rules) of phenotypic values for two traits, with a given correlation between mates. Selection efficiency after t generations of assortative mating without selection was expressed as a function of random mating genetic parameters, economic values, the mating rule, and the correlation between mates. Selection efficiency was maximized with respect to the coefficients in the mating rule. Because the objective function was nonlinear, a computer routine was used for maximizing it. Two cases were considered. When random mating heritabilities for the two traits were h _X ² =0.25 and h _Y ² =0.50, the genetic correlation r_XY=-0.60, and the economic values were a_X=3 and a_Y=1, continued assortative mating based on the optimal mating rule for 31 generations (with a correlation of 0.80 between mates) increased selection efficiency by 29%. Heritabilities changed to 0.38 and 0.66, respectively, and the genetic correlation became – 0.79. When h _X ² =0.60, h _Y ² =0.60, r_XY=– 0.20, a₁=1 and a₂=1, 36 generations of continued assortative mating with the optimal mating rule increased the efficiency of selection by 17%, heritabilities became h _X ² = h _Y ² =0.71, and the genetic correlation changed to 0.25. Only three generations of assortative mating were required to change the sign of the genetic correlation.     

Direct and indirect assortative mating: a multivariate approach to plant flowering schedules

This paper develops methods to partition the phenotypic correlation between mates for a focal trait--the standard measure for assortative mating--into a direct component and additional indirect components. Indirect assortative mating occurs when a nonassorting trait is correlated within individuals to a directly assorting trait. Direct and indirect assortative mating is assessed for flowering phenology in Brassica rapa. The flowering time of pollen recipients (mothers) was strongly correlated (rho=0.67) to that of potential pollen donors (fathers). Similarly, recipients and donors were correlated for duration of their flowering periods (rho=0.32) and stem diameters (rho=0.52). A partitioning of between-mate correlations revealed direct assortative mating for flowering time and period duration. However, assortment for stem diameter is explained solely through its correlation to flowering time. Examination of standard quantitative genetic theory shows that indirect assortative mating inflates genetic variance in a focal trait and the genetic covariance between focal and phenotypically correlated traits.     

Assortive mating for personaltiy traits, educational level, religious affiliation, height, weight, adn body mass index in parents of Korean twin sample.

The degree of assortative mating for psychological and physical traits in Asian societies in relatively unknown. The present study examined assortative mating for educational level, personality traits, religious affiliation, height, weight, and body mass index in a korean sample. Age-adjusted spouse correlations were high for educational level (r = .63) and religious affiliation (r = .67), modest for most personality traits (rs = -.01 to .26), and trivial for height (r = .04), weight (r = .05)m and body mass index (r = .11). These results were remarkably similar to those found from the western samples. Implications of the present findings in behavior genetic studies and human mating patterns were briefly discussed.     

Genetic variation in flowering time induces phenological assortative mating: quantitative genetic methods applied to Brassica rapa

It has been argued from first principles that plants mate assortatively by flowering time. However, there have been very few studies of phenological assortative mating, perhaps because current methods to infer paternal phenotype are difficult to apply to natural populations. Two methods are presented to estimate the phenotypic correlation between mates-the quantitative genetic metric for assortative mating-for phenological traits. The first method uses individual flowering schedules to estimate mating probabilities for every potential pairing in a sample. These probabilities are then incorporated into a weighted phenotypic correlation between all potential mates and thus yield a prospective estimate based on mating opportunities. The correlation between mates can also be estimated retrospectively by comparing the regression of offspring phenotype over one parent, which is inflated by assortative mating, to the regression over mid-parent, which is not. In a demonstration experiment with Brassica rapa, the prospective correlation between flowering times (days from germination to anthesis) of pollen recipients and their potential donors was 0.58. The retrospective estimate of this correlation strongly agreed with the prospective estimate. The prospective method is easily employed in field studies that explore the effect of phenological assortative mating on selection response and population differentiation.     

Assortative mating and the cost of inbreeding — A simulation approach

Assortative mating is an important factor in the process of speciation. Models of speciation frequently deal with small founder populations often with mating preferences based on ecological traits or habitat preferences. Small populations, on the other hand might suffer from inbreeding. However, few studies have explored the combined effects of assortative mating and inbreeding in such populations. Can they speciate, or are they doomed to eventually go extinct? With this simulation we show that assortative mating based on similarities increases the possibility for change in a population, as long as the population does not suffer from inbreeding depression. Inbred populations seem not to be able to cope with strong assortative mating, as this is likely to elevate the level of inbreeding, increasing the risks of inbreeding depression and as a result decreasing population mean fitness. This in turn hinders the possibility of change, and instead might drive the population to extinction.     

Assortative mating counteracts the evolution of dispersal polymorphisms

Polymorphic dispersal strategies are found in many plant and animal species. An important question is how the genetic variation underlying such polymorphisms is maintained. Numerous mechanisms have been discussed, including kin competition or frequency-dependent selection. In the context of sympatric speciation events, genetic and phenotypic variation is often assumed to be preserved by assortative mating. Thus, recently, this has been advocated as a possible mechanism leading to the evolution of dispersal polymorphisms. Here, we examine the role of assortative mating for the evolution of trade-off-driven dispersal polymorphisms by modeling univoltine insect species in a metapopulation. We show that assortative mating does not favor the evolution of polymorphisms. On the contrary, assortative mating favors the evolution of an intermediate dispersal type and a uni-modal distribution of traits within populations. As an alternative, mechanism dominance may explain the occurrence of two discrete morphs.     

Assortative mating patterns of multiple phenotypic traits in a long‐lived seabird

Choosing the right mate is crucial for successful breeding, particularly in monogamous species with long and extensive bi‐parental care, and when the breeding pair is presumed to last many seasons. We investigated the degree of assortative mating in the Little Auk Alle alle, a long‐lived seabird with long‐term pair bonds and bi‐parental care for fixed (morphological) and labile (physiological, behavioural) traits. Using randomization tests, we suggest assortative mating with respect to wing length, extent of the white area on the upper eyelid and hormonal stress response (the difference between stress‐induced and baseline corticosterone levels). We discuss how the assortative mating patterns that we found in the Little Auk may be adaptive.     

On the origin of species by means of assortative mating.

Assortative mating may split a population even in the absence of natural selection. Here, we study when this happens if mating depends on one or two quantitative traits. Not surprisingly, the modes of assortative mating that can cause sympatric speciation without selection are rather strict. However, some of them may occur in nature. Slow elimination of intermediate individuals caused by the gradual tightening of assortative mating, which evolves owing to relatively weak disruptive selection, provides the alternative scenario for sympatric speciation, in addition to fast elimination of intermediate individuals as a result of the direct action of strong disruptive selection under an invariant mode of assortative mating. Even when assortative mating alone cannot split an initially coherent population, it may be able to prevent the merging of species after their secondary contact.