Selection for altruism through random drift in variable size populations

Background

Seed storage proteins are a major source of dietary protein, and the content of such proteins determines both the quantity and quality of crop yield. Significantly, examination of the protein content in the seeds of crop plants shows a distinct difference between monocots and dicots. Thus, it is expected that there are different evolutionary patterns in the genes underlying protein synthesis in the seeds of these two groups of plants.

Results

Gene duplication, evolutionary rate and positive selection of a major gene family of seed storage proteins (the 11S globulin genes), were compared in dicots and monocots. The results, obtained from five species in each group, show more gene duplications, a higher evolutionary rate and positive selections of this gene family in dicots, which are rich in 11S globulins, but not in the monocots.

Conclusion

Our findings provide evidence to support the suggestion that gene duplication and an accelerated evolutionary rate may be associated with higher protein synthesis in dicots as compared to monocots.     

Selection versus random drift: long-term polymorphism persistence in small populations (evidence and modelling)

Our data on a subterranean mammal, Spalax ehrenbergi, and other evidence, indicate that appreciable polymorphism can be preserved in small isolated populations consisting of several dozens of, or a hundred, individuals. Current theoretical models predict fast gene fixation in small panmictic populations without selection, mutation, or gene inflow. Using simple multilocus models, we demonstrate here that moderate stabilizing selection (with stable or fluctuating optimum) for traits controlled by additive genes could oppose random fixation in such isolates during thousands of generations. We also show that in selection-free models polymorphism persists only for a few hundred generations even under high mutation rates. Our multi-chromosome models challenge the hitchhiking hypothesis of polymorphism maintenance for many neutral loci due to close linkage with few selected loci.     

Selection for increased brood size in historical human populations

Human twinning rates are considered to either reflect the direct fitness effects of twinning in variable environments, or to be a maladaptive by-product of selection for other maternal reproductive traits (e.g., polyovulation). We used historical data (1710-1890) of Sami populations from Northern Scandinavia to contrast these alternative hypotheses. We found that women who produced twins started their reproduction younger, ceased it later, had higher lifetime fecundity, raised more offspring to adulthood, and had higher fitness (individual lambda) than mothers of singletons in all populations studied. For example, an average of 1.2 offspring survived to adulthood from a twin delivery, irrespective of its sex ratio, whereas only 0.8 offspring survived to adulthood from a singleton delivery. Only if mothers started reproduction at very late age (> 37 yr), or had a very long reproductive life span (> 20 yr), was it more beneficial to produce only singletons. These findings suggest that twin deliveries among Sami could not be explained as a maladaptive by-product of selection for other maternal reproductive traits. In contrast, our results suggest that twinning was under natural selection, although the strength of selection was likely to have been context dependent.     

Stalk size and altruism investment within and among populations of the social amoeba

Reproductive division of labour is common in many societies, including those of eusocial insects, cooperatively breeding vertebrates, and most forms of multicellularity. However, conflict over what is best for the individual vs. the group can prevent an optimal division of labour from being achieved. In the social amoeba Dictyostelium discoideum, cells aggregate to become multicellular and a fraction behaves altruistically, forming a dead stalk that supports the rest. Theory suggests that intra‐organismal conflict over spore–stalk cell fate can drive rapid evolutionary change in allocation traits, leading to polymorphisms within populations or rapid divergence between them. Here, we assess several proxies for stalk size and spore–stalk allocation as metrics of altruism investment among strains and across geographic regions. We observe geographic divergence in stalk height that can be partly explained by differences in multicellular size, as well as variation among strains in clonal spore–stalk allocation, suggesting within‐population variation in altruism investment. Analyses of chimeras comprised of strains from the same vs. different populations indicated genotype‐by‐genotype epistasis, where the morphology of the chimeras deviated significantly from the average morphology of the strains developed clonally. The significantly negative epistasis observed for allopatric pairings suggests that populations are diverging in their spore–stalk allocation behaviours, generating incompatibilities when they encounter one another. Our results demonstrate divergence in microbial social traits across geographically separated populations and demonstrate how quantification of genotype‐by‐genotype interactions can elucidate the trajectory of social trait evolution in nature.     

Variable rates of random genetic drift in protected populations of English yew: implications for gene pool conservation

Protecting populations in their natural habitat allows for the maintenance of naturally evolved adaptations and ecological relationships. However, the conservation of genetic resources often requires complementary practices like gene banks, translocations or reintroductions. In order to minimize inbreeding depression and maximize the adaptive potential of future populations, populations chosen for ex situ conservation should be selected according to criteria that will result in a reduction of global coancestry in the population. Generally, large populations should reveal lower coancestry and higher genetic variation than small populations. If detailed knowledge about coancestry is lacking, census population number (N _c ) can be used as a proxy for required characteristics. However, a simple measure of N _c may be misleading in particular cases as genetic processes rely on effective population size (N _e ) rather than N _c and these two measures may differ substantially due to demographic processes. We used an example of English yew to address whether N _c can be a good predictor of genetic parameters when used in conservation programs. Using microsatellite markers, we estimated allelic richness, inbreeding and coancestry coefficients of six relatively large yew populations in Poland. Each population was characterized by N _e using the linkage disequilibrium method. Our results showed that populations of English yew were subject to substantial divergence and genetic drift, with both being inversely proportional to the effective subpopulation size (N _e ). Additionally, allelic richness appeared proportional to N _e but not to N _c . However, the N _e /N ratio differed greatly among populations, which was possibly due to different population histories. From the results we concluded that choosing source populations based only on their census size can be fairly misleading. Implications for conservation are briefly discussed.     

The continuous prisoner's dilemma and the evolution of cooperation through reciprocal altruism with variable investment

Understanding the evolutionary origin and persistence of cooperative behavior is a fundamental biological problem. The standard "prisoner's dilemma," which is the most widely adopted framework for studying the evolution of cooperation through reciprocal altruism between unrelated individuals, does not allow for varying degrees of cooperation. Here we study the continuous iterated prisoner's dilemma, in which cooperative investments can vary continuously in each round. This game has been previously considered for a class of reactive strategies in which current investments are based on the partner's previous investment. In the standard iterated prisoner's dilemma, such strategies are inferior to strategies that take into account both players' previous moves, as is exemplified by the evolutionary dominance of "Pavlov" over "tit for tat." Consequently, we extend the analysis of the continuous prisoner's dilemma to a class of strategies in which current investments depend on previous payoffs and, hence, on both players' previous investments. We show, both analytically and by simulation, that payoff-based strategies, which embody the intuitively appealing idea that individuals invest more in cooperative interactions when they profit from these interactions, provide a natural explanation for the gradual evolution of cooperation from an initially noncooperative state and for the maintenance of cooperation thereafter.     

Modelling disease spread through random and regular contacts in clustered populations

An epidemic spreading through a network of regular, repeated, contacts behaves differently from one that is spread by random interactions: regular contacts serve to reduce the speed and eventual size of an epidemic. This paper uses a mathematical model to explore the difference between regular and random contacts, considering particularly the effect of clustering within the contact network. In a clustered population random contacts have a much greater impact, allowing infection to reach parts of the network that would otherwise be inaccessible. When all contacts are regular, clustering greatly reduces the spread of infection; this effect is negated by a small number of random contacts.     

Natural selection of altruism in inelastic viscous homogeneous populations

Biological explanations are given of three main uninterpreted theoretical results on the selection of altruism in inelastic viscous homogeneous populations, namely that non-overlapping generations hinder the evolution of altruism, fecundity effects are more conducive to altruism than survival effects, and one demographic regime (so-called death-birth) permits altruism whereas another (so-called birth-death) does not. The central idea is ‘circles of compensation’, which measure how far the effects of density dependence extend from a focal individual. Relatednesses can then be calculated that compensate for density dependence. There is very generally a ‘balancing circle of compensation’, at which the viscosity of the population slows up selection of altruism, but does not affect its direction, and this holds for altruism towards any individual, not just immediate neighbours. These explanations are possible because of recent advances in the theory of inclusive fitness on graphs. The assumption of node bitransitivity in that recent theory is relaxed to node transitivity and symmetry of the dispersal matrix, and new formulae show how to calculate relatedness from dispersal and vice versa.     

The adaptive dynamics of altruism in spatially heterogeneous populations

Abstract.— We study the spatial adaptive dynamics of a continuous trait that measures individual investment in altruism. Our study is based on an ecological model of a spatially heterogeneous population from which we derive an appropriate measure of fitness. The analysis of this fitness measure uncovers three different selective processes controlling the evolution of altruism: the direct physiological cost, the indirect genetic benefits of cooperative interactions, and the indirect genetic costs of competition for space. In our model, habitat structure and a continuous life cycle makes the cost of competing for space with relatives negligible. Our study yields a classification of adaptive patterns of altruism according to the shape of the costs of altruism (with decelerating, linear, or accelerating dependence on the investment in altruism). The invasion of altruism occurs readily in species with accelerating costs, but large mutations are critical for altruism to evolve in selfish species with decelerating costs. Strict selfishness is maintained by natural selection only under very restricted conditions. In species with rapidly accelerating costs, adaptation leads to an evolutionarily stable rate of investment in altruism that decreases smoothly with the level of mobility. A rather different adaptive pattern emerges in species with slowly accelerating costs: high altruism evolves at low mobility, whereas a quasi-selfish state is promoted in more mobile species. The high adaptive level of altruism can be predicted solely from habitat connectedness and physiological parameters that characterize the pattern of cost. We also show that environmental changes that cause increased mobility in those highly altruistic species can beget selection-driven self-extinction, which may contribute to the rarity of social species.     

Species selection and random drift in macroevolution

Species selection resulting from trait‐dependent speciation and extinction is increasingly recognized as an important mechanism of phenotypic macroevolution. However, the recent bloom in statistical methods quantifying this process faces a scarcity of dynamical theory for their interpretation, notably regarding the relative contributions of deterministic versus stochastic evolutionary forces. I use simple diffusion approximations of birth‐death processes to investigate how the expected and random components of macroevolutionary change depend on phenotype‐dependent speciation and extinction rates, as can be estimated empirically. I show that the species selection coefficient for a binary trait, and selection differential for a quantitative trait, depend not only on differences in net diversification rates (speciation minus extinction), but also on differences in species turnover rates (speciation plus extinction), especially in small clades. The randomness in speciation and extinction events also produces a species‐level equivalent to random genetic drift, which is stronger for higher turnover rates. I then show how microevolutionary processes including mutation, organismic selection, and random genetic drift cause state transitions at the species level, allowing comparison of evolutionary forces across levels. A key parameter that would be needed to apply this theory is the distribution and rate of origination of new optimum phenotypes along a phylogeny.     

Phenotypic evolution in prehistoric Ohio Amerindians: natural selection versus random genetic drift in tooth size reduction

Many anthropologic investigations involve measurement and analysis of polygenic skeletal and dental traits in prehistoric populations from which genetic details cannot be inferred. However, population genetics concepts can be applied productively to analyses of phenotypic variation in prehistoric human populations. One potentially useful approach, derived from basic quantitative genetics (Lande 1976, p. 314), models the effects of natural selection and random genetic drift on the evolution of the average phenotype in a population. We apply this model to the problem of dental size reduction in three prehistoric Amerindian populations from Ohio. Conversion of mean log-transformed buccolingual diameters for six permanent teeth (maxillary and mandibular I1, M1, and M2) to phenotypic standard deviation units reveals significant size reduction in the maxillary teeth only. By assuming 40 generations (t) between the 2 populations and a narrow heritability (h2) range of 0.30-0.70, the estimated minimum selective mortality required to produce the reductions is 1.8 deaths per 100 persons per generation. Given the same t and h2 values, the effective population size (Ne) needed to reject the neutral hypothesis (i.e., random genetic drift) with 95% confidence is approximately 150. Because paleodemographic and ethnographic studies suggest minimum effective sizes of this magnitude for these populations, we tentatively reject random genetic drift and conclude that selective mortality is most probably responsible for the maxillary tooth size reduction observed.     

Selection in plant populations of effectively infinite size VI. Overlapping generations

A selection model for iteroparous, monoecious, or hermaphroditic plant populations is considered which encompasses viabilities, pollen fertilities, ovule fertilities, and rates of self-fertilization which may arbitrarily depend on both age and genotype. The general conditions for establishment (which are also those for protectedness) of an allele are derived. The classical conjecture that the conditions of protectedness are the same for separated and overlapping generations if the intrinsic rates of increase are applied is discussed. For this purpose it is necessary to introduce two new intrinsic values: the intrinsic rate of self-fertilization and the intrinsic pollen-to-ovule ratio. The significance of the intrinsic values is demonstrated for complete self-fertilization, selection restricted to differential, partial self-fertilization, and sexual asymmetry (absence of proportionality between pollen and ovule production), including selection restricted to one sex. With the exception of asymmetric selection in both sexes, it turns out that the intrinsic values suffice to state the conditions for protectedness, but more information about the life histories is required to determine the exact speed of establishment. For asymmetric selection in both sexes, the concept of intrinsic value is inadequate for investigating the problem of establishment and thus the evolution of life histories. Since sexual asymmetry is rather the rule than the exception and selfing is common in plants, the consequences for finding optimal life histories are outlined.     

Selection for recombination in structured populations

In finite populations, linkage disequilibria generated by the interaction of drift and directional selection (Hill-Robertson effect) can select for sex and recombination, even in the absence of epistasis. Previous models of this process predict very little advantage to recombination in large panmictic populations. In this article we demonstrate that substantial levels of linkage disequilibria can accumulate by drift in the presence of selection in populations of any size, provided that the population is subdivided. We quantify (i) the linkage disequilibrium produced by the interaction of drift and selection during the selective sweep of beneficial alleles at two loci in a subdivided population and (ii) the selection for recombination generated by these disequilibria. We show that, in a population subdivided into n demes of large size N, both the disequilibrium and the selection for recombination are equivalent to that expected in a single population of a size intermediate between the size of each deme (N) and the total size (nN), depending on the rate of migration among demes, m. We also show by simulations that, with small demes, the selection for recombination is stronger than both that expected in an unstructured population (m = 1 - 1/n) and that expected in a set of isolated demes (m = 0). Indeed, migration maintains polymorphisms that would otherwise be lost rapidly from small demes, while population structure maintains enough local stochasticity to generate linkage disequilibria. These effects are also strong enough to overcome the twofold cost of sex under strong selection when sex is initially rare. Overall, our results show that the stochastic theories of the evolution of sex apply to a much broader range of conditions than previously expected.     

Numerical analysis of random drift in a cline

The equilibrium state of a diffusion model for random genetic drift in a cline is analyzed numerically. The monoecious organism occupies an unbounded linear habitat with constant, uniform population density. Migration is homogeneous, symmetric and independent of genotype. A single diallelic locus with a step environment is investigated in the absence of dominance and mutation. The flattening of the expected cline due to random drift is very slight in natural populations. The ratio of the variance of either gene frequency to the product of the expected gene frequencies decreases monotonically to a nonzero constant. The correlation between the gene frequencies at two points decreases monotonically to zero as the separation is increased with the average position fixed; the decrease is asymptotically exponential. The correlation decreases monotonically to a positive constant depending on the separation as the average position increasingly deviates from the center of the cline with the separation fixed. The correlation also decreases monotonically to zero if one of the points is fixed and the other is moved outward in the habitat, the ultimate decrease again being exponential. Some asymptotic formulae are derived analytically.——The loss of an allele favored in an environmental pocket is investigated by simulating a chain of demes exchanging migrants, the other assumptions being the same as above. For most natural populations, provided the allele would be maintained in the population deterministically, this process is too slow to have evolutionary importance.     

Three components of genetic drift in subdivided populations

Wright's metaphor of sampling is extended to consider three components of genetic drift: those occurring before, during, and after migration. To the extent that drift at each stage behaves like an independent random sample, the order of events does not matter. When sampling is not random, the order does matter, and the effect of population size is confounded with that of mobility. The widely cited result that genetic differentiation of local groups depends only on the product of group size and migration rate holds only when nonrandom sampling does not occur prior to migration in the life cycle.