Evolution: plastic sociality in a sweat bee

How and why do bees become social? A transplant experiment shows that sweat bees can adopt a solitary or social lifestyle in response to their environment.     

Means,variances, and ranges in reproductive success: comparative evidence

Data on reproductive success in traditional cultures suggest that for men, but not for women, range and variance rise as subsistence intensifies. For hunter–gatherers, ranges and variances tend to cluster in single digits: they reach 15 or 16, at the high end. For herder-gardeners, ranges and variances are more consistently in double digits: they get as high as 80 or 85. And for full-time agriculturalists in the first civilizations, ranges consistently ran to triple digits: emperors from Mesopotamia to Peru were the fathers of hundreds of children. In human societies, as in other animal societies, reproductive skew goes up with a more sedentary life.     

Giving your daughters the edge: bequeathing reproductive dominance in a primitively social bee

The fitness associated with behavioural strategies is usually estimated in terms of offspring number and size. However, in group-living animals the reproductive value of offspring may also depend on their social rank. We show here that in an allodapine bee Exoneura robusta, dominant mothers can behaviourally influence their daughters'' reproductive rank by controlling insemination of other potential mothers. In E. robusta, group living is near mandatory and reproductive dominance among female nestmates is determined by order of adult emergence. Nests are single, undivided burrows and the dominant female assumes a guarding position closest to the nest entrance. We show that before the egg-laying period, subordinate females who have been absent from the nest are ''screened'' by the reproductive guard upon attempted re-entry. Those who have been in contact with foreign males are less likely to be granted access back into the nest than those who have been in contact with foreign females or with no bees at all. We argue that by controlling insemination patterns of their nestmates, dominant females ensure that their own daughters eclose first and are therefore more likely to assume dominance in the next generation. This presents a situation where dominance is bequeathed to daughters by behavioural means. The ability of mothers to influence social hierarchies in subsequent generations introduces a fitness component additional to the number and size of offspring produced.     

Effects on heterozygosity and reproductive fitness of inbreeding with and without selection on fitness in Drosophila melanogaster

The effects of inbreeding, with (IS) and without selection (IO) for reproductive fitness, on inbreeding depression and heterozygosity were evaluated in 20 lines of each treatment inbred over seven generations using full-sib mating. The survival of lines was significantly greater in IS (20/20) than in IO (15/20). The competitive index measure of reproductive fitness was significantly lower in the inbred lines than in the outbred base population, but not significantly different in surviving IS and IO lines. There was a trend for higher fitness in the IS treatment as relative fitnesses were 19% higher in IS than IO for surviving lines and 59% higher for all lines. Heterozygosities were lower in the inbred lines than in the base population, and significantly higher in the IS than the IO lines. Consequently, the reduction of inbreeding depression in IS has been achieved, at least in part, by slowing the rate of fixation.     

Reproductive fitness and artificial selection in animal breeding: culling on fitness prevents a decline in reproductive fitness in lines of Drosophila melanogaster selected for increased inebriation time

Summary The maintenance of reproductive fitness in lines subjected to artificial selection is one of the major problems in animal breeding. The decline in reproductive performance has neither been predictable from heritabilities and genetic correlations, nor have conventional selection indices been adequate to avoid the problem. Gowe (1983) has suggested that the heritabilities of reproductive traits are non-linear, with heritabilities being higher on the lower fitness side. Consequently, he has predicted that culling on reproductive fitness in artificial selection lines will be effective in preventing the usual declines in fitness. An experimental evaluation of Gowe's prediction has been carried out by comparing fitnesses of replicated lines of three treatments: selection for increased inebriation time without culling on fitness (HO), selection for inebriation time with culling of 20% (4/20) of selected females on reproductive fitness (HS), and unselected controls (C). Response to selection for inebriation time in the two selection treatments was similar. After 25 generations, the competitive index, a measure of reproductive fitness, was significantly lower in the HO treatment than the HS treatment, while the HS treatment did not differ from the control lines or the base population. These results demonstrate for the first time that culling on reproductive fitness in selection lines can be used to prevent the usual decline in reproductive performance.     

The effects of reproductive specialization on energy costs and fitness genetic variances in cyclical and obligate parthenogenetic aphids

Organisms with coexisting sexual and asexual populations are ideal models for studying the consequences of either reproductive mode on the quantitative genetic architecture of life-history traits. In the aphid Rhopalosiphum padi, lineages differing in their sex investment coexist but all share a common parthenogenetic phase. Here, we studied multiple genotypes of R. padi specialized either for sexual and asexual reproduction and compared their genetic variation in fitness during the parthenogenetic phase. Specifically, we estimated maintenance costs as standard metabolic rate (SMR), together with fitness (measured as the intrinsic rate of increase and the net reproductive rate). We found that genetic variation (in terms of broad-sense heritability) in fitness was higher in asexual genotypes compared with sexual genotypes. Also, we found that asexual genotypes exhibited several positive genetic correlations indicating that body mass, whole-animal SMR, and apterous individuals production are contributing to fitness. Hence, it appears that in asexual genotypes, energy is fully allocated to maximize the production of parthenogenetic individuals, the simplest possible form of aphid repertoire of life-histories strategies.     

Evolution of culture-dependent discriminate sociality: a gene-culture coevolutionary model

Animals behave cooperatively towards certain conspecifics while being indifferent or even hostile to others. The distinction is made primarily according to kinship as predicted by the kin selection theory. With regards to humans, however, this is not always the case; in particular, humans sometimes exhibit a discriminate sociality on the basis of culturally transmitted traits, such as personal ornaments, languages, rituals, etc. This paper explores the possibility that the human faculty of cultural transmission and resultant cultural variation among individuals may have facilitated the evolution of discriminate sociality in humans. To this end, a gene-culture coevolutionary model is developed focusing on competition over control of resource as a context in which discriminate sociality may have evolved. Specifically, two types of culture-dependent discriminate sociality are considered: ingroup favouritism, with ingroup and outgroup being distinguished by the presence or absence of a cultural trait; and prestige hierarchies, with the prestige being conferred on the bearer of a cultural trait. The model specifies the conditions under which emergence and evolutionary stability of the two types of discriminate sociality are promoted by the presence of cultural variation among individuals.     

Processing power limits social group size: computational evidence for the cognitive costs of sociality

Sociality is primarily a coordination problem. However, the social (or communication) complexity hypothesis suggests that the kinds of information that can be acquired and processed may limit the size and/or complexity of social groups that a species can maintain. We use an agent-based model to test the hypothesis that the complexity of information processed influences the computational demands involved. We show that successive increases in the kinds of information processed allow organisms to break through the glass ceilings that otherwise limit the size of social groups: larger groups can only be achieved at the cost of more sophisticated kinds of information processing that are disadvantageous when optimal group size is small. These results simultaneously support both the social brain and the social complexity hypotheses.     

Riding across the selection landscape: fitness consequences of annual variation in reproductive characteristics

Evolutionary models estimating phenotypic selection in character size usually assume that the character is invariant across reproductive bouts. We show that variation in the size of reproductive traits may be large over multiple events and can influence fitness in organisms where these traits are produced anew each season. With data from populations of two orchid species, Caladenia valida and Tolumnia variegata, we used Bayesian statistics to investigate the effect on the distribution in fitness of individuals when the fitness landscape is not flat and when characters vary across reproductive bouts. Inconsistency in character size across reproductive periods within an individual increases the uncertainty of mean fitness and, consequently, the uncertainty in individual fitness. The trajectory of selection is likely to be muddled as a consequence of variation in morphology of individuals across reproductive bouts. The frequency and amplitude of such changes will certainly affect the dynamics between selection and genetic drift.     

Darwinian fitness and the intensity of natural selection: studies in sensitivity analysis

Darwinian fitness, the capacity of a variant type to establish itself in competition with the resident population, is determined by evolutionary entropy, a measure of the uncertainty in age of the mother of a randomly chosen newborn. This article shows that the intensity of natural selection, as measured by the sensitivity of entropy with respect to changes in the age-specific fecundity and mortality variables, is a convex function of age, decreasing at early and increasing at later ages. We exploit this result to provide quantitative evolutionary explanations of the large variation in survivorship curves observed in natural populations. Previous studies to explain variation in survivorship curves have been based on the proposition that Darwinian fitness is determined by the Malthusian parameter. Hence the intensity of natural selection will be determined by the sensitivity of the Malthusian parameter with respect to changes in the age-specific fecundity and mortality variables. This measure of the selection gradient is known to be a decreasing function of age, with implications which are inconsistent with empirical observations of survivorship curves in human and animal populations. The analysis described in this paper point to the mitigated import of sensitivity studies based on the Malthusian parameter. Our analysis provides theoretical and empirical support for the ecological and evolutionary significance of sensitivity analysis based on entropy, which is the appropriate measure of Darwinian fitness.     

Evolution: natural selection in the evolution of humans and chimps

We now have more or less full sequences of both human and chimp genomes, allowing comparison that sheds light on their evolution. A few hundred genes show significant evidence for adaptive evolution in the two lineages, but the actual number might be much higher. Natural selection has eliminated about 75% of amino acid changes in coding sequence since the split of the human and chimpanzee genomes.     

Effectiveness of sexual selection in preventing fitness deterioration in bulb mite populations under relaxed natural selection

Under the 'good genes' mechanism of sexual selection (SS), females benefit from mate choice indirectly: their offspring inherit genes of the preferred, high quality fathers. Recent models assume that the genetic variance for male quality is maintained by deleterious mutations. Consequently, SS can be predicted to remove deleterious mutations from populations. We tested this prediction by relaxing selection in populations of the bulb mite, thus increasing their rate of accumulation of deleterious mutation. SS, allowed to operate in half of these populations, did not prevent the fitness decline observed in the other half of the relaxed selection lines. After 11 generations of relaxed selection, female fecundity in lines in which males were allowed to compete for females declined compared with control populations by similar amount as in monogamous lines (17.5 and 14.5%, respectively), whereas other fitness components (viability, longevity, male reproductive success) did not differ significantly between both types of lines and control populations.     

Marmot phylogeny revisited: molecular evidence for a diphyletic origin of sociality

We established the phylogeny of 11 species of the genus Marmota based on the entire sequence of the mitochondrial cytochrome b ( cyt-b ) gene (1.1 kb) and a partial sequence of the NADH dehydrogenase subunit 4 ( ND4 ) gene (1.2 kb). In three species ( Marmota caligata , Marmota olympus , and Marmota bobac ) full-sized nuclear pseudogenes of the mitochondrial cyt-b were identified. The mitochondrial cyt-b genes and the three pseudogenes form separate clusters in the maximum parsimony dendrogram. This finding suggests that the pseudogenes originated from a single transfer to the nucleus that may have occurred prior to the radiation of the genus Marmota . Notably, compared with their functional mitochondrial equivalents the pseudogenes show a much lower substitution rate. In the dendrograms deduced from the mitochondrial sequences two distinct clusters become apparent: one cluster consists of the North-west American species, the other contains the Eurasian species together with the North American species Marmota monax . The position of M. monax as a member of the Eurasian clade is in accordance with the evolution of chromosome numbers. The results are of special interest with respect to the evolution of social systems in the genus that vary from solitary species ( M. monax ) to highly social species living in family groups (e.g. Marmota marmota ). The molecular phylogeny suggests a diphyletic origin of high sociality in the genus Marmota .     

Alzheimer disease susceptibility loci: evidence for a protein network under natural selection

Recent genome-wide association studies have identified a number of susceptibility loci for Alzheimer disease (AD). To understand the functional consequences and potential interactions of the associated loci, we explored large-scale data sets interrogating the human genome for evidence of positive natural selection. Our findings provide significant evidence for signatures of recent positive selection acting on several haplotypes carrying AD susceptibility alleles; interestingly, the genes found in these selected haplotypes can be assembled, independently, into a molecular complex via a protein-protein interaction (PPI) network approach. These results suggest a possible coevolution of genes encoding physically-interacting proteins that underlie AD susceptibility and are coexpressed in different tissues. In particular, PICALM, BIN1, CD2AP, and EPHA1 are interconnected through multiple interacting proteins and appear to have coordinated evidence of selection in the same human population, suggesting that they may be involved in the execution of a shared molecular function. This observation may be AD-specific, as the 12 loci associated with Parkinson disease do not demonstrate excess evidence of natural selection. The context for selection is probably unrelated to AD itself; it is likely that these genes interact in another context, such as in immune cells, where we observe cis-regulatory effects at several of the selected AD loci.