Variation in two phases of post-winter development of a butterfly.

The temporal aspects of life cycle characteristics, such as diapause development, are under strong selection in seasonal environments. Fine-tuning of the life cycle may be particularly important to match the phenology of potential mates and resources as well as for optimizing abiotic conditions at eclosion. Here, we experimentally study the spring phenology of the orange tip butterfly, Anthocharis cardamines, by analysing post-winter pupal development in three populations along a latitudinal cline in each of Sweden and the United Kingdom. These countries differ substantially in their seasonal temperature profile. By repeatedly recording pupal weights, we established that post-winter development has two separate phases, with a more rapid weight loss in the second phase than in the first, likely corresponding to a ramping up of the rate of development. Variation in the duration of the first phase contributed more strongly than the second phase to the differences in phenology between the localities and sexes. We found that insects from Sweden had a faster overall rate of development than those from the United Kingdom, which is consistent with countergradient variation, as Sweden is colder during the spring than the United Kingdom. Similar trends were not observed at the within-country scale, however. A cogradient pattern was found within Sweden, with populations from the north developing more slowly, and there was no clear latitudinal trend within the United Kingdom. In all localities, males developed faster than females. Our results point to the importance of variation in the progression of post-winter development for spring phenology.

Heterospecific courtship, minority effects and niche separation between cryptic butterfly species.

Species interacting in varied ecological conditions often evolve in different directions in different local populations. The butterflies of the cryptic Leptidea complex are sympatrically distributed in different combinations across their Eurasian range. Interestingly, the same species is a habitat generalist in some regions and a habitat specialist in others, where a sibling species has the habitat generalist role. Previous studies suggest that this geographically variable niche divergence is generated by local processes in different contact zones. By varying the absolute and relative densities of Leptidea sinapis and Leptidea juvernica in large outdoor cages, we show that female mating success is unaffected by conspecific density, but strongly negatively affected by the density of the other species. Whereas 80% of the females mated when a conspecific couple was alone in a cage, less than 10% mated when the single couple shared the cage with five pairs of the other species. The heterospecific courtships can thus affect the population fitness, and for the species in the local minority, the suitability of a habitat is likely to depend on the presence or absence of the locally interacting species. If the local relative abundance of the different species depends on the colonization order, priority effects might determine the ecological roles of interacting species in this system.

Evolution of cooperation through indirect reciprocity.

How can cooperation through indirect reciprocity evolve and what would it be like? This problem has previously been studied by simulating evolution in a small group of interacting individuals, assuming no gene flow between groups. In these simulations, certain 'image scoring' strategies were found to be the most successful. However, analytical arguments show that it would not be in an individual's interest to use these strategies. Starting with this puzzle, we investigate indirect reciprocity in simulations based on an island model. This has an advantage in that the role of genetic drift can be examined. Our results show that the image scoring strategies depend on very strong drift or a very small cost of giving help. As soon as these factors are absent, selection eliminates image scoring. We also consider other possibilities for the evolution of indirect reciprocity. In particular, we find that the strategy of aiming for 'good standing' has superior properties. It can be an evolutionarily stable strategy and, even if not, it usually beats image scoring. Furthermore, by introducing quality variation among individuals into the model, we show that the standing strategy can be quality revealing, adding a new dimension to indirect reciprocity. Finally, we discuss general problems with currently popular modelling styles.

Unpredictable food and sexual size dimorphism in insects.

The evolution of sexual size dimorphism is likely to be affected by the amount of resources each sex invests in offspring. Male nuptial gifts, occurring in many insect species, might reduce the value of large size in females and increase the value for males. For large nuptial gifts and an accompanying shift in dimorphism to evolve, males with larger gifts should be rewarded, in effect trading the larger gift for more offspring. We suggest that food variability, causing some males to have much to provide and some females to be in great need, would be conducive to the evolution of such a mating system, and we present comparative data on butterflies supporting the suggestion. In a gift-giving mating system, growing male and female juveniles should react differently to food shortage. A female maturing at small size can to some extent buffer her disadvantage through nuptial gifts, whereas a male maturing at small size will suffer from his inability to provide substantial gifts. Thus, males benefit more than females from continued growth in the face of food shortage, leading to a shift in size dimorphism. Here we confirm this prediction in a butterfly, Pieris napi, with large nuptial gifts.

Sexual conflict and cooperation in butterfly reproduction: a comparative study of polyandry and female fitness.

Most butterfly species can be characterized as capital breeders, meaning that reproductive output is strongly coupled to the amount of resources they have procured during the larval stage. Accordingly, female fecundity is generally correlated with female mass, both within and across species. However, the females of some species can be partly characterized as income breeders, in the sense that their reproductive output is dependent not only on larval-derived capital but also on resources acquired during the adult stage. These adult resources can be derived from female feeding or from male-transferred nuptial gifts. Recent studies on the within-species effects of multiple matings on female fitness show that females generally gain directly from multiple matings in terms of increased lifetime offspring production. Here, we test whether the positive effects of multiple mating on female fitness also hold at a comparative level, by conducting a laboratory study of female reproductive output in eight pierid species that differ in life-time female mating frequency. Female reproductive output, measured as cumulative egg mass divided by female mass, increased significantly with polyandry (r = 0.942, p < 0.001), demonstrating that the positive effect of mating rate on female reproductive fitness also holds between species. The positive effect of male nutrient contribution is substantial, and the per capita reproductive output is more than twice as high in the most polyandrous species as in the most monandrous ones. Hence, the positive net effect of the ejaculates is highly substantial, although males and females can have sexual interests that run counter to each other, setting the stage for sexually antagonistic coevolution, so that the various component parts of the male ejaculate-sperm, nutrients, anti-aphrodisiacs, and gonadotrophic hormones-may each correspond to a separate conflict-cooperation balance between the sexes. Two scenarios for the evolution of nuptial gifts in butterflies are discussed, one arguing that variation in larval food is the underlying factor and the other arguing that sexually antagonistic coevolution is the driving force. The two views are complementary rather than mutually exclusive, although the former hypothesis predicts that variation in female mating rate depends on variation in larval food availability, whereas the latter suggests that variation in female mating rate between species results from species-specific idiosyncrasies.

Sex-biased dispersal in sperm whales: contrasting mitochondrial and nuclear genetic structure of global populations.

The social organization of most mammals is characterized by female philopatry and male dispersal. Such sex-biased dispersal can cause the genetic structure of populations to differ between the maternally inherited mitochondrial DNA (mtDNA) and the bi-parental nuclear genome. Here we report on the global genetic structure of oceanic populations of the sperm whale, one of the most widely distributed mammalian species. Groups of females and juveniles are mainly found at low latitudes, while males reach polar waters, returning to tropical and subtropical waters to breed. In comparisons between oceans, we did not find significant heterogeneity in allele frequencies of microsatellite loci (exact test; p = 0.23). Estimates of GST = 0.001 and RST = 0.005 also indicated negligible if any nuclear DNA differentiation. We have previously reported significant differentiation between oceans in mtDNA sequences. These contrasting patterns suggest that interoceanic movements have been more prevalent among males than among females, consistent with observations of females being the philopatric sex and having a more limited latitudinal distribution than males. Consequently, the typical mammalian dispersal pattern may have operated on a global scale in sperm whales.

A simple fitness proxy for structured populations with continuous traits, with case studies on the evolution of haplo-diploids and genetic dimorphisms.

For structured populations in equilibrium with everybody born equal, ln(R (0)) is a useful fitness proxy for evolutionarily steady strategy (ESS) and most adaptive dynamics calculations, with R (0) the average lifetime number of offspring in the clonal and haploid cases, and half the average lifetime number of offspring fathered or mothered for Mendelian diploids. When individuals have variable birth states, as is, for example, the case in spatial models, R (0) is itself an eigenvalue, which usually cannot be expressed explicitly in the trait vectors under consideration. In that case, Q(Y| X):=-det (I-L(Y| X)) can often be used as fitness proxy, with L the next-generation matrix for a potential mutant characterized by the trait vector Y in the (constant) environment engendered by a resident characterized by X. If the trait space is connected, global uninvadability can be determined from it. Moreover, it can be used in all the usual local calculations like the determination of evolutionarily singular trait vectors and their local invadability and attractivity. We conclude with three extended case studies demonstrating the usefulness of Q: the calculation of ESSs under haplo-diploid genetics (I), of evolutionarily steady genetic dimorphisms (ESDs) with a priori proportionality of macro- and micro-gametic outputs (an assumption that is generally made but the fulfilment of which is a priori highly exceptional) (II), and of ESDs without such proportionality (III). These case studies should also have some interest in their own right for the spelled out calculation recipes and their underlying modelling methodology.

The distribution of the paternity index as a basis for evaluation of sequential testing in paternity analysis.

Several procedures for evaluation of paternity testing data have been suggested in the literature, the majority of them being based on the paternity index statistic (L) or some transform of it. A major problem has been that the true distribution of the paternity index has not been known, making it difficult to perform quantitative evaluations of different procedures. We present an algorithm for computation of the distribution of the paternity index within the limits of a completely controlled amount of approximation. Using this algorithm we evaluate the power and the rate of erroneous classifications of a standard routine test based on a fixed number of genetic marker systems. The efficiency of this standard test procedure is compared to a stepwise (sequential) procedure where in each step one or several marker systems are scored for the mother-child-putative father trio. We suggest that a sequential strategy for testing may be more efficient than one that is based on a fixed number of systems. A sequential procedure can provide information about the accused man's state of paternity in a considerably larger fraction of cases without a substantial increase of the frequency of incorrect classifications. In addition, the cost measured as the average number of marker systems that has to be tested for each trio may be lower in the case of sequential testing than with a fixed number of systems.

Low diversity and biased substitution patterns in the mitochondrial DNA control region of sperm whales: implications for estimates of time since common ancestry.

The mitochondrial DNA (mtDNA) control region was sequenced in 37 sperm whales from a large part of the global range of the species. Nucleotide diversity was several-fold lower than that reported for control regions of abundant and outbred mammals, but similar to that for populations known to have experienced bottlenecks. Relative neck tests did not suggest that the low diversity is due to a lower substitution rate in sperm whale mtDNA. Rather, it is more likely that demographic factors have reduced diversity. The pattern of nucleotide substitutions was examined by cladistic methods, facilitated by the apparent monophyly of lineages from the Southern Hemisphere, as defined by a single base pair deletion. Substitutions were nonrandom in nature, confined to a few "hot spots," and parallel substitutions constituted a majority of the inferred changes. The substitution pattern fitted a negative binomial distribution better than a Poisson distribution, and the bias in number of substitutions among sites was considerably higher than previously reported for the mtDNA control region of any species. A novel method of estimating time since common ancestry was developed, which utilizes the transition/transversion ratio R and the number of substitutions inferred from a parsimony analysis. Using this method, we estimated the age of sperm whale mtDNA diversity to be about 6,000-25,000 years, and when the uncertainty of R was accounted for, a range of about 1,000-100,000 years was obtained.

Did aggregation favour the initial evolution of warning coloration? A novel world revisited.

From experiments using novel prey signals to avoid innate reactions to traditional signals, Alatalo & Mappes (1996, Nature, 382, 708-710) concluded that gregariousness would have selected for warning coloration as it originated for the first time, whereas a solitary prey distribution would not. We have investigated this suggestion in experiments using the same novel prey and background symbols and wild-caught great tit, Parus major, predators. We compared the attack rate on cryptic unpalatable and aposematic unpalatable prey in either a solitary or an aggregated treatment. In the aggregated treatment we found no difference in attack rate on cryptic and aposematic prey. In the solitary treatment the attack rate on aposematic prey was significantly lower after one attack and at the end of the experiment. Thus, we conclude that, in so far as these experiments mimic an original predator-prey relationship, they do not give support to the idea that aggregation would have favoured the evolution of warning coloration in unpalatable prey. Copyright 2000 The Association for the Study of Animal Behaviour.