Simulated emergence of cyclic sexual-asexual reproduction

Motivated by the cyclic pattern of reproductive regimes observed in some species of green flies (``{\it aphids}''), we simulate the evolution of a population enduring harsh seasonal conditions for survival. The reproductive regime of each female is also seasonal in principle and genetically acquired, and can mutate for each newborn with some small probability. The results show a sharp transition at a critical value of the survival probability in the winter, between a reproductive regime in the fall that is predominantly sexual, for low values of this probability, or asexual, for high values.Comment: 9 pages, 4 figures, requires RevTe

The advantage of being slow: the quasi-neutral contact process

According to the competitive exclusion principle, in a finite ecosystem, extinction occurs naturally when two or more species compete for the same resources. An important question that arises is: when coexistence is not possible, which mechanisms confer an advantage to a given species against the other(s)? In general, it is expected that the species with the higher reproductive/death ratio will win the competition, but other mechanisms, such as asymmetry in interspecific competition or unequal diffusion rates, have been found to change this scenario dramatically. In this work, we examine competitive advantage in the context of quasi-neutral population models, including stochastic models with spatial structure as well as macroscopic (mean-field) descriptions. We employ a two-species contact process in which the "biological clock" of one species is a factor of $\alpha$ slower than that of the other species. Our results provide new insights into how stochasticity and competition interact to determine extinction in finite spatial systems. We find that a species with a slower biological clock has an advantage if resources are limited, winning the competition against a species with a faster clock, in relatively small systems. Periodic or stochastic environmental variations also favor the slower species, even in much larger systems.Comment: Reviewed extended versio

Bethe ansatz solution of the Osp(1∣2n)Osp(1|2n) invariant spin chain

We have applied the analytical Bethe ansatz approach in order to solve the $Osp(1|2n)$ invariant magnet. By using the Bethe ansatz equations we have calculated the ground state energy and the low-lying dispersion relation. The finite size properties indicate that the model has a central charge $c=n$.Comment: 9 page

Linking Farmers to Markets: The Case of Grain Marketing Information in Western Kenya

Market liberalization created a situation where there are no guaranteed grain prices, no central information source and the need for marketing information increased. Unfortunately, most farmers have little or no access to marketing information. This study evaluates farmers’ perceptions of importance of marketing information; identifies farmers’ sources of grain marketing information; determine farmers’ confidence in and use of marketing information; and assesses determinants farmers’ willingness (WTP) to pay for marketing information. Data used in this study were generated using a structured questionnaire in a survey that covered a random sample of 120 households in traditionally grain surplus-and deficit zones of Kenya. The data were analyzed by descriptive statistics and logit model. Results show that 68% and 55% of the households in grain surplus and deficit zones, respectively, recognized that marketing information was very important. Farmers received marketing information from multiple sources, mainly from traders and other farmers. Most of the farmers who received the information were not utilizing the information due to perceived unreliability of the information and poor access to complementary infrastructure. Education level of the household was the most significant factor that positively affected farmers’ WTP for marketing information. In view of farmers’ perception that information provided by the private sources is unreliable, the public sector ought to provide marketing information as a public service. Smallholder farmers should be catalyzed to form strong associations so as to enjoy economies of scale in accessing marketing information and markets.Farmer, Grains, Logit, Markets., Agricultural and Food Policy, Consumer/Household Economics, Demand and Price Analysis, Environmental Economics and Policy, Farm Management, Food Consumption/Nutrition/Food Safety, Food Security and Poverty, International Relations/Trade, Marketing, Productivity Analysis, Research and Development/Tech Change/Emerging Technologies,

Fragmentation Experiment and Model for Falling Mercury Drops

The experiment consists of counting and measuring the size of the many fragments observed after the fall of a mercury drop on the floor. The size distribution follows a power-law for large enough fragments. We address the question of a possible crossover to a second, different power-law for small enough fragments. Two series of experiments were performed. The first uses a traditional film photographic camera, and the picture is later treated on a computer in order to count the fragments and classify them according to their sizes. The second uses a modern digital camera. The first approach has the advantage of a better resolution for small fragment sizes. The second, although with a poorer size resolution, is more reliable concerning the counting of all fragments up to its resolution limit. Both together clearly indicate the real existence of the quoted crossover. The model treats the system microscopically during the tiny time interval when the initial drop collides with the floor. The drop is modelled by a connected cluster of Ising spins pointing up (mercury) surrounded by Ising spins pointing down (air). The Ising coupling which tends to keep the spins segregated represents the surface tension. Initially the cluster carries an extra energy equally shared among all its spins, corresponding to the coherent kinetic energy due to the fall. Each spin which touches the floor loses its extra energy transformed into a thermal, incoherent energy represented by a temperature used then to follow the dynamics through Monte Carlo simulations. Whenever a small piece becomes disconnected from the big cluster, it is considered a fragment, and counted. The results also indicate the existence of the quoted crossover in the fragment-size distribution.Comment: 6 pages, 3 figure