The genetic mating system of a sea spider with male-biased sexual size dimorphism: evidence for paternity skew despite random mating success

Male-biased size dimorphism is usually expected to evolve in taxa with intense male–male competition for mates, and it is hence associated with high variances in male mating success. Most species of pycnogonid sea spiders exhibit female-biased size dimorphism, and are notable among arthropods for having exclusive male parental care of embryos. Relatively little, however, is known about their natural history, breeding ecology, and mating systems. Here we first show that Ammothella biunguiculata, a small intertidal sea spider, exhibits male-biased size dimorphism. Moreover, we combine genetic parentage analysis with quantitative measures of sexual selection to show that male body size does not appear to be under directional selection. Simulations of random mating revealed that mate acquisition in this species is largely driven by chance factors, although actual paternity success is likely non-randomly distributed. Finally, the opportunity for sexual selection (Is), an indirect metric for the potential strength of sexual selection, in A. biunguiculata males was less than half of that estimated in a sea spider with female-biased size dimorphism, suggesting the direction of size dimorphism may not be a reliable predictor of the intensity of sexual selection in this group. We highlight the suitability of pycnogonids as model systems for addressing questions relating parental investment and sexual selection, as well as the current lack of basic information on their natural history and breeding ecology

A MODEL OF MATE GUARDING

Precopulatory mate guarding is the habit, practised by many species, of the two sexes' joining together in intimate pairs for some time, usually days, before mating. It is mainly found in species in which mating is confined to a very short period of the female's reproductive cycle. A mathematical model confirms that precopula will evolve when mating is restricted in time. The model also specifies the evolutionary stable duration of precopula. There are two models: in the simpler, males cannot take over paired females; in the second, larger males can take over females from smaller males. In the model with takeovers, larger males guard for less time than smaller males, and the average guarding duration for all males is shorter than in the model in which there were no takeovers. © 1983

Non-independence in statistical tests for discrete cross-species data

Four methods have been proposed that can be used to test for associations between the states of discrete characters in cross-species data and that do not suffer from non-independence due to overcounting of data points. The tests are those of Ridley (1983), Burt (1989), Grafen (1989), and a new test called the ICDE test. The aim of the paper is to measure the Type I error rates for these methods with simulated null distributions of discrete characters. The null data is generated by a model of discrete character evolution, using three shapes of phylogeny: tetratomous, dichotomous, and realistic. Ridley’s and Burt’s tests are both reasonably valid with the realistic phylogeny but biased with the tetratomous and dichotomous phylogenies. Grafen’s phylogenetic regression is reasonably valid with all tree shapes. One version of the ICDE test was valid, the other less so. The invalid results are explained in terms of two kinds of statistical non-independence that arise in discrete data: non-independence due to the reconstruction of character states by parsimony, and the ‘‘family problem’ ’ in which similar patterns are found in null data in many separate radiations because all the radiations began from the same ancestral state. � 1996 Academic Press Limited 1

A new model for discrete character evolution.

The paper provides an explicit justification for the principle that a uniform taxon should contribute only one datapoint in comparative analyses with discrete variables. The justification is that phylogenetic patterns in variables unincluded in the proposed test vitiate the assumption of independence, both at the level of species and at the level of branch segments. The consequence is that a uniform taxon cannot safely be counted as more than one datapoint. The arguments use a branching discrete Markov process in continuous time, with the new feature that the tested variables are only a subset of the evolving characters. This model is proposed as a useful criterion for measuring the merit of proposed tests, and illustrates the necessity for models in evaluating comparative methods