Single nucleotide polymorphisms (SNPs) have been proposed by some as the new frontier for population studies, and several papers have presented theoretical and empirical evidence reporting the advantages and limitations of SNPs. As a practical matter, however, it remains unclear how many SNP markers will be required or what the optimal characteristics of those markers should be in order to obtain sufficient statistical power to detect different levels of population differentiation. We use a hypothetical case to illustrate the process of designing a population genetics project, and present results from simulations that address several issues for maximizing statistical power to detect differentiation while minimizing the amount of effort in developing SNPs. Results indicate that (i) while ~30 SNPs should be sufficient to detect moderate (F(ST)  = 0.01) levels of differentiation, studies aimed at detecting demographic independence (e.g. F(ST) < 0.005) may require 80 or more SNPs and large sample sizes; (ii) different SNP allele frequencies have little affect on power, and thus, selection of SNPs can be relatively unbiased; (iii) increasing the sample size has a strong effect on power, so that the number of loci can be minimized when sample number is known, and increasing sample size is almost always beneficial; and (iv) power is increased by including multiple SNPs within loci and inferring haplotypes, rather than trying to use only unlinked SNPs. This also has the practical benefit of reducing the SNP ascertainment effort, and may influence the decision of whether to seek SNPs in coding or noncoding regions.

In the most extensive analysis of body size in marine invertebrates to date, we show that the size-frequency distributions of northeastern Pacific bivalves at the provincial level are surprisingly invariant in modal and median size as well as size range, despite a 4-fold change in species richness from the tropics to the Arctic. The modal sizes and shapes of these size-frequency distributions are consistent with the predictions of an energetic model previously applied to terrestrial mammals and birds. However, analyses of the Miocene-Recent history of body sizes within 82 molluscan genera show little support for the expectation that the modal size is an evolutionary attractor over geological time. Body size influences almost every aspect of the biology of a species, from physiology to life history (1-4), and plays an important role in the organization of ecological communities (5-8). Size-frequency distributions (SFDs) of species within clades and regional biotas represent a macroecological and macroevolutionary expression of the forces operating on body sizes over large temporal and spatial scales, and several models have attempted to explain the shapes of these distributions (9-13). However, little is known about how SFDs of marine invertebrates vary along major environmental gradients such as latitude, and contradictory predictions exist. For example, some authors have argued that size should increase with latitude within and among species even for ectotherms (14-16), whereas species-energy theory predicts decreasing size with latitude (16,17), and clade-specific or region-specific effects might overwhelm any general trends (18). In the most extensive biogeographic analysis of body size in marine invertebrates to date, we compare the SFDs of northeastern Pacific bivalve faunas among four biogeographic provinces, spanning 75 o of latitude. We then compare the shapes of these SFDs to the predictions of a theoretical model of body size based on energetics (10) and test the evolutionary predictions of this and other optimization models. Latitudinal Trends in Body SizeMethods. The latitudinal ranges and body sizes of 915 of the Ϸ950 species of marine bivalves recorded from the tropics to the Arctic along the northeastern Pacific continental shelf (depth Ͻ 200 m) were compiled through an extensive search of the primary literature and from major museum collections (19-21). All bivalve trophic groups are represented, including depositfeeding protobranchs, epifaunal pterioid and infaunal veneroid suspension-feeders, chemosymbiotic lucinoids, and carnivorous septibranchs. As a measure of body size for each species, we used the geometric mean of length and height of the largest known specimen. This standard metric for living and fossil mollusks (22-24) correlates closely with body mass [for the limited mass data available on the bivalves used in this study, log 2 [(length) 1͞2ϫ height] ϭ 5.507 ϩ 0.316 ϫ log 2 (mass), r 2 ϭ 0.81, P Ͻ 0.0001; highly significant relationships between linear shell measurements and body mass...

Taxonomy is an imprecise science that delimits the evolutionary continuum into discrete categories. For marine mammals, this science is complicated by the relative lack of morphological data for taxa that inhabit remote and often vast ranges. We provide guidelines to promote consistency in studies relying primarily on molecular genetic data to delimit cetacean subspecies from both populations and species. These guidelines identify informational needs: basis for the taxonomic hypothesis being tested, description of current taxonomy, description of relevant life history, sample distribution, sample size, number and sequence length of genetic markers, description of measures taken to ensure data quality, summary statistics for the genetic markers, and analytical methods used to evaluate the genetic data. We propose an initial set of quantitative and qualitative standards based on the types of data and analytical methods most readily available at present. These standards are not expected to be rigidly applied. Rather, they are meant to encourage taxonomic arguments that are consistent and transparent. We hope professional societies, such as the Society for Marine Mammalogy, will adopt quantitative standards that evolve as new data types and analytical methods become widely available.