ARE PARTIAL MANTEL TESTS ADEQUATE?

Partial Mantel tests were designed to test for correlation among three matrices of pairwise distances. We show through an example that these tests may be inadequate, because the associated P-value is not indicative of the type I error.     

WHAT IS THE GOOD OF HEALTH CARE?

This paper sets out to discuss what precisely is meant by 'benefit" when we talk of the requirement that the health care system concern itself with health gain or with maximizing beneficial health care. In particular I argue that in discharging the duty to do what is most beneficial we need to choose between rival conceptions of what is meant by beneficial. One is the patient's conception of benefit and the second is the provider's or funder's conception of benefit. I argue that it is the patient's conception of benefit which is paramount and that if this is followed it commits us to a conception of patient care which must be blind to prognosis in so far as prognosis is thought to bear upon issues of prioritisation or resource allocation.     

IS THE PRODUCTION OF MULTIPLE SPERM TYPES ADAPTIVE?

Males of many species concurrently produce more than one sperm type, now called sperm heteromorphism. In the Drosophila obscura group, all species examined to date produce multiple sperm types that differ in sperm length. Short sperm types in at least three obscura group species do not participate in fertilization, leading to questions regarding the adaptive value of sperm heteromorphism. The common and pervasive inheritance of this trait in the obscura group, however, may indicate that sperm heteromorphism is phylogenetically constrained and therefore does not reflect an adaptive response to selection pressures. I measured interspecific variation in sperm length and determined the number of sperm types produced in 10 obscura group species. I subsequently tested if interspecific variation in sperm length is significantly associated with phylogeny by using an autoregressive comparative method. All obscura group species examined produce two visually distinct sperm lengths, short and long. Phylogenetic autoregression analyses indicated that 22% of the interspecific variation in long sperm is related to phylogeny, whereas short sperm are not significantly correlated with phylogeny. These results suggest different selection pressures on the two sperm length types; long sperm have evolved in response to fertilization demands and short sperm have been decoupled from these requirements.     

PARTIAL RESISTANCE IN THE LINUM‐MELAMPSORA HOST–PATHOGEN SYSTEM: DOES PARTIAL RESISTANCE MAKE THE RED QUEEN RUN SLOWER?

Five levels of disease expression were scored in a cross-inoculation study of 120 host and 60 pathogen lines of wild flax Linum marginale and its rust fungus Melampsora lini sampled from six natural populations. Patterns of partial resistance showed clear evidence of gene-for-gene interactions, with particular levels of partial resistance occurring in specific host-pathogen combinations. Sympatric and putatively more highly coevolved host-pathogen combinations had a lower frequency of partial resistance types relative to allopatric combinations. Sympatric host-pathogen combinations also showed a lower diversity of resistance responses, but there was a trend toward a greater fraction of this variance being determined by pathogen-genotype × host-genotype interactions. In this system, there was no evidence that partial resistances slow host-pathogen coevolution. The analyses show that if variation is generated by among population host or pathogen dispersal, then coevolution occurs largely by pathogens overcoming the partial resistances that are generated.     

TROPAEOLUM UMBELLATUM: IS IT EXTINCT IN THE WILD?

The brief history of Tropaeolum umbellatum Hooker in cultivation is described, and the original illustration ( Fig. 1 ) is republished. Only three collections of this very distinct species appear to have been made in the wild, all in Ecuador around 1846, and no further records are noted either in the literature, or on the internet.

Figure 1 Open in figure viewer PowerPoint Tropaeolum tuberosum. From a hand‐coloured lithograph by W. H. Fitch in Curtis's Botanical Magazine 74, t. 4337(1848).     

IS EVOLVABILITY INVOLVED IN THE ORIGIN OF MODULAR VARIATION?

Abstract.— Lipson et al. (2002) presented an elegant linear algebraic formalism to define and study the evolution of modularity in an artificial evolving system. They employed simulation data to support their suggestion that modularity arises spontaneously in temporally fluctuating systems in response to selection for enhanced evolvability. We show analytically and by simulation that their correlate of modularity is itself under selection and so is not a reliable indicator of selection for modularity per se. In addition, we question the relation between modularity and evolvability in their simulations, suggesting that this modularity cannot confer enhanced evolvability.     

IS THE GROWTH RATE HYPOTHESIS APPLICABLE TO MICROALGAE?1

The growth rate hypothesis (GRH) asserts, from known biochemistry, that maintaining high growth rates requires high concentrations of ribosomes. Since ribosomes are rich in phosphorus (P), the GRH predicts a positive correlation between growth rate and P content; this correlation is observed in some organisms. We consider the application of the GRH to phytoplankton and identify several key problems that require further research before the hypothesis can be accepted for these organisms. There are severe methodological problems that confound interpretation of data for testing the GRH. These problems include the measurement of protein and nucleic acids (such that ratio of these components carries a high level of uncertainty), studies of steady‐state versus dynamic systems, and the presentation of data per cell (especially as cell size varies with growth rate limitations) and the calculation of growth rates. In addition, because of the short generation times and rapid responses of these organisms to perturbations, ribosome and RNA content is expected to vary in response to (de)repression of various systems; content may increase on application of growth‐limiting stress. Finally, that most phytoplankton accumulate P when not P stressed conflicts with the GRH. In consequence, the value of the GRH for any sort of predictive role in nature appears to be severely limited. We conclude that the GRH cannot be assumed to apply to phytoplankton taxa without first performing experimental tests under transient conditions.     

IS THE NUCLEAR DNA CONTENT OF MATURE ROOT CELLS PRESCRIBED IN THE ROOT MERISTEM?

Cells of the mature root exhibit arrest within the G1 and G2 periods of the mitotic cycle. The number of cells arrested with a 2C or 4C DNA amount in mature tissue was compared with that in meristems of excised primary root tips deprived of carbohydrate. Results from four plant species are described. Cells in mature tissue of seedling roots of Vicia and Pisum exhibited arrest predominately at the 4C while those of Triticum and Helianthus arrested preponderantly at the 2C DNA level. The proportion of cells arrested at the 2C and 4C levels in mature root tissue was specific for each species tested. In each species the cycle stage where most cells arrested was the same in carbohydrate-deficient root meristems as in mature root tissue; consequently, most meristematic cells are preconditioned or predetermined to arrest in a specific mitotic period. A test system was developed in Pisum in which the predominant period of arrest was altered by the removal of the cotyledons. The predominant arrest period changed from 4C to 2C in both mature root tissue and carbohydrate-deficient root meristems with cotyledon removal and indicated that mature root cells are preconditioned while meristematic as to where they will eventually arrest in the mitotic cycle.     

IS YOUR PHYLOGENY INFORMATIVE? MEASURING THE POWER OF COMPARATIVE METHODS

Phylogenetic comparative methods may fail to produce meaningful results when either the underlying model is inappropriate or the data contain insufficient information to inform the inference. The ability to measure the statistical power of these methods has become crucial to ensure that data quantity keeps pace with growing model complexity. Through simulations, we show that commonly applied model choice methods based on information criteria can have remarkably high error rates; this can be a problem because methods to estimate the uncertainty or power are not widely known or applied. Furthermore, the power of comparative methods can depend significantly on the structure of the data. We describe a Monte Carlo-based method which addresses both of these challenges, and show how this approach both quantifies and substantially reduces errors relative to information criteria. The method also produces meaningful confidence intervals for model parameters. We illustrate how the power to distinguish different models, such as varying levels of selection, varies both with number of taxa and structure of the phylogeny. We provide an open-source implementation in the pmc ("Phylogenetic Monte Carlo") package for the R programming language. We hope such power analysis becomes a routine part of model comparison in comparative methods.