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Please don't hesitate to contact me if you would like a reprint of any of the publications below, but don't have access to the PDF.

 

 

E. Postma, 2007. Inflated impact factors? The true impact of evolutionary papers in non-evolutionary journals. PLoS ONE 2: e999

 

 

Abstract Amongst the numerous problems associated with the use of impact factors as a measure of quality are the systematic differences in impact factors that exist among scientific fields. While in theory this can be circumvented by limiting comparisons to journals within the same field, for a diverse and multidisciplinary field like evolutionary biology, in which the majority of papers are published in journals that publish both evolutionary and non-evolutionary papers, this is impossible. However, a journal's overall impact factor may well be a poor predictor for the impact of its evolutionary papers. The extremely high impact factors of some multidisciplinary journals, for example, are by many believed to be driven mostly by publications from other fields. Despite plenty of speculation, however, we know as yet very little about the true impact of evolutionary papers in journals not specifically classified as evolutionary. Here I present, for a wide range of journals, an analysis of the number of evolutionary papers they publish and their average impact. I show that there are large differences in impact among evolutionary and non-evolutionary papers within journals; while the impact of evolutionary papers published in multidisciplinary journals is substantially overestimated by their overall impact factor, the impact of evolutionary papers in many of the more specialized, non-evolutionary journals is significantly underestimated. This suggests that, for evolutionary biologists, publishing in high-impact multidisciplinary journals should not receive as much weight as it does now, while evolutionary papers in more narrowly defined journals are currently undervalued. Importantly, however, their ranking remains largely unaffected. While journal impact factors may thus indeed provide a meaningful qualitative measure of impact, a fair quantitative comparison requires a more sophisticated journal classification system, together with multiple field-specific impact statistics per journal.

 

 

E. Postma, Jan Visser and Arie J. van Noordwijk, 2007. Strong artificial selection in the wild results in predicted small evolutionary change. Journal of Evolutionary Biology 20: 1823-1832

 

 

 

 

 

 

 

 

E. Postma, S.C. Griffith and R. Brooks, 2006. Brief Communications Arising: Evolution of mate choice in the wild. Nature 444: E16. Arising from Qvarnström et al. Nature 441, 84-86 (2006)

 

 

Abstract Qvarnström et al. test whether the preference of female collared flycatchers (Ficedula albicollis) for males with large forehead patches could have evolved as a by-product of selection acting on male patch size. They find that the crucial genetic correlation between female choice and male patch size is not significant, and conclude that preference for large patches must have been shaped directly by selection. However, their use of the patch size of a female's social partner as a measure of choice is incomplete, and will result in low estimates of the potential for direct selection to shape female preference. Their study is therefore unable to resolve the question of how female preference for large forehead patches has evolved.

 

 

P. Gienapp, E. Postma and M.E. Visser, 2006. Why breeding time has not responded to selection for earlier breeding in a songbird population. Evolution 60: 2381-2388

 

PDF (BioOne), PDF (Allenpress)

 

Abstract A crucial assumption underlying the breeders' equation is that selection acts directly on the trait of interest, and not on an unmeasured environmental factor which affects both fitness and the trait. Such an environmentally induced covariance between a trait and fitness has been repeatedly proposed as an explanation for the lack of response to selection on avian breeding time. We tested this hypothesis using a long-term dataset from a Dutch great tit (Parus major) population. Although there was strong selection for earlier breeding in this population, egg-laying dates have changed only marginally over the last decades. Using a so-called animal model, we quantified the additive genetic variance in breeding time and predicted breeding values for females. Subsequently, we compared selection at the phenotypic and genetic levels for two fitness components, fecundity and adult survival. We found no evidence for an environmentally caused covariance between breeding time and fitness or counteracting selection on the different fitness components. Consequently, breeding time should respond to selection but the expected response to selection was too small to be detected.

 

E. Postma, 2006. Implications of the difference between true and predicted breeding values for the study of natural selection and micro-evolution. Journal of Evolutionary Biology 19: 309-320

 

 

Abstract The ability to predict individual breeding values in natural populations with known pedigrees has provided a powerful tool to separate phenotypic values into their genetic and environmental components in a nonexperimental setting. This has allowed sophisticated analyses of selection, as well as powerful tests of evolutionary change and differentiation. To date, there has, however, been no evaluation of the reliability or potential limitations of the approach. In this article, I address these gaps. In particular, I emphasize the differences between true and predicted breeding values (PBVs), which as yet have largely been ignored. These differences do, however, have important implications for the interpretation of, firstly, the relationship between PBVs and fitness, and secondly, patterns in PBVs over time. I subsequently present guidelines I believe to be essential in the formulation of the questions addressed in studies using PBVs, and I discuss possibilities for future research.

 

E. Postma, 2005. The evolutionary genetics of life-history traits in a structured environment: Understanding variation in clutch size and laying date in great tits (Parus major). Thesis Utrecht University

 

 

Abstract  Understanding variation in clutch size and laying date in great tits (Parus major). Some of the key questions evolutionary biology aims to answer are how genetic variation is being maintained in traits that are closely related to fitness, how populations are adapted to their environment, and, if their environment changes, how they are able to cope with these changes. Especially the question of how well natural populations can cope with rapid environmental changes is becoming increasingly important given that a continuously increasing human population accompanied by a rapid expansion of human activities is resulting in unprecedented rates of environmental change.

Answering these fundamental questions requires the quantification of not only the genetic variances (or heritabilities) and covariances (or genetic correlations) for a suite of traits, but also of the selection pressures acting upon these traits. In this thesis I deal with the evolutionary genetics of natural populations, and thus with genetic variation, selection, and the interaction between the two. More specifically, I investigate how genetic variation and selection are shaped by their environmental and structural context. The goal of this thesis is to obtain a better insight into the evolutionary quantitative genetics of wild populations, with special reference to the role of environmental variation within and among years, as well as within and among populations. I discuss different methodological approaches to deal with such environmental structuring, and I apply these to great tit clutch size and laying date, using both long-term and experimental data. I investigate how both genetic means and variances differ among populations, and how such differences (or their absence) can be explained in the light of gene flow and selection. I provide empirical evidence that population structure, and thereby environmental and genetic structure, may exist on a small spatial scale, and that, when such structure is overlooked, this may have large effects on estimates of quantitative genetic parameters and selection. However, provided we employ the appropriate methods and take into account the interplay between the different processes, then estimates of genetic variation and selection can provide us with valuable insights into both the direction and the potential rate of life-history evolution in natural populations, and thus into their capacity to cope with both random and directional environmental change.

Spatial and temporal variation at both the individual and the population level in major life-history traits like clutch size and laying date are shaped by genes and the environment, and by selection and gene flow. Although these can all be studied independently from each other, I argue here that it is in particular the interplay between genetic and environmental variation, and between population structure, gene flow and selection that is shaping genetic patterns across time and space. We can therefore only understand variation in clutch size and laying date, and life-history traits in general, if on the one hand we take into account small scale genetic and environmental variation within single populations, and if on the other hand we do not treat these populations as independent units that are isolated from the rest of the world.

 

P. Edelaar, E. Postma, P. Knops and R. Phillips, 2005. No support for a genetic basis for mandible crossing direction in crossbills (Loxia spp.). Auk 122: 1123-1129

 

 

Abstract  Unusual among birds, the bill tips in crossbills (Loxia spp.) overlap in the vertical plane, with the tip of the lower mandible to either the left or right of the tip of the upper mandible when viewed from above. Patterns observed in wild populations and experimental foraging data suggest that a 1:1 ratio of left - to rightcrossing individuals is maintained by frequency-dependent natural selection in some populations, and that genetic drift causes deviation from a 1:1 ratio in other populations. Both processes require a genetic basis for this remarkable polymorphism, yet few data are available that address whether, and how, mandible crossing direction is heritable. To test for a genetic basis of this trait (single or quantitative, autosomal or sex-linked), we analyzed resemblance in mandible crossing direction between related captive-bred individuals of several crossbill taxa with standard statistical techniques as well as modern animal model methodology. Surprisingly, we did not find statistically significant support for a genetic basis of mandible crossing direction. Comparisons of the ratio of left - to right-crossing males and females in wild populations also did not support a sex-linked quantitative genetic basis. We conclude that mandible crossing direction may have uncharacteristically low heritability, but we cannot rule out that it is nongenetically determined.

 

D.H. Nussey, E. Postma, P. Gienapp and M.E. Visser, 2005. Selection on heritable phenotypic plasticity in a wild bird population. Science 310: 304-306

 

 

Abstract  Theoretical and laboratory research suggests that phenotypic plasticity can evolve under selection. However, evidence for its evolutionary potential from the wild is lacking. We present evidence from a Dutch population of great tits (Parus major) for variation in individual plasticity in the timing of reproduction, and we show that this variation is heritable. Selection favoring highly plastic individuals has intensified over a 32-year period. This temporal trend is concurrent with climate change causing a mismatch between the breeding times of the birds and their caterpillar prey. Continued selection on plasticity can act to alleviate this mismatch.

 

E. Postma and A.J. van Noordwijk, 2005. Genetic variation for clutch size in wild populations of birds from a reaction norm perspective. Ecology 86: 2344-2357

 

 

Abstract  Genetic variation for ecologically important traits determines the potential for evolutionary changes and should be measured directly. Such measurements of genetic variation based on quantitative genetic theory rely on assumptions of environmental constancy. These assumptions are not likely to hold in nature. Instead, natural environments are structured, and systematic variation in environmental conditions is an important determinant of phenotypic variation. Here we provide an introduction to quantitative genetics using a reaction norms approach, because we believe that this provides us with a good framework for combining ecology and genetics. We subsequently review the literature on genetic variation for clutch size of birds, and we show that, in spite of the inherent limitations of the methods employed, there is strong evidence that clutch size has a heritable component in natural populations of several species. However, the number of studies on the amount of genetic variation for clutch size in different species and across a range of environmental conditions is still far too small to study patterns in the relationship between heritable variation and properties of species and/or their environments. Furthermore, the role of both correlations and interactions with the environment in these estimates requires much more attention. Above all else, however, we need more information on the structure and magnitude of the environmental variation present in these studies. Future work should focus on how to obtain such data, and how to subsequently incorporate them into the proposed reaction norm framework. This requires the search for, and measurement of, relevant ecological variables. Also a more detailed investigation of the within-individual variation and the use of animal model methodology may prove to be valuable. Such additional data are essential for interpreting the amounts of genetic variation present for clutch size as a model system in the general problem of better understanding the maintenance of genetic variation in heterogeneous environments and the estimation of evolutionary potential.

 

E. Postma and A.J. van Noordwijk, 2005. Gene flow maintains a large genetic difference in clutch size at a small spatial scale. Nature 433: 65-68

 

 

Abstract  Understanding the capacity of natural populations to adapt to their local environment is a central topic in evolutionary biology. Phenotypic differences between populations may have a genetic basis, but showing that they reflect different adaptive optima requires the quantification of both gene flow and selection. Good empirical data are rare. Using data on a spatially structured island population of great tits (Parus major), we show here that a persistent difference in mean clutch size between two subpopulations only a few kilometres apart has a major genetic component. We also show that immigrants from outside the island carry genes for large clutches. But gene flow into one subpopulation is low, as a result of a low immigration rate together with strong selection against immigrant genes. This has allowed for adaptation to the island environment and the maintenance of small clutches. In the other area, however, higher gene flow prevents local adaptation and maintains larger clutches. We show that the observed small-scale genetic difference in clutch size is not due to divergent selection on the island, but to different levels of gene flow from outside the island. Our findings illustrate the large effect of immigration on the evolution of local adaptations and on genetic population structure.

 

P. Edelaar,T. Piersma and E. Postma, 2005. Retained non-adaptive plasticity: Gene flow or small inherent costs of plasticity. Evolutionary Ecology Research 7: 489-495

 

 

Question: Do clams from populations not exposed to a predator retain the ability to respond to that predator?

Motivation: If maintaining the potential for phenotypic plasticity involves a significant cost, plasticity should be selected against in constant environments.

Background: Clams of the species Macoma balthica (a burrowing bivalve) respond to shore crabs by burrowing deeper in the sediment. Norwegian M. balthica are not exposed to crabs naturally, whereas Dutch M. balthica are naturally exposed to variable crab densities.

Sites: Collection: the Balsfjord near Tromsø, Norway, and the Wadden Sea near Harlingen, The Netherlands. Holding tanks: outdoor basins with a continuous flow of unfiltered water from the Wadden Sea.

Method: We introduced a mixture of clams from both sites into experimental aquaria with a thick layer of sandy sediment. Twelve aquaria contained one shore crab; twelve had none. We measured burrowing depth 7 days after the start of each experiment.

Result: Clams from the two sites show similar burrowing responses after exposure to predatory crabs, supporting the hypothesis that maintaining the potential for plasticity costs very little.

 

E. Postma, L.E.B. Kruuk, J. Merilä, A.J. van Noordwijk and B.C. Sheldon, 2003. Old wine in a new but defective bottle. Comment on Quantitative genetic analysis of natural populations by Allen J. Moore & Penelope F. Kukuk. Nature Reviews Genetics Published online on 1 May 2003

 

 

E. Postma, W.F. van Hooft, S.E. van Wieren and L. van Breukelen, 2001. Microsatellite variation in Dutch roe deer (Capreolus capreolus) populations. Netherlands Journal of Zoology 51: 85-95

 

 

Abstract
In this study we investigated microsatellite variation in Dutch roe deer (Capreolus capreolus) populations. We used 65 tissue samples from culled animals from three populations (Amsterdamse Waterleidingduinen, National Park Zuid-Kennemerland and Flevopolder). The first two are dune populations and are about 3.5 kilometers apart. In both populations, roe deer have been present since the early 1950's. In the Flevopolder roe deer were first observed in 1959. From theoretical predictions, a decrease in heterozygosity of 20% due to genetic drift could be expected in the small Amsterdamse Waterleidingduinen population, compared to the much larger Flevopolder population. However, the expected heterozygosity (H_e) at seven loci was 0.56 on average with no significant differences in H_e between populations. The probability that a decrease of 20% or more did occur but that it went undetected is smaller than 2.2%. All populations were significantly differentiated from each other and showed positive FST and Rho values, suggesting limited gene flow between populations. The fact that the decrease of genetic variation was smaller than expected is probably due to gene flow between the Amsterdamse Waterleidingduinen and National Park Zuid-Kennemerland, effective population size at the time of introduction being larger than assumed, and/or because the animals which were used for stocking came from different populations. Considering the amount of variation still present in the different populations, negative effects of reduced genetic variation are currently not expected, but in both dune populations the decrease in the amount of variation will continue.