"Target Review" in Journal of Evolutionary Biology about hybridization and speciation and a comment

Posted by Erik Svensson

In the latest issue of Journal of Evolutionary Biology there is a so-called "Target Review" by a large group of evolutionary biologists entitled: "Hybridization and speciation".  This review, as well as the comments on it, are published "Open Acccess", meaning that anyone can read and download them, even if you are not in a university library. One of the co-authors of this multi-authored paper is by the way Fabrice Eroukhmanoff, former PhD-student in Lund and past member of the EXEB lab, and currently postdoc in Oslo (Norway).  Below is the Abstract:

Abstract

Hybridization has many and varied impacts on the process of speciation. Hybridization may slow or reverse differentiation by allowing gene flow and recombination. It may accelerate speciation via adaptive introgression or cause near-instantaneous speciation by allopolyploidization. It may have multiple effects at different stages and in different spatial contexts within a single speciation event. We offer a perspective on the context and evolutionary significance of hybridization during speciation, highlighting issues of current interest and debate. In secondary contact zones, it is uncertain if barriers to gene flow will be strengthened or broken down due to recombination and gene flow. Theory and empirical evidence suggest the latter is more likely, except within and around strongly selected genomic regions. Hybridization may contribute to speciation through the formation of new hybrid taxa, whereas introgression of a few loci may promote adaptive divergence and so facilitate speciation. Gene regulatory networks, epigenetic effects and the evolution of selfish genetic material in the genome suggest that the Dobzhansky–Muller model of hybrid incompatibilities requires a broader interpretation. Finally, although the incidence of reinforcement remains uncertain, this and other interactions in areas of sympatry may have knock-on effects on speciation both within and outside regions of hybridization.

The Target Review is, as usual for these types of invited reviews,  followed by a number of comments, some of them critical, by several evolutionary biologists, including myself. My comment can be found here and is entitled: "Beyond hybridization: diversity of interactions with heterospecifics, direct fitness consequences and the effects on mate preferences".  There are also contributions by Nick Barton, Servedio, Hermisson and Dorn, Seehausen, Björklund and Shaw and Mendelson, to mention only a few comments of what seems to be an interesting discussion around a controversial topic, namely the role of hybridization in speciation. Enjoy!

Does sexual and natural selection operate against each other or in the same direction?

Posted by Erik Svensson

During last week's lab-meeting we talked a little about the relationship between natural and sexual selection, and to what extent these processes are opposed to each other vs. operate in the same direction and favours the same phenotypic trait values. Next week we will continue discussing this theme based on a recent article about sexual and natural selection in fruitflies (Drosophila melanogaster) in Current Biology. This article by Long, Agrawal and Rowe does also have some important implications for what kind of inferences that can be made to natural populations based on studies in laboratory settings, with a cautionary tale.

Time and place as usual: "Argumentet "(2nd floor, Ecology Building) at 10.30, Tuesday, February 5.

Below, you will find Abstract and link to the article.

The Effect of Sexual Selection on Offspring Fitness Depends on the Nature of Genetic Variation

• Tristan A.F. Long^1, 2, , , 
• Aneil F. Agrawal¹, 
• Locke Rowe¹

• ¹ Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON M5S 3B2, Canada
• ² Department of Biology, Wilfrid Laurier University, Waterloo, ON N2L 3C5, Canada

• http://dx.doi.org.ludwig.lub.lu.se/10.1016/j.cub.2011.12.020, How to Cite or Link Using DOI

• Permissions & Reprints

---

Summary

Whether the changes brought about by sexual selection are, on the whole, congruent or incongruent with the changes favored by natural selection is a fundamentally important question in evolutionary biology. Although a number of theoretical models have assumed that sexual selection reinforces natural selection [1 and 2], others assume these forces are in opposition [3, 4 and 5]. Empirical results have been mixed (see reviews in [1, 6, 7 and 8]) and the reasons for the differences among studies are unclear. Variable outcomes are expected if populations differ in their evolutionary histories and therefore harbor different amounts and types of segregating genetic variation. Here, we constructed populations of Drosophila melanogaster that differed in this regard to directly test this hypothesis. In well-adapted populations, sexually successful males sired unfit daughters, indicating sexual and natural selection are in conflict. However, in populations containing an influx of maladaptive alleles, attractive males sired offspring of high fitness, suggesting that sexual selection reinforces natural selection. Taken together, these results emphasize the importance of evolutionary history on the outcome of sexual selection. Consequently, studies based on laboratory populations, cultured for prolonged periods under homogeneous conditions, may provide a skewed perspective on the relationship between sexual and natural selection.

"The Polyandry Revolution" and a new paper

The latest issue of the review journal Phil. Trans. R. Soc. Lond. B. has an exciting theme issue about the evolution of polyandry, entitled "The Polyandry Revolution". There are several interesting papers that should be "a must" read for those interested in the evolutionary causes and consequences of multiple mating by females and the selection pressures behind female mating behaviour. The cover issue is lovely, isn't it?

We now know that polyandry is common, indeed the norm rather than the exception in many organisms, including most insects. Indeed, it is rather monogamy that needs to be explained, as there are many adaptive and non-adaptive factors that could lead to multiple matings by females. Together with my Canadian colleague and fish biologist Bryan Neff, I have a review paper in the very same issue entitled "Polyandry and alternative mating tactics". Below you will find the Abstract and the topics that we adress:

Polyandry and alternative mating tactics

Many species in the animal kingdom are characterized by alternative mating tactics (AMTs) within a sex. In males, such tactics include mate guarding versus sneaking behaviours, or territorial versus female mimicry. Although AMTs can occur in either sex, they have been most commonly described in males. This sex bias may, in part, reflect the increased opportunity for sexual selection that typically exists in males, which can result in a higher probability that AMTs evolve in that sex. Consequently, females and polyandry can play a pivotal role in governing the reproductive success associated with male AMTs and in the evolutionary dynamics of the tactics. In this review, we discuss polyandry and the evolution of AMTs. First, we define AMTs and review game theoretical and quantitative genetic approaches used to model their evolution. Second, we review several examples of AMTs, highlighting the roles that genes and environment play in phenotype expression and development of the tactics, as well as empirical approaches to differentiating among the mechanisms. Third, ecological and genetic constraints to the evolution of AMTs are discussed. Fourth, we speculate on why female AMTs are less reported on in the literature than male tactics. Fifth, we examine the effects of AMTs on breeding outcomes and female fitness, and as a source, and possibly also a consequence, of sexual conflict. We conclude by suggesting a new model for the evolution of AMTs that incorporates both environmental and genetic effects, and discuss some future avenues of research.

Sexual versus natural selection

 
For next week's lab meeting I thought we could discuss a paper about the effect of natural selection on sexual selection.  Byers and Dunn found that sexual selection in pronghorn seemed to be strongest when natural selection was weakest.  I thought this paper would be a good choice because it's quite short yet deals with several important questions evolutionary biology, such as whether natural and sexual selection usually act in the same or in opposing directions, the role of stochastic effects in determining the response to evolution, and the relationship between the strength of sexual selection and the evolution of sexual dimorphism.

We will also hear an "R tip of the week" from John, and feel free to bring along your favorite biology-related books again, as I gather that this was appreciated last week.

Bateman in Nature: Predation on Offspring Reduces the Potential for Sexual Selection

Abstract: Sexual selection is driven by competition for mates, and the advantage of a competitor is determined by the number of offspring it produces. Early experiments by Angus Bateman characterized this interaction, and the quantitative relationship between a male’s number of mates and number of offspring is known as the Bateman slope. Sexual dimorphism, one of the most obvious results of sexual selection, largely requires a positive Bateman relationship, and the slope provides an estimate of the potential for sexual selection. However, natural selection from the environment can also influence male success, as can random effects, and some have argued for inclusion of the latter in calculations of mate success. Data from pronghorn (Antilocapra americana) reveal the presence of a positive Bateman slope in each year of a 10-year study. We found no evidence that random effects skewed male mating success; however, substantial yearly variation in the Bateman slope due to predation on fawns was evident. These results support the validity of the Bateman relationship, yet they also demonstrate that environmental or extrinsic influences can limit the potential for sexual selection.                          

Purging of the genetic load in the Skyros wall lizard?

Posted by Anna Runemark

In our new Molecular Ecology-paper Has the inbreeding load for a condition-dependent sexual signalling trait been purged in insular lizard populations? we present a pattern which indicates that purging of the genetic load may have taken place in islet populations of the Skyros wall lizard Podarcis gaigeae. We find a pattern of heterozygosity-dependence for a sexually selected ornament, blue side patches, among the populations on the main island (see Figure). In spite of small population sizes and lower heterozygosity we do, however, not find any such pattern among islet populations. In previous studies we have found that the effective population sizes of these populations are small (Runemark et al. 2012) and that genetic drift may affect throat color morph frequency (Runemark et al. 2010) and pheromone composition (Runemark et al. 2011), indicating that stochastic fixation of deleterious alleles could occur at these islets. We find the pattern of heterozygosity dependence for this sexually selected signalling character intriguing and compare it to the patterns of characters mainly subjected to natural selection. We discuss purging and other non-exclusive explanations to the pattern, see abstract below.

This paper happened to be written because I started teasing Marcus Ljungqvist, telling him that the system of islets with small populations of the Skyros wall lizard was a lot better for studying inbreeding than his study system, a not too inbred meta-population of nest-box blue tits. I challenged him to do something with my data, and on Research School in Genomic Ecology meeting we decided to try to study inbreeding together. 

We found out that the blue side patches seemed to be condition dependent in Podarcis hispanica, so we asked Mikkel Brydegaard to do some Matlab-magic to quantify their size from photographs, and used my microsatellite data to estimate between population variation in heterozygosity, and if heterozygosity affected mean patch size. The results were not quite what we expected: we did not find any effects of heterozygosity among the islet populations, which had significantly lower heterozygosity than had the mainland populations. We started pondering about whether the data could be interesting in spite of this. After some thorough statistical scrutiny together with my supervisor Erik Svensson and my coadvisor Bengt Hansson and some comparisons with other data from the study system the project resulted in this paper. I do still – maybe even more now – think that the Skyros wall lizard is a suitable system for addressing topics regarding inbreeding. 

Abstract:

Sexually selected traits are often condition-dependent and are expected to be affected by genome-wide distributed deleterious mutations and inbreeding. However, sexual selection is a powerful selective force that can counteract inbreeding through purging of deleterious mutations. Inbreeding and purging of the inbreeding load for sexually selected traits has rarely been studied across natural populations with different degrees of inbreeding. Here we investigate inbreeding effects (measured as marker-based heterozygosity) on condition-dependent sexually selected signalling trait and other morphological traits across islet- and mainland populations (n = 15) of an endemic lizard species (Podarcis gaigeae). Our data suggest inbreeding depression on a condition-dependent sexually selected signalling character among mainland subpopulations with low or intermediate levels of inbreeding, but no sign of inbreeding depression among small and isolated islet populations despite their higher overall inbreeding levels. In contrast, there was no such pattern among ten other morphological traits which are primarily naturally selected and presumably not involved in sexual signalling. These results are in line with purging of recessive deleterious alleles, or purging in combination with stochastic fixation of alleles by genetic drift, for a sexual signalling character in the islet environment, which is characterized by low population sizes and strong sexual selection. Higher clutch sizes in islet populations also raise interesting questions regarding the possibility of antagonistic pleiotropy. Purging and other non-exclusive explanations of our results are discussed.

Lab meeting Tuesday January 22nd: On the evidence for species coexistence: a critique

Posted by Maren Wellenreuther

Next week, we will discuss a paper by Adam Siepielski and Mark McPeek on the evidence for species coexistence. The paper was published in 2010 as part of the Concepts & Synthesis section in Ecology, which aims towards stimulating new research in ecology. In a  nutshell, the authors survey the literature for empirical studies of coexistence on a local scale and evaluate how many of these studies also actually test the underlying assumptions of species coexistence (such as invasibility). 

In addition, I would like to ask everybody to bring  either your favorite biology book or a biology/science book that you think might be of interest to somebody else in the EXEB group.  The idea is that we can get inspirations and ideas from each other, and maybe some healthy book swapping will result from it. 

Time and place as usual: Argumentet (2nd floor, Ecology Building) at 10.30

I will bring fika! 

 

Below is the abstract of the paper that we will discuss.

Siepielski, Adam M., and Mark A. McPeek. 2010. On the evidence for species coexistence: a critique of the coexistence program. Ecology 91:3153–3164. http://dx.doi.org/10.1890/10-0154.1 

 

Abstract

A major challenge in ecology is to understand how the millions of species on Earth are organized into biological communities. Mechanisms promoting coexistence are one such class of organizing processes, which allow multiple species to persist in the same trophic level of a given web of species interactions. If some mechanism promotes the coexistence of two or more species, each species must be able to increase when it is rare and the others are at their typical abundances; this invasibility criterion is fundamental evidence for species coexistence regardless of the mechanism. In an attempt to evaluate the level of empirical support for coexistence mechanisms in nature, we surveyed the literature for empirical studies of coexistence at a local scale (i.e., species found living together in one place) to determine whether these studies satisfied the invasibility criterion. In our survey, only seven of 323 studies that drew conclusions about species coexistence evaluated invasibility in some way in either observational or experimental studies. In addition, only three other studies evaluated necessary but not sufficient conditions for invasibility (i.e., negative density dependence and a trade-off in performance that influences population regulation). These results indicate that, while species coexistence is a prevalent assumption for why species are able to live together in one place, critical empirical tests of this fundamental assumption of community structure are rarely performed. These tests are central to developing a more robust understanding of the relative contributions of both deterministic and stochastic processes structuring biological communities

Get the paper and read more: http://www.esajournals.org/doi/full/10.1890/10-0154.1

Lab meeting Tuesday January 15th: Linking assortative mating, fitness and sexually antagonistic genetic variation

Posted by Anna Nordén

For the lab meeting next week I thought we could read an interesting article about how assortative mating by fitness effects sexually antagonistic genetic variation. Although it is a modelling paper, and quite theoretical, it is fairly short and I hope most of you have time to read it. You can find it here. In addition, John will show some of his work in the program MARK on damselfly survival data, how fitness is linked to morphology. Hopefully I will also show some results on the strength of assortative mating for different morphological traits in Calopteryx splendens and C. virgo damselflies. I will bring fika. Hope to see you there!

Time and place as usual: Argumentet (2nd floor, Ecology Building) at 10.30

Title: ASSORTATIVE MATING BY FITNESS AND SEXUALLY ANTAGONISTIC GENETIC VARIATION

Author: Göran Arnqvist
Abstract: Recent documentations of sexually antagonistic genetic variation in fitness have spurred an interest in the mechanisms that may act to maintain such variation in natural populations. Using individual-based simulations, I show that positive assortative mating by fitness increases the amount of sexually antagonistic genetic variance in fitness, primarily by elevating the equilibrium frequency of heterozygotes, over most of the range of sex-specific selection and dominance. Further, although the effects of assortative mating by fitness on the protection conditions of polymorphism in sexually antagonistic loci were relatively minor, it widens the protection conditions under most reasonable scenarios (e. g., under heterozygote superiority when fitness is averaged across the sexes) but can also somewhat narrow the protection conditions under other circumstances. The near-ubiquity of assortative mating in nature suggests that it may contribute to upholding standing sexually antagonistic genetic variation in fitness.