More on speciation and extinction rates and latitudinal diversity gradients in amphibians

Common frog (Rana temporaria). Photo by Erik Svensson

Posted by Erik Svensson

It is spring time, and some of you have already noted the frogs that mate in the pond outside the Ecology Building. Then it seems highly timely to discuss some classical problems in ecology and evolutionary biology that have been subject to some previous lab-meetings, but which continue to fascinate many (including me): why are there more species in the tropics?

Is it because of evolutionary history, such as higher speciation rates in the tropics or lower extinction rates over millions of years? Or is it mainly due to ecological factors such as more energy through the sun and higher humidity in the tropics? Or a combination of ecological and evolutionary factors? What about the roles of niche conservatism and diversity dependence, and how do these affect tropical and temperate diversity?

We will discuss a recent paper  about this by Alexander Pyron and John Wiens about latitudinal diversity gradients in amphibians in relation to speciation, extinction and phylogeny. You can find the paper here, and it is a phylogenetic comparative study, and the abstract is appended below.

When: Tuesday, April  15, at 10.30
Where: "Argumentet", 2nd floor, Ecology Building.

Large-scale phylogenetic analyses reveal the causes of high tropical amphibian diversity

Abstract

Many groups show higher species richness in tropical regions but the underlying causes remain unclear. Despite many competing hypotheses to explain latitudinal diversity gradients, only three processes can directly change species richness across regions: speciation, extinction and dispersal. These processes can be addressed most powerfully using large-scale phylogenetic approaches, but most previous studies have focused on small groups and recent time scales, or did not separate speciation and extinction rates. We investigate the origins of high tropical diversity in amphibians, applying new phylogenetic comparative methods to a tree of 2871 species. Our results show that high tropical diversity is explained by higher speciation in the tropics, higher extinction in temperate regions and limited dispersal out of the tropics compared with colonization of the tropics from temperate regions. These patterns are strongly associated with climate-related variables such as temperature, precipitation and ecosystem energy. Results from models of diversity dependence in speciation rate suggest that temperate clades may have lower carrying capacities and may be more saturated (closer to carrying capacity) than tropical clades. Furthermore, we estimate strikingly low tropical extinction rates over geological time scales, in stark contrast to the dramatic losses of diversity occurring in tropical regions presently.

On speciation, the species problem and the role of species in evolution

This week's lab-meeting will be dedicated to the classical "species problem" in evolutionary biology and the role of species in ecology. We will start off with a brief presentation by Maren Wellenreuther about molecular identification of (putative) hybrid phenotypes between the two calopterygid damselflies (Calopteryx splendens and C. virgo) that she has been working on lately. I will also say a few words about my research trip to Texas, and the remarkable species diversity of odonates in this state (> 260 species in the state of Texas, about five times more than entire Sweden!).

Then, I was thinking we should discuss two recent idéa-articles, which should perhaps be a relatively easy read, and would hopefully be stimulating. One is on the state of the so-called "neutral theory" of species diversity in ecology, and the other is about species concepts and the ephemeral role of species in evolution. Phylogenetic comparative biologist Luke J. Harmon is co-author on both these papers, and one of the other authors is Rampall Etienne, who will be a plenary speaker at our ESF-funded meeting "The role of behaviour in non-adaptive and non-ecological speciation" in August this year. Here you can sign up to this meeting, which is free of charge and will take place on August 18 2012.

Our  lab-meeting  this coming week will take place on May 2, at 13.30 in the seminar room "Argumentet". Below, I provide the abstracts and links to these two interesting articles. You can download them here and here and also by clicking on the Abstract-links below. Enjoy!

The case for ecological neutral theory

• James Rosindell^1, 2, 3, , 
• Stephen P. Hubbell^4, 5, 
• Fangliang He^6, 7, 
• Luke J. Harmon^2, 8,
• Rampal S. Etienne⁹

• Ecological neutral theory has elicited strong opinions in recent years. Here, we review these opinions and strip away some unfortunate problems with semantics to reveal three major underlying questions. Only one of these relates to neutral theory and the importance of ecological drift, whereas the others involve the link between pattern and process, the tradeoff between simplicity and complexity in modeling, and the role of stochasticity and drift in ecology. We explain how neutral theory cannot be simultaneously used both as a null hypothesis and as an approximation. However, we also show how neutral theory always has a valuable use in one of these two roles, even though the real world is not neutral.

Trends Ecol. Evol. 27: 203-208.

Goldilocks Meets Santa Rosalia: An Ephemeral Speciation Model Explains Patterns of Diversification Across Time Scales

Erica Bree Rosenblum1, 2, 3  , Brice A. J. Sarver1, Joseph W. Brown1, Simone Des Roches1, Kayla M. Hardwick1, Tyler D. Hether1, Jonathan M. Eastman1, Matthew W. Pennell1 and Luke J. Harmon1, 3 

Understanding the rate at which new species form is a key question in studying the evolution of life on earth. Here we review our current understanding of speciation rates, focusing on studies based on the fossil record, phylogenies, and mathematical models. We find that speciation rates estimated from these different studies can be dramatically different: some studies find that new species form quickly and often, while others find that new species form much less frequently. We suggest that instead of being contradictory, differences in speciation rates across different scales can be reconciled by a common model. Under the “ephemeral speciation model”, speciation is very common and very rapid but the new species produced almost never persist. Evolutionary studies should therefore focus on not only the formation but also the persistence of new species. 

Welcome Lesley Lancaster, our new postdoc

I am pleased to welcome Lesley Lancaster, our new incoming postdoc, funded by BECC ("Biodiversity and Ecosystem Services in Changing Climate"). Lesley will arrive to Lund in late May or early June, and she will work with me and Bengt Hansson on the population genetics and ecology of range limit evolution, particularly using our favourite model organism: the damselfly Ischnura elegans ("Common Bluetail") as the main study object. Both Bengt and I are very excited about this project and about recruiting Lesley, who will bring with her new skills and perspectives from her previous research.

Currently, Lesley is a postdoctoral scholar at  the National Center for Ecological Analysis and Synthesis (NCEAS) in Santa Barbara (California, USA), where she has been since 2009. Her main focus of research has been to reconstruct historical evolutionary processes of adaptation, speciation, extinction and migration using time-calibrated molecular phylogenies of various Californian plant clades. She is also interested in historical habitat tolerances of the unique California chaparral habitat. Her postdoctoral research has resulted in some interesting papers in BMC Evolutionary Biology and Systematic Biology.

Lesley's thesis research was on a quite different topic: maternal effects, reproductive strategies and evolutionary ecology of a colour polymorphic lizard (Uta stansburiana), where she worked in the laboratory of Barry Sinervo at University of California, Santa Cruz (UCSC). Her thesis work also resulted in a number of interesting and impressive publications in American Naturalist, Ecology Letters, Evolution and PNAS. 

Lesley is thus an extremely broad and well-rounded biologist and a very experienced postdoc, who has worked at quite different levels of biological organization, and moved her research focus from studies of individual behaviour and evolutionary ecology, to broader macroevolutionary and macroecological questions. It is for precisely these reasons we are excited to bring her in to Lund; she has both sufficient much in common with our already ongoing research,  yet has many complementary skills that will be of interest to us and our research.

On density-dependent diversification and speciation in birds

Posted by Erik Svensson


This coming Wednesday (March 21 at 13.30 NOTE TIME!!!!), we will discuss a paper about the (possible) density-dependent slowdown of cladogenesis in birds with the progress of adaptive radiation. It is a paper that was published relatively recently, although it is not entirely new, in PLoS Biology by Albert Phillimore and Trevor D. Price. The title of the paper is:
"Density-Dependent Cladogenesis in Birds"
The authors have used molecular phylogenies from various avian groups to test the hypothesis that diversification rates decline with the progress of adaptive radiation, and as we are approaching the present. Here is the Abstract to the article, which can be downloaded here (PLoS Biology is a "Open Acess"-publisher, which makes everything easier):

Abstract 

A characteristic signature of adaptive radiation is a slowing of the rate of speciation toward the present. On the basis of molecular phylogenies, studies of single clades have frequently found evidence for a slowdown in diversification rate and have interpreted this as evidence for density dependent speciation. However, we demonstrated via simulation that large clades are expected to show stronger slowdowns than small clades, even if the probability of speciation and extinction remains constant through time. This is a consequence of exponential growth: clades, which, by chance, diversify at above the average rate early in their history, will tend to be large. They will also tend to regress back to the average diversification rate later on, and therefore show a slowdown. We conducted a meta-analysis of the distribution of speciation events through time, focusing on sequence-based phylogenies for 45 clades of birds. Thirteen of the 23 clades (57%) that include more than 20 species show significant slowdowns. The high frequency of slowdowns observed in large clades is even more extreme than expected under a purely stochastic constant-rate model, but is consistent with the adaptive radiation model. Taken together, our data strongly support a model of density-dependent speciation in birds, whereby speciation slows as ecological opportunities and geographical space place limits on clade growth.

We meet at 13.30 (not 13.00!) to discuss this interesting article.

Lab-meeting on adaption, extinction and GIS and field excursion next week

This coming week, I was thinking that we should discuss two interesting and general papers. The first one is an essay entitled Adaptation, extinction and plasticity in a changing environment, and it is published in the journal PLoS Biology. One of the authors is Russel Lande, one of the pioneers in developing statistical methods to study natural and sexual selection in natural populations. The current paper outlines a new research programme in how to apply these methods to study ongoing adaptation and evolutionary change in response to rapid environmental change, e. g. due to anthroprogenic global warming.

The second paper is published in Trends in Ecology & Evolution and it is entitled Integrating GIS-based environmental data in to evolutionary biology. One of the authors is John Wiens, who has used GIS extensively in some recent impressive and interesting studies on niche conservatism in salamanders and amphibians, some of which we have discussed in previous lab-meetings. The choice of this latter paper is motivated by the fact that Maren Wellenreuther and Keith Larson have done some interesting new analyzes using GIS that we might take a look at, depending on time and if Maren kan make it to the lab-meeting.

For Thursday (May 20), I was thinking that some of us should make a field trip to Klingavälsåns Naturreservat and other damselfly sites. Although spring is late, we could at least go out in the field and look at the sites and do some planning. We can decide about time and practical details on Wednesday.

Time and place for our regular lab-meeting:

Where: "Darwin-room", 2nd floor, Ecology Building
When: Wednesday, May 19, 10.15.

Any fika-volunteer?

Goodbye to 20 % of all lizards by 2080?

An interesting, but depressing study was published in Science this Friday. A research team lead by Barry Sinervo, and also including my colleagues Donald Miles (University of Ohio) and Jean Clobert (CNRS, France) have shown an alarming high rate of local population extinctions in Mexico over the last 35 years. Based on these observed real-time extinctions and biophysical modelling of lizard body temperatures in the field (based on experiments), they conclude that the rate of climate change and increasing temperatures are too high for the lizards to have time to adapt.

Lizards and other ectotherms are constrained in their foraging time in hot climates because they must avoid overheating. In particular, viviparous lizards suffer, since pregnant females are especially sensitive: they carry embryos in their bodies which easily die at high temperatures. It is therefore not surprising that most viviparous lizard populations are found at higher latitudes and altitudes, i. e. in colder climates. As temperatures increase dramatically in these environments, these viviparous lizards face a significantly higher extinction risk compared to oviparous lizards, largely because their foraging time becomes drastically reduced and they have to spend a larger part of the day in the shade, to avoid overheating.

The research team estimate, conservatively, that about 20 % of all lizard species on Earth run a significant and serious risk of becoming extinct before 2080, unless the current global warming trend is reversed. This is rather alarming, as lizards is only one of several organismal groups that are likely to have passed the "extinction" threshold determined by anthroprogenic global warming. Other groups that have been discussed are amphibians.

Perhaps we are now entering the next (the sixth) massextinction, which will also drag humans away from this planet? Keep in mind that one of the most famous massextinctions about 251 milllion years ago (The Perman-Triassic massextinction event) killed between 70 and 96 % of all living species, and happened after a global temperature increase of about 6 degrees. Incidentally, a six-degree temperature increase is one of the scenarios outlined by UN:s intergovernmental panel IPCC in their most pessimistic scenario over the coming 100 years (although some climate scientists consider this a rather realistic scenario). If so, Homo sapiens might not have long time left on this planet.

You can read more about this lizard study here, and do not forget to watch the video, where Jean Clobert and Barry Sinervo discuss their findings.