More than just a metaphor, Wright’s Adaptive Landscape provides inspiration

[Originally appeared on Nothing in Biology Makes Sense]

Review of The Adaptive Landscape in Evolutionary Biology edited by Erik Svensson and Ryan Calsbeek

Have you ever wished you could go back in time to be present at a particular historical event? The 1932 International Congress of Genetics sounds perfect, right? There R. A. Fisher, J. B. S. Haldane, and Sewall Wright all presented papers of their recent research. If you’re a student of population genetics, you probably recognize these names as some of the founders of the field. At this meeting, Wright was asked to condense some of his more technical mathematical framework into a form that was more widely accessible to the audience of biologists. The result was his conceptualization of the Adaptive Landscape where an analogy is made between the fitness of an individual or population and the varied topographic landscape (pictured on the cover of the book). Wright used this metaphor to describe aspects evolutionary dynamics of populations.

The editors of a recent book, The Adaptive Landscape in Evolutionary Biology, gathered together contributions from evolutionary biologists, ecologists, and philosophers to demonstrate the impact that the Adaptive Landscape has had on the field of biology. This book embraces an 80 year old metaphor created by one of the founders of the modern synthesis to explore the breadth and depth of research generated in evolutionary biology. Unlike a recent book addressing aspects of the modern synthesis, Evolution: The Extendend Synthesis (Pigliucci and Müller, 2010) which called for a revolution, Svensson and Calsbeek have assembled authors that explore the innovations and contributions that build upon the fundamental ideas of population genetics and seek to grow the field. Early in this book, Pigliucci asks about the utility of the Adaptive Landscape metaphors, even titling his chapter with the question, “what are they good for?” I think the rest of the book provides a more than sufficient answer to his question.

Living at the edge, range expansion is a losing battle with mutations

[I originally posted this at the blog Nothing in Biology Makes Sense]

Environments can vary substantially in habitat quality, local population abundance, or carrying capacity. Under some climate change scenarios, new, higher quality habitats become available along the margin of a species’ range (e.g. higher latitudes or altitudes) (Thomas et al 2001). These new habitats may be able to support larger population sizes. Factors of demography, evolution, and qualities of the abiotic and biotic communities all interact to determine where a species is found and may influence the ability of a species to expand its range. New research is building genetically explicit models in order to understand how the interplay of these different factors influence evolutionary changes,

The authors of a recent study focus on how the interaction of the demographic process of range expansion changes the way that natural selection favors beneficial and deleterious mutations (Peischl et al 2013). Using both computer simulations as well as mathematical approximations, the authors find that at the range margins, individuals carry a substantial load of deleterious mutations.

Experiments beyond evolution, a primer on coevolution in the laboratory

A review paper on coevolution published online ahead of the print version of Trends in Ecology and Evolution caught my eye a couple of weeks ago. Brockhurst and Koskella (2013) review that state of affairs of experimental coevolution research. Both of these researchers have a rich experience in this field and present a concise review of the field.

The major contributions of experimental coevolution thus far have been to provide direct evidence of the tempo and mode of antagonistic coevolutionary dynamics, the role of antagonistic coevolution in increasing diversity within and among populations, including the role of parasitism in maintaining sexual recombination, and the structure of specificity in coevolving antagonistic interactions.

Within the article, the authors summarize the expected outcomes or results from common experimental coevolution studies.

Approaches to quantifying reciprocal adaptation.
(Modified figure 2 from Brockhurst and Koskella, 2013). 

The process of rapid reciprocal adaptation inherent to antagonistic coevolution can be driven by at least two contrasting modes of reciprocal selection: ‘Fluctuating Selection Dynamics’ (FSD) where changing allele frequencies in host and parasite populations are driven by parasite-mediated selection against common host resistance alleles or ‘Arms Race Dynamics’ (ARD) where recurrent selective sweeps of novel host resistance and parasite infectivity alleles occur through time, leading to increases in the host range of the parasite and the subsequent host resistance traits. Experimental coevolution has revealed evidence for the operation of both of these modes of reciprocal selection.

The authors also do a good job of pointing to a path rich in research aims for understanding coevolutionary interactions. To date, most experimental coevolution studies have focused on single pair, antagonistic interactions. Beyond the common critique of laboratory experiments (a need to increase “reality”), they suggest that studying more complex communities as well as different forms of interactions.

Don’t wait for the article to show up in print, check out the review online now. The Brockhurst lab website can be found here. You can also see what Dr. Koskella is up by reading her blog, Nature's Microcosm.


Reference

Brockhurst MA, Koskella B (2013) Experimental Coevolution of Species Interactions. Trends in Ecology & Evolution: DOI: http://dx.doi.org/10.1016/j.tree.2013.02.009

When mummies attack! Why specificity matters for coevolution

Evolutionary change by means of Natural Selection needs a couple of things in order to happen: heritability and variation in fitness. That is, offspring need to resemble their parents at least a little (heritability) and individuals need to differ in their survival and offspring production (fitness). We’ll worry about heritability in another post, but variation is something that seems like it might be hard to maintain. Some forms of Natural Selection will reduce variation as more fit individuals become frequent and all the different kinds of less fit individuals are eliminated from the population. However, there is a force, common in nature, which may maintain variation, parasites.

Interactions between hosts and parasites can generate strong selective pressures on each player, especially if your life depends on infecting a host. Often, biologists make an analogy to an arms race where players are developing bigger and better defenses or weapons. Antagonistic interactions may also generate negative frequency dependence where a rare host type is favored because the parasites are adapted to a common type. You can learn more by checking out two posts over at Nothing in Biology makes Sense (CJ’s post on the Red Queen Hypothesis or Jeremy’s post on a different coevolutionary puzzle). A key component for maintaining variation via negative frequency dependent selection is specificity. There must variation in the interaction among different host genotypes and parasite genotypes. This is sometimes referred to as a GxG interaction. If parasites can infect all the hosts, there is no specificity. Specificity allows different hosts to be favored over time depending on the composition of the parasite population.

Theoreticians love to use different models of interactions between hosts and parasites, but without empirical evidence, there seems little point. In a recent paper by Rouchet and Vorburger (2012), the authors looked for evidence of just the kind of genetic specificity would result in the maintenance of genetic variation.