Encyclopedia of Evolutionary Biology Exclusive Preview Articles Page 2 - Preface Page 3 - The Evolutionary Ecology of Mutualism Page 10 - Recombination and Molecular Evolution Page 16 - Sex Chromosome Evolution: Birth, Maturation, Decay, and Rebirth PREFACE The Encyclopedia of Evolutionary Biology was developed to pro- Smocovitis and Norman Johnson) cover the history of evo- vide an authoritative overview of the current state of evo- lutionary biology and applications of evolutionary biology. lutionary biology. It was an ambitious goal, especially given Readers of the encyclopedia will find that entries are gen- that the field did not pause for the two and a half years needed erally pitched at a somewhat advanced level, although with to complete the project. The encyclopedia’s 15 section editors great effort by authors to make entries as accessible as possible collaborated to ensure that content gaps were kept to a min- to a broad audience. Encyclopedias, like living organisms, are imum, and their efforts show. When the project was com- compromises. If all entries could be readily understood in pleted, we had compiled 256 entries, covering a broad range their entirety by first-year university students, this encyclopedia of topics selected by the editors to ensure a comprehensive would be of limited value to experts. At the other extreme, resource. It was a privilege to read every one of these entries, if entries were extremely technical – and our authors were and I was truly humbled by the collective efforts of hundreds undoubtedly capable of producing such entries – the en- of authors to communicate the excitement and sophistication cyclopedia might be inaccessible to students. While there is, by of a field of study that touches on every conceivable topic necessity, variation among entries in this regard, we settled on in biology today. a general target: the majority of an entry should be accessible There are many ways to envision an encyclopedia of evo- to a motivated, advanced undergraduate. Readers are, of lution, and we had to choose an approach that would lead course, directed to additional resources, with authors pro- to a cohesive resource. Readers will note that, in the more viding bibliographies and lists of further reading. organismal-focused entries (edited by David Guttman, Amy As with any undertaking of this scale, there are many Litt, and Claudia Russo), there is an emphasis on diversification individuals who should be recognized for their roles in of life. We did not set out to provide an overview of the the development of this encyclopedia. Special thanks go to diversity of life, as such a goal would be untenable; rather, Norman Johnson for early discussions that helped us develop we focused on the evolutionary processes and key events the general structure of the encyclopedia. The dedicated and responsible for diversity. Numerous entries deal with speci- distinguished team of section editors deserves the credit for ation, life history evolution, evolutionary biogeography, and drafting the table of contents, recruiting authors, and working coevolution. These entries (edited by Daniel Ortiz-Barrientos, extensively with authors to ensure the highest quality product. Tim Coulson, Rosemary Gillespie, and Andrew Forbes) bring It should go without saying that the high quality of this en- to light how the evolution and diversification of life is cyclopedia ultimately reflects the efforts of the editors and intimately entwined with ecology. Of course, there is extensive authors. Finally, the project management and development coverage of population genetics, quantitative genetics, evo- teams at Academic Press were always ready to assist, and lutionary developmental biology, the evolution of sex and while it is not possible to name everyone who contributed to mating systems, molecular/genome evolution, and phylo- the effort, I am particularly indebted to Simon Holt, Will genetic analysis (edited by Maria Orive, Jason Wolf, Karen Bowden-Green, Paula Davies, and Justin Taylor. Sears, Nina Wedell, Hiroshi Akashi, and Laura Kubatko), all fundamental to our understanding of evolutionary processes. Richard Kliman And as thematic bookends, several entries (edited by Betty Editor in Chief xxxi Mutualism, the Evolutionary Ecology of DM Althoff and KA Segraves, Syracuse University, Syracuse, NY, USA r 2016 Elsevier Inc. All rights reserved. Glossary individuals of the other species and in return receives By-product mutualism An association among species in nutrients or resources needed for growth and reproduction. which the resources exchanged are cost free to at least one Exploiter A species from outside a mutualistic interaction partner and are produced as by-products of other that takes the resources or services provided by mutualistic organismal functions such as metabolism. species without providing anything in return. Cheater An exploiter species that evolved from a Geographic mosaic of coevolution The concept that most mutualistic one and no longer reciprocates with its partner. of the coevolution among species occurs at different rates in Coevolution Reciprocal evolutionary change caused by an different populations with some populations being interaction among species. hotspots of coevolutionary change. Coevolutionary hotspot Populations where all interacting Host sanctions A mechanism by which a mutualist can species are evolving in response to one another. reduce the trade of resources or services in response to a Connectedness In a community of interacting mutualists, partner that is of inferior quality. connectedness is the proportion of the total possible Nutritional mutualism An interaction among species in interactions among species that are observed within an which partners exchange nutrients needed for growth and interaction network. reproduction. Cooperation The act of individuals working together for a Partner choice A mechanism by which mutualistic common benefit. Mainly applied to interactions among partners preferentially interact with high-quality partners. individuals within the same species. Partner fidelity feedback Mutualisms where the increase Defensive mutualism An interaction among partner in fitness of one mutualistic partner causes an automatic species in which one partner defends the other in exchange increase in the other resulting in a positive feedback loop. for a place to live or for nutrients needed for growth and This is usually only possible between individuals of reproduction. different mutualist species that associate long enough to Dispersive mutualism An interaction among species in experience changes in fitness. which one partner moves the gametes, offspring, or Introduction can apply to mutualistic interactions, but not all mutualisms include cooperation or facilitation, and not all symbioses are Species interact with one another in myriad ways with the mutualisms. ultimate goal of obtaining resources or services needed for Mutualistic interactions are traditionally divided into three survival and reproduction. Many of these interactions are an- different types based on the resources or services that are tra- tagonistic in which one species gains a benefit but the other ded between species. Nutritional mutualism involves inter- suffers a cost. Predators, herbivores, and parasites either kill actions among species that are trading resources that are their hosts for energy and nutrients or take some resources needed for growth and reproduction. A classic example is the directly from their hosts. In contrast, there are interactions in trade of carbon and minerals between plants and mycorrhizal which the species involved all gain reciprocal benefits. Van fungi. Plant roots form intimate associations with the hyphae Beneden (1873) was the first to call these interactions mutu- of fungi that are very fine and can forage efficiently for phos- alisms. For example, bees visiting a flower gain energy from phorus, nitrogen, and micronutrients in the soil (Courty et al., nectar rewards while providing pollen exchange for the plant. 2010). The roots exude rich carbon compounds that are used Since the term was coined, we have come to realize that as food by the fungi which in return provide the plant with mutualisms are the foundation of most ecosystems (Figure 1). scavenged water and minerals. Dispersive mutualism involves For instance, in terrestrial systems, plants interact with a variety one partner species distributing gametes, offspring, or indi- of species that help in the procurement of nutrients, the dis- viduals of another species in return for a resource. The persal of pollen and seeds, and defense against herbivores. In diversification of angiosperm plants is partly attributed to their aquatic systems, coral and their symbiotic algae build the ex- mutualism with pollinating insects that act as directed pollen tensive reef ecosystems that harbor much of the ocean’s bio- dispersal agents (Grimaldi, 1999). Likewise, plants also often diversity. Furthermore, mutualisms involve a taxonomically trade food rewards with a diversity of mammals, birds, and diverse set of species, occur in every environment, and occur insects that inadvertently disperse plant seeds. Lastly, defensive among highly mobile free-living species as well as those that mutualism involves one species defending another in ex- live on or within other species. For this reason, many add- change for resources and/or a place to live. For example, itional terms have been synonymized with mutualism such as ants protect acacia plants from