Speciation in Birds

tags: Speciation in Birds, Trevor Price, book review, evolution, birds

The question of what is a species and how they arise has generated numerous discussions and tremendous controversy throughout the decades. This interest is more than academic, as any bird watcher will tell you since the American Ornithologists' Union (AOU) routinely splits one species into two or lumps two species into one, thereby wreaking havoc with many birders' "life lists"; that master list of species seen that is kept by each birder. More than once, I have heard birders question the validity of one or another of the AOU's decisions based on their personal personal experience or on the scientific data. However, a new book has been published, Speciation in Birds by Trevor Price (Roberts & Company; Greenwood Village, Colorado: 2007) that will provide the reader -- whether an academic, student or birder -- with a clearer knowledge of the extensive research that underlies and contributes to the dynamic concept of what is a species.

In this book, the author examines the empirical evidence documenting the complex and often subtle relationships between birds and their environment; including birds' extraordinary ability to disperse over long distances and into new habitats, their often complex song and reproductive behaviors and how all these traits combine to drive phenotypic and cultural differences that mold them into different forms, ultimately giving rise to the genetic divergence associated with speciation. To this end, the book evaluates the contributions of sexual and natural selection in bird speciation, along with the roles of geographical isolation, ecological differentiation and genetic drift.

Instead of presenting the reader with discussions of the long-term evolutionary history, global biogeography and fossil record of birds, this book details the microevolutionary processes of speciation, hybridization and trait divergence. In addition to an introductory and concluding chapter, the book has 15 chapters, divided into four main topic areas; the geography of speciation (chapters 2-4 & 8), the ecology of speciation (chapters 5-7), mechanisms of social and sexual selection (chapters 9-12), and other pertinent topics, including species recognition (chapter 13), mate choice after speciation (chapter 14), hybrid zones (chapter 15), and genetic incompatability (chapter 16).

The introductory chapter reveals the author's emphasis (bias?) when he defines species as "groups of interbreeding populations reproductively isolated from other such groups". This is, in short, the Biological Species Concept (BSC), which has many problems -- although the author takes some time to discuss when the BSC works well and to point out when it needs revision by comparing it with other species definitions. Fortunately, the more modern Phylogenetic Species Concept, especially as estimated via DNA technology, is well-described (albeit, briefly) in several appendices to chapters one and two.

Chapter two discusses how ecological factors influence the rate of speciation, beginning with geographic isolation, progressing to development of phenotypic divergence, and ending with the establishment of distinct populations in new locations. The chapters that follow examine factors that either promote or retard geographic variation and parapatric speciation. Next are chapters that focus on ecological speciation and environmental controls over speciation on continents; the relationship between ecology and behavior, especially feeding innovations, predation and nest building; and how geographical isolation leads to high rates of island endemism.

In my opinion, the highlight of the entire book, especially for behavioral ecologists and birders, are the comprehensive and clearly written discussions of the effects that social selection has on speciation. Social selection is based on social interactions, especially intra- and intersexual communication, and it is explored extensively because it represents an exception to the rule that ecological differences alone drive divergence and subsequent speciation. Because birds are highly visual and auditory animals, especially when choosing mates and recognizing species, they respond to a large variety of social signals, such as mating cues and breeding systems, sexual selection, song acquisition and divergence, plumage dimorphism and many additional "nonecological" factors -- all which potentially drive speciation. In these chapters, Price reviews several elegant and compelling examples of the link between behavior and speciation; for example, the classic experiments that revealed female preference in zebra finches and their relatives for males that possess novel ornaments. The author observes that female preference for particular novel traits can actually drive speciation, even if such traits have no discernible effect on male fitness.

Chapter thirteen examines the various contexts for learning: sexual imprinting; species recognition, filial recognition and individual recognition, and cultural speciation. The book then ends with chapters devoted to mate choice and a review of the constraints against hybridization; hybrid zones and their role in creation of new species; and the nature of pre- and post-zygotic isolation.

Even though I enjoyed this book, I noticed several shortcomings. For example, the author does not devote separate chapters to either sympatric speciation nor, surprisingly, to avian phylogeography. This is understandable in the case of sympatic speciation since book publishing is a slow process while the literature is relatively new and is rapidly expanding, but I think that avian phylogeography certainly deserves a concentrated discussion. Additionally, several chapters include so-called thought experiments and even a few claims that have not been adequately tested, and they are presented without even mentioning alternative hypotheses.

But I thought that the author included one chapter that is a valuable contribution to the book, even though some researchers might question its inclusion. Chapter eleven compares differences between domestic breeds of birds that arose from artificial (human) selection to naturally-occurring differences among related wild species. The author asks how captive populations have diversified into distinct breeds over time and whether divergence in domestic breeds can be a valid model for studying speciation in wild populations.

This 480 page paperback book, which took eight years to write, is printed on heavy glossy paper that will easily withstand years of use. It is generously filled with 120 remarkable color diagrams and illustrations by Emiko-Rose Paul and Susan Young that are so well-chosen and designed that "studying the pictures" alone can often provide the reader with a general overview of what is presented in the text. The book contains a 7-page glossary that will be useful for those who are unfamiliar with the jargon used in the field. As one would expect, this masterful volume is extensively researched, listing more than 1,000 research papers and books as its references.

Speciation in Birds represents a significant intellectual advance in our understanding of the origin and maintenance of biological diversity by recognizing and reviewing the contributions that ecological and behavioral studies are making to the ongoing study of speciation. The combination of clear and insightful writing with numerous and well-chosen illustrations make this book accessible to a large audience. I think it is an essential addition to the science and natural history libraries of universities and colleges and is a "must read" for professional ornithologists, naturalists, and behavioral, ecological and evolutionary biologists, bird watchers and for anyone who has an interest in natural history, evolution and biology. Additionally, I think it would make a superb textbook for a university course focused on speciation. Highly recommended.

Trevor Price earned a bachelor's degree from the University of Cambridge and a Ph.D. from the University of Michigan. He was a postdoctoral fellow at the University of Chicago in 1984 when he received the American Society of Naturalists Young Investigator's Award. He worked at the universities of Houston and California, San Diego before returning to Chicago in 2003. He was awarded the Guggenheim fellowship for research in 2005 and the E.O. Wilson Naturalist Award in 2007. His fieldwork has been based in India for the past 30 years, specifically in north east India, an area with many endemic bird species and one of the world's top areas in bird diversity.