Speciation (review)

This intellectually challenging book provides a comprehensive overview and synthesis of speciation research. In 12 chapters plus an appendix, the authors detail the history as well as the current state of knowledge on the categories and origins of reproductive isolating barriers (RIBS), geographic and genetic aspects of speciation, the roles of selection and drift in species formation, connections between speciation modes and macroevolutionary patterns, and alternative species concepts. Jerry Coyne and Allen Orr (formerly mentor and student) are preeminent scholars on biological speciation, so it is appropriate that they have rejoined forces to compose this first authoritative treatise on speciation (apart from edited symposium volumes) in more than two decades.

Ironically, Darwin's own tome On the Origin of Species (1859) dealt primarily with the origin of intraspecific adaptations and largely sidestepped issues of how new species arise by cladogenesis (lineage splitting). Darwin explicated no clear distinction between adaptation and speciation, viewing the latter somehow as merely another evolutionary repercussion of the struggle for existence as organisms jostle over finite resources (especially in the cramped ecological quarters of sympatry). A century later, architects of the Modern Synthesis had gained clearer notions about cladogenesis: that the process entails the emergence of genetic RIBS, and that it usually requires an initial allopatric phase. Nonetheless, the exact roles of natural selection (and sexual selection) in biological speciation, and the magnitudes and types of genetic alterations involved, remained poorly understood. Conundrums abounded, such as: how could selection promote the evolution of postzygotic RIBS (e.g., offspring inviability or infertility) when these conditions obviously reduce genetic fitness? And, how could informative analyses of the genetics of speciation be conducted when the RIBS that define organisms as belonging to different biological species preclude the necessary experimental crosses?

In the ensuing decades, researchers addressed these and many other questions by elaborating verbal and graphical models of speciation, by using molecular techniques to examine genic and genomic changes associated with cladogenesis, and by adopting comparative evolutionary approaches. But rather than culminating in a grand new synthesis, in several respects these efforts fractionated the discipline more than ever. Questions resurfaced as to whether species are real entities or just arbitrary constructs of the human mind. Debates continued over the relative roles of natural selection, sexual selection, and stochastic processes in cladogenesis. Proponents of sympatric speciation claimed that for many taxa, allopatric separation had been grossly overrated as a prerequisite to lineage diversification. And in certain arenas of systematics, revolutionaries sought to overthrow the biological species concept and replace it with one or another version of a phylogenetic species concept, in which reproductive isolation is rejected outright as the theoretical and operational hallmark of cladogenesis.

Speciation critically evaluates such issues in the light of modern as well as classical evidence. The authors demand that any speciation concept or scenario worthy of merit must be able to withstand penetrating logic as well as rigorous empirical assessment. Coyne and Orr thus claim to set the bar higher than had sometimes been the case in past speciation research, and many readers may be unhappy with various of their conclusions—for example, that "despite the perennial popularity of models based on genetic drift, there is little evidence that drift plays an important role in speciation" (p. 8); that "reproductive barriers are the currency of speciation" and "how these barriers arise is the solution to the species problem" (p. 31); that available data "have not supported the view that sympatric speciation is frequent in nature, either overall or in specific groups" (p. 175); that we cannot yet "conclude with confidence if the pattern of enhanced isolation seen in sympatry is usually . . . caused by reinforcement" (p. 381); and that "those who continue to debate the possibility of species selection fail to realize that comparative analyses have already settled the issue" in the affirmative (p. 445).

Regardless of your personal stance on such issues, I highly recommend this deeply insightful book. The field of speciation research was in need of a sagacious update, and this benchmark work will provide...