One of the foundational ideas for modern organismal biology is that species can be characterized by a multidimensional niche, in which the dimensions correspond to conditions and resources that are necessary for individuals to survive and reproduce and for populations to persist. From this idea grew the study of specialization, which is the study of niche “breadth” along one or many axes of the ecological niche. A species is said to be “specialized” if, for example, the range of temperatures under which it thrives or prey species on which it feeds is narrower than another species. Thus specialization and generalization are relative terms, and they are also dynamic in ecological and evolutionary time and space. In some cases, proximate ecological factors (such as the presence of a competitor or predator) can be identified that determine the position of a species along a continuum of specialization and generalization. One of the primary challenges in the study of specialization has been to identify and separate ecological factors from evolutionary factors that constrain species or entire lineages to narrow niche breadth. Are species constrained to specialize more by their evolutionary past (i.e., limited fundamental niche breadth) or more by their ecological circumstance (i.e., limited realized niche breadth)? Interest in this issue has increased in recent decades and is currently fueled at least in part by the increasing possibility (via new genomic tools) of understanding the genetic architecture of specialized phenotypes in wild populations. Another factor fueling continued interest is the need to understand forest and agricultural pests, as well as the accumulation of large datasets of trophic interactions across communities or for species-rich lineages. Here we cover the roots of the study of specialization and point to some of the new directions and even classic questions that might be answered within the next ten years. Although relevant to the study of the evolution of specialization, a number of related subfields are not covered, including optimal foraging theory, plasticity through learning of trophic or habitat preferences, and community-level impacts of specialized interactions. Throughout, we emphasize (although not exclusively) trophic specialization in plant-herbivore systems, partly because this is the expertise of the authors but also because many important examples and theory have come from this area.
The landmark paper for the field in the late 20th century is Futuyma and Moreno 1988 (also see the historical overview in Berenbaum 1996). The study of specialization was growing rapidly in the 1980s, and condensing around a set of core hypotheses (e.g., the trade-off hypothesis, the dead-end hypothesis; see Trade-Offs and Specialization as an Evolutionary Dead End). In subsequent decades, there have been many more review papers (e.g., Poisot, et al. 2011), which is not surprising given the breadth of the challenge of understanding evolutionary dynamics of the ecological niche. These overview papers continue to be important for setting research agendas in a diverse field (e.g., Bolnick, et al. 2003), but they are not without their biases. For example, the field has often focused on specialization in trophic interactions to the exclusion of specialization in habitat or abiotic associations; and many workers in the field (the present authors included) have often focused on herbivorous insects (e.g., Forister, et al. 2012, Jaenike 1990), not only for their extreme diversity and specialization but also for the ease with which they can be manipulated and exposed to alternative resources. Without doubt, examples from plants and other animals have been influential in a great many cases, as can be seen in many of the reviews cited here.
Berenbaum, M. R. 1996. Introduction to the symposium: On the evolution of specialization. The American Naturalist 148:S78–S83.
This paper provides a short introduction to the evolution of specialization with a succinct historical overview up to the mid-1990s. It focuses on trophic interactions and outlines some of the major difficulties in studying specialization, including how our human and cultural biases may influence how it is defined.
Bolnick, D. I., R. Svanbäck, J. A. Fordyce, et al. 2003. The ecology of individuals: Incidence and implications of individual specialization. The American Naturalist 161.1: 1–28.
This highly influential paper challenges the assumption that populations are normally homogeneous collections of individuals with identical degrees and types of specialization. To the contrary, strong empirical evidence suggests that relatively generalized populations are often heterogeneous collections of more specialized individuals. The theoretical, ecological, evolutionary, and conservation implications of these insights are explored.
Brooks, D. R., and D. A. McLennan. 2002. The nature of diversity: An evolutionary voyage of discovery. Chicago: Univ. of Chicago Press.
This book gives an expansive and in-depth overview of the history, theory, methods, and empirical work related to the concept of ecological specialization. Among many important ideas, it introduces the concept of “faux generalists” to describe species with apparently wide diet breadth that are actually specialized on a specific resource and tracking that same resource in multiple host or prey species.
Forister, M. L., L. A. Dyer, M. S. Singer, J. O. Stireman, and J. T. Lill. 2012. Revisiting the evolution of ecological specialization, with emphasis on insect-plant interactions. Ecology 93.5: 981–991.
This paper provides a more recent perspective on many of the issues addressed in the seminal review of the topic thirty years earlier in Futuyma and Moreno 1988. Insect-plant systems are used as a model to address theoretical and technological advancements in research on some of the major ideas related to the evolution of specialization. A prospectus for future research to guide the field is provided.
Futuyma, D. J., and G. Moreno. 1988. The evolution of ecological specialization. Annual Review of Ecology and Systematics 19:207–233.
This is the landmark review that synthesized major ideas for the field, and lessons continue to be learned from this paper. For example, the authors pointed out that interspecific comparisons (of, for example, performance on different resources) tend to tell us more about the consequences rather than the causes of specialization, which is still an important idea for researchers working on the “-omics” of specialized phenotypes in a comparative framework.
Jaenike, J. 1990. Host specialization in phytophagous insects. Annual Review of Ecology and Systematics 21:243–273.
This paper provides a classic overview from the perspective of the most extensively studied group of organisms in specialization research: plant-feeding insects. Although relatively narrow in focus, the lessons learned are general, including important distinctions between density-dependent and density-independent mechanisms of specialization and an early quantitative treatment of factors related to the frequency distribution of diet breadth.
Johnson, S. D., and K. E. Steiner. 2000. Generalization versus specialization in plant pollination systems. Trends in Ecology & Evolution 15.4: 140–143.
Along with the review of plant-feeding insects in Jaenike 1990, this paper provides a broad overview from the perspective of another relatively well-studied group: plants and pollinators. The question of whether the degree of specialization has been historically overestimated is addressed with the important conclusion that plant pollination systems occupy all points along a continuum from extremely generalized to extremely specialized.
Poisot, T., J. D. Bever, A. Nemri, P. H. Thrall, and M. E. Hochberg. 2011. A conceptual framework for the evolution of ecological specialisation. Ecology Letters 14.9: 841–851.
Like Forister, et al. 2012, this paper provides a recent overview in the three decades since the seminal work Futuyma and Moreno 1988 with special emphasis on biotic dimensions to specialization. The authors develop an ambitious conceptual framework that synthesizes many ideas to understand how and when specialization evolves and, particularly, how specialized species interactions emerge.
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