Rishi De-Kayne1, Rowan Schley2, Julia M.I. Barth3, Luke C. Campillo4, Catalina Chaparro-Pedraza5, Jahnavi Joshi6,7, Walter Salzburger3, Bert Van Bocxlaer8, Darko D. Cotoras9,10, Carmelo Fruciano11,12,13, Anthony J. Geneva14, Rosemary Gillespie15, Joseph Heras16, Stephan Koblmüller17, Blake Matthews5, Renske E. Onstein18,19, Ole Seehausen5,20, Pooja Singh5,20, Erik I. Svensson21, David Salazar-Valenzuela22, Maarten P.M. Vanhove23, Guinevere O.U. Wogan24, Ryo Yamaguchi25,26, Anne D. Yoder27 and José Cerca28 1Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, California 95060, USA 2University of Exeter, Exeter, Devon EX4 4QE, United Kingdom 3Zoological Institute, Department of Environmental Science, University of Basel, CH-4051 Basel, Switzerland 4Department of Biology, University of Kentucky, Lexington, Kentucky 40508, USA 5Department of Fish Ecology and Evolution, EAWAG, Swiss Federal Institute for Aquatic Science and Technology, CH-6047 Kastanienbaum, Switzerland 6CSIR-Centre for Cellular and Molecular Biology, Hyderabad, Telangana 500007, India 7Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, Uttar Pradesh 201002, India 8CNRS, Univ. Lille, UMR 8198 - Evo-Eco-Paleo, F-59000 Lille, France 9Department of Terrestrial Zoology, Senckenberg Research Institute and Natural History Museum, 60325 Frankfurt am Main, Germany 10Department of Entomology, California Academy of Sciences, San Francisco, California 94118, USA 11Institute for Marine Biological Resources and Biotechnology (IRBIM), National Research Council (CNR), 98122 Messina, Italy 12National Biodiversity Future Center, 61 90133 Palermo, Italy 13Department of Biological, Geological and Environmental Sciences, University of Catania, 95124 Catania, Italy 14Department of Biology and Center for Computational and Integrative Biology, Rutgers University–Camden, Camden, New Jersey 08103, USA 15Department of Environmental Science, Policy and Management, University of California Berkeley, Berkeley, California 94720, USA 16Department of Biology, California State University, San Bernardino, California 92407, USA 17Institute of Biology, University of Graz, 8010 Graz, Austria 18Naturalis Biodiversity Center, 2333CR Leiden, The Netherlands 19German Center for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, 04103 Leipzig, Germany 20Aquatic Ecology Division, Institute of Ecology and Evolution, University of Bern, 3012 Bern, Switzerland 21Department of Biology, Lund University, SE-223 62 Lund, Sweden 22Centro de Investigación de la Biodiversidad y Cambio Climático (BioCamb) e Ingeniería en Biodiversidad y Recursos Genéticos, Facultad de Ciencias de Medio Ambiente, Universidad Indoamérica, Machala y Sabanilla, Quito EC170103, Ecuador 23Research Group Zoology: Biodiversity and Toxicology, Centre for Environmental Sciences, Hasselt University, 3590 Diepenbeek, Belgium 24Department of Integrative Biology, Oklahoma State University, Stillwater, Oklahoma 74078, USA 25Department of Advanced Transdisciplinary Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan 26Department of Zoology and Biodiversity Research Centre, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada 27Department of Biology, Duke University, Durham, North Carolina 27710, USA 28Department of Biosciences, Centre for Ecological and Evolutionary Synthesis (CEES), University of Oslo, NO-0316 Oslo, Norway Correspondence: rishidekgmail.com

Understanding the processes that drive phenotypic diversification and underpin speciation is key to elucidating how biodiversity has evolved. Although these processes have been studied across a wide array of clades, adaptive radiations (ARs), which are systems with multiple closely related species and broad phenotypic diversity, have been particularly fruitful for teasing apart the factors that drive and constrain diversification. As such, ARs have become popular candidate study systems for determining the extent to which ecological features, including aspects of organisms and the environment, and inter- and intraspecific interactions, led to evolutionary diversification. Despite substantial past empirical and theoretical work, understanding mechanistically how ARs evolve remains a major challenge. Here, we highlight a number of understudied components of the environment and of lineages themselves, which may help further our understanding of speciation and AR. We also outline some substantial remaining challenges to achieving a detailed understanding of adaptation, speciation, and the role of ecology in these processes. These major challenges include identifying factors that have a causative impact in promoting or constraining ARs, gaining a more holistic understanding of features of organisms and their environment that interact resulting in adaptation and speciation, and understanding whether the role of these organismal and environmental features varies throughout the radiation process. We conclude by providing perspectives on how future investigations into the AR process can overcome these challenges, allowing us to glean mechanistic insights into adaptation and speciation.