The biological variation for which genomics researchers currently seek to understand underlying genetic features exists at many scales. This essay will focus on two of them:
Intraspecific variation refers to traits that differ among individuals of a given species. In humans, examples include hair color, height, and presence or absence of diabetes. In plants, such traits include flower color or the ability to tolerate high salinity conditions. James Mielke, Lyle Konigsberg, and John Relethford emphasize both commonalities and variation, including in disease propensities, among people living in different regions of the world in their book Human Biological Variation, with specific attention to features such as lactose tolerance that reflect region-specific, historical selective pressures. Further, A. C. Westerband, J. L. Funk, and K. E. Barton review how both genetic and environment-induced variation drive within-species variation in plant features such as desiccation tolerance in their recent article “Intraspecific Trait Variation in Plants.”
Interspecific variation refers to variation across species, and so its study encompasses a much larger set of traits than studies of intraspecific variation. For just one example, an internal skeleton is one anatomical feature common to all healthy humans, and so its presence/absence does not vary within species. However, this trait is variable across animal species; an internal skeleton is universally present in mammals but universally absent in insects. Visual treatments such as the Animal Anatomy Coloring Book can be useful for helping students to appreciate the vastness of interspecific variation in anatomical features.
Variation in physiological traits—the set of biological processes that together shape the function of an organism and its response to the environment—is generally less visible than anatomical variation, but is often at least as important for species robustness with respect to stressors such as high temperature and drought. For a whirlwind tour of physiological variation among mammalian species, readers of all levels will appreciate Knut Schmidt-Neilsen’s classic Animal Physiology, which includes some striking figures highlighting the physiological tolerance of species that encounter, for example, exceptionally low-oxygen conditions as in the high Himalayas, or large daily fluctuations in temperature, as are typical in a desert. A number of more focused, classic resources will appeal to readers interested in a comparative approach to specific processes, such as C. E. Stevens and Ian Hume’s Comparative Physiology of the Vertebrate Digestive System, and G. M. Hughes’s Comparative Physiology of Vertebrate Respiration.
Animal Anatomy Coloring Book: Incredibly Detailed Self-Test Veterinary Anatomy Color Workbook by 
Comparative Physiology of the Vertebrate Digestive System by 
