Hispanic as a term, with various uses, dates back to the 16th c.

No one is arguing the "one race of man." But there are important genetic differences among the groups. Nothing is clear cut because of variation and intermixing but grouping at a level other than population groups is also important to organization of material for research.

Looking at some work with 'Hispanics'-
From the National Cancer Institute- "First workshop on Cancer epidemiology and genomics variation in Hispanic/Latino Populations within the American Continents." http://epi.grants.cancer.gov/admixture/index.html
-Differences in FM02*1 allelic frequency between hispanics of Puerto Rican and Mexican descent. http://dmd.aspetjournals.org/cgi/content/full/32/12/1337
-National Coalition for Health Professional Education in Genetics; several articles; "Genetics, Race and Healthcare," "Genes, Culture and Medicines: Bridging Gaps in Treatment for Hispanic Americans," etc.
-"A genomewide admixture mapping panel for Hispanic/Latino populations," www.citeulike.org/user/daforerog/article/4531691
etc. etc.