TABLE 3.

Defining Features of Populations of Living Beings, Including Humans, Relevant to Public Health and Population Sciences

			Intrinsic (Constitutive) Relationships (Internal and External)
Example	Boundaries	Individuals	Genealogical	Internal and Economical: Relationships among Individuals in the Population	External and Ecological: With Other Populations	Teleological (for Humans and Possibly Some Other Species)
Human beings: U.S. population	Political and geographic, i.e., nation-state with citizenship criteria established by politics and territory; although “cultural” boundaries also exist, they are predicated on nationality.	Individual persons, in legally defined groups demarcated by historically contingent citizenship status: (a) U.S. free nonindigenous citizensa; (b) U.S. indigenous citizensb (who may have legally defined dual citizenship with sovereign tribal nations); and (c) noncitizens: legal “permanent residents” (and “permanent aliens”c), legally defined refugees, and undocumented persons.	Direct genealogy: U.S. citizenship by being born to U.S. citizens (jus sanguisd); citizenship by place of birth (jus soli, for persons not otherwise born to U.S. citizens) can become genealogical citizenship for subsequent generations.	As in any polity (political-geographic entity), the economic, legal, political, and social relationships in the United States between individuals that produce, reproduce, and transform the daily conditions of life (e.g., involving work, commerce, property, and the production, exchange and consumption of material goods; establishing and maintaining family life from birth to death), which individuals are legally permitted to engage in these relationships is historically contingent (e.g., banning of child labor in the early 20th century; legal racial discrimination in employment and housing until the mid-1960s; current legal restriction of marriage to heterosexual couples in most U.S. states)	U.S. foreign and domestic policy, along with international treaties the United States has signed, shape political, territorial, legal, social, economic, cultural, and ecosystem relationships both (a) between the U.S. population and populations elsewhere in the world (including who is and is not allowed to immigrate, cf. the 1882 Chinese exclusion act and the 1924 immigration restriction act)c and (b) within the United States.	U.S. domestic and foreign policy sets parameters of who counts as the U.S. population and the conditions in which the U.S. population (and its component groups) lives.
Human beings: Social classes	Economic, political, and legal, set by rules and relationships involving property and labor (within and across boundaries of nation-states).	Individual persons and/or individuals in households and/or family structures that live as an economic unit.	Direct genealogy: class origins at birth; political system and legal rules determine if class position is solely hereditary or if class mobility is allowed.	Social classes are established and maintained through their intrinsic relationships to one another as established by the prevailing political system and its legal rules involving property and labor (e.g., cannot have employer without employee); individuals within particular classes can form groups to advance their class interest (whether in conflict or cooperation with the other classes).	Political, legal, and economic relationships among social classes generated by underlying political economy, shaping ways of living, and rights of each social class.	Political philosophies and economic interests shape how individuals view social classes and act to maintain or alter the political and economic systems that give rise to them.
Populations within human beings: human cells and the microbiome	Biological: cell surfaces (and surfaces of cells as organized in tissues, and of tissues as organized in organs).	Human cells (∼10% of cells within a human) and microbial cells (∼90% of the cells within and on a human).	Human cells: from fertilized ovum. Microbiome: initiated by exposure to mother's microbial ecology via birth (vaginal if vaginal delivery, epidermal if Cesarean section); bacteria then primarily reproduce asexually and new bacteria may be introduced (e.g., by fecal-oral transmission).	Example of gut microbiome: symbiotic (mutualistic) extension of human gut cell faculties, in which diverse types of bacteria (represented by different phyla and their species in the oral cavity, stomach, small intestine, and large intestine) receive (and compete for) nourishment, aid with digestion, produce vitamins, and modulate inflammatory response.	Relationships within and on body: among bacteria (intraspecies, interspecies, and gene transfer) and with human cells. Relationships across body boundary: exposure to exogenous bacteria.	Deliberate alteration of microbiome composition by use of antibiotics, probiotics, changes in diet, and changes in water supply and sanitation.
Nonhuman population: example of the eastern cottonwood (Populus deltoids), a hardwood tree native to North America that grows best near streams, and one of 35+ tree species that are poplars.	Biological: a tree species, one that has the ability to produce hybrids with other species in the same genus, including Populus trichocarpa, whose genome was sequenced in 2006, thereby establishing it as the first tree model system for plant biology.	Individual tree (dioecious, i.e., tree is typically female or male).	Sexual reproduction: via wind-driven pollination of flowers on female tree by pollen from flowers on male tree (whereby a female tree may annually produce millions of seeds fertilized by pollen from thousands of male trees), and the seeds (which have long wispy tufts, resembling cotton) are dispersed by both wind and water. Asexual reproduction: via broken branches (e.g., due to storms and floods); people can also propagate via unrooted cuttings.	Typically grows in pure stands, with dominant trees determining spacing between trees (since the trees are very intolerant of shade). Communication to counter predation: self-signaling and between-tree communication via plant volatiles (airborne chemicals) released by herbivore-damaged leaves (e.g., eaten by gypsy moth larvae) that prime defenses (e.g., to attract parasitoids that prey on the larvae) in other leaves (within tree and, if close enough, those of adjacent trees).	In ecosystem context of growing in riverine environment (flood plains with alluvial soil), relationships with —insect predators, —fungal pathogens: —herbivores (e.g., rabbits, deer, and livestock, who both browse and trample the seedlings and saplings) —other animals (e.g., beavers, which build dams out of the saplings; cavities in living cottonwoods used for nesting and winter shelter by wood ducks, woodpeckers, owls, opossums, raccoons) —other tree species: compete with willows (which grow in same areas).	Nontelological (on part of trees) but can be affected by purpose-driven animal behavior (e.g., beavers fell poplars for dams) and by human activity (e.g., human damming and diversions of river waters).

Notes:

^aBefore Emancipation, neither U.S. slaves nor their children were granted citizenship rights, and they became citizens only after passage of the 1866 Bill of Rights and, in 1868, the Fourteenth Amendment (Steinman 2011).

^bIt was not until 1924 that the U.S. government extended citizenship to all American Indians born within the territorial limits of the United States; reflecting this change, in the 1930 census the terminology shifted from Indians “in” the USA to Indians “of” the USA. Before 1924, the status of “citizen” was applied only to those American Indians granted citizenship by specific treaties, naturalization proceedings, and military service in World War I (Steinman 2011).

^cThe 1882 Chinese exclusion act, which banned Chinese immigration for 10 years and also imposed new restrictions on reentry (including reassignment from citizen to “permanent alien”) was renewed repeatedly and reversed only in 1943. The 1924 Immigration Act, designed to control “undesirable immigration” (especially by Jews and also by Asians), set quotas and restrictions (in relation to the U.S. composition, by national origins, in 1880) that were in effect until 1965 (Foner 1997; Zinn 2003).

^dAccording to the U.S. government, the criterion “to become a citizen at birth” is that the person must “have been born in the United States or certain territories or outlying possessions of the United States, and subject to the jurisdiction of the United States; OR had a parent or parents who were citizens at the time of your birth (if you were born abroad) and meet other requirements; people can also become a citizen after birth if they “apply for ‘derived’ or ‘acquired’ citizenship through parents” or “apply for naturalization” (U.S. Citizenship and Immigration Services 2012). For discussion of the changing complexities of conceptualizing and defining nation-states and who counts as belonging to them, see Wimmer and Schiller 2002.