264 



POPULATIONS 



emerge at the group level as new biological 

 expressions. These are common property of 

 the group as a whole. The point merits 

 further elaboration. Below are discussed the 

 biological attributes possessed by a popula- 

 tion and shared with an organism and cer- 

 tain of the population's unique features. 

 For an extension of this discussion, the 

 papers of Pearl (1937) and of Thomas 

 Park (1939) may be consulted. 



At least five general attributes are exhib- 

 ited by population and organism alike. 

 These are: 



1. A definite structure and composition is 

 constant for any moment of time, but fluc- 

 tuates with age. 



2. The population is ontogenetic. It ex- 

 hibits (as does an organism) growth, differ- 

 entiation and division of labor, mainte- 

 nance, senescence, and death. 



3. The population has a heredity. 



4. The population is integrated by both 

 genetic and ecologic factors that operate as 

 interdependent mechanisms. 



5. Like the organism, the population is 

 a unit that meets the impact of its environ- 

 ment. This is a reciprocal phenomenon, 

 since the population is altered as a conse- 

 quence of this impact, and, in time, it 

 alters its effective environment. 



While it does stress the analogies be- 

 tween organism and population, this fist 

 fails to indicate certain of the dissimilarities. 

 Pearl (1937, pp. 52-53) spoke to this point 

 when he said: 



"A population is a group of living individ- 

 uals set in a frame that is limited and defined 

 in respect of both time and space. The biology 

 of populations is consequently a division or 

 department of group biology in general. The 

 essential and differentiating feature of group 

 biology is that it considers groups as wholes. 

 It aims to describe the attributes and behavior 

 of a group as such, that is as an entity in it- 

 self, and not as the simple sum of the separate 

 attributes and behavior of the single in- 

 dividual organisms that together make up the 

 group. The concept of group attributes, separate 

 and different from the attributes of the com- 

 ponent individuals, is a familiar one in other 

 fields. For example, the familiar measures of 

 variation such as the standard deviation and 

 coefficient of variation, are quantitative ex- 

 pressions of a group attribute, namely, the 

 shape of the distribution of frequency of the 

 component individuals in respect of the char- 

 acter measured. Similarly, birth rates and death 

 rates are quantitative expressions of group 



attributes, meaningless relative to any in- 

 dividual." 



These "group attributes" are not Umited 

 to birth rates and death rates and should be 

 examined further. Another such character 

 concerns numerical distribution in space 

 and in time— dispersion. The geographic 

 position of an individual organism is hardly 

 a matter of much import. But the dispersion 

 of its group may, and usually does, have 

 great significance both in terms of survival 

 of the group and of its impact on the envi- 

 rorunent it exploits. Thus dispersion emerges 

 as a population attribute. 



Population density is also a group phe- 

 nomenon. One of the tasks of this section 

 will be to make this point. It is self-evident 

 that density is a numerical concept and a 

 population product. Any density analysis 

 yields data that are directly concerned with 

 population mechanics. Population growth- 

 form and equihbrium are also group attri- 

 butes, and the fact tliat populations have 

 functioned as units of selection in evolution 

 adds another convincing argument for the 

 reahty of such attributes (p. 684). 



Perhaps the point is best made by taking 

 an illustration from the field of genetics. 

 Mr. Doe is a white man living, say, in the 

 town of Richmond, Indiana. In terms of his 

 blood groups he is in group O, which means 

 that he has no isoagglutinogen in his blood 

 cells and has a and b isoagglutinins in his 

 serum. In respect of this character Mr. Doe 

 has the genotype ii, the other possible 

 alleles being I" and L. Although this is a 

 precise description of him as a person, it 

 has no validity as a description of the popu- 

 lation to which he belongs. There, the de- 

 scription has reality only when stated as a 

 frequency; i.e., in population terms. On the 

 basis of a 20,000 sample the genetic de- 

 scription of Mr. Doe's group would be, not 

 ii, but 26 per cent P, 7 per cent P and 

 67 per cent i. (For further reference see 

 Strandskov, 1941.) 



The ultimate in population attributes is 

 attained in the truly social insects. Ter- 

 mites, for example, have a division of 

 labor fixed by heredity and by coactions 

 with other members of the colony in which 

 certain castes are differentiated functionally 

 and structurally for the survival of the 

 colony as a whole. This special phase of 

 the problem is discussed later and need 

 only be mentioned here (Chap. 24). 



