ECOLOGY AND GENETIC VARIATION 



599 



adaptation, defined in its strict sense (p. 

 630), is likely to be involved in the origin 

 of many species, we admit evolutionary fac- 

 tors that have not led to increasing adapta- 

 tion. 



The main factors influencing evolution 

 are hereditary variation, reproductive isola- 

 tion, and natural selection. We shall discuss 

 each in turn and then give a broad inter- 



pretation of their interaction. We shall also 

 attempt to substantiate the ecological prin- 

 ciples of evolution with supporting evi- 

 dence. In this section, reference is usually 

 made to recent investigations rather than to 

 the historical development of the subject. 

 The cited literature often contains historical 

 bibliographies, and Chapters 2 and 3 give a 

 brief historical perspective. 



31. ECOLOGY AND GENETIC VARIATION 



INHERITANCE OF ACQUIRED 

 CHARACTERS 



With the evidence available in the early 

 nineteenth century, Lamarck's well-known 

 theor)' of the inheritance of acquired char- 

 acters gave a plausible explanation of the 

 origin of adaptation through the supposed 

 direct action of the environment, often 

 with the intentional cooperation of the or- 

 ganism. One objection to the classical La- 

 marckian concept is that, with the excep- 

 tion of certain biochemical effects dis- 

 cussed in the following para Graphs, no 

 mechanism has been discovered by which 

 an acquired character of the soma in a mul- 

 ticellular organism is able to modify the 

 genes or chromosomes in a directive man- 

 ner (response adapted to stimulus). Inher- 

 itance of the somatically acquired charac- 

 ter depends upon the presence of such a 

 mechanism, and our knowledge of the 

 physiology of the genetic system makes it 

 seem unlikely that any such Lamarckian 

 mechanism exists. (Various controversial 

 aspects of Lamarckism are discussed by 

 Detlefsen, 1925; Morgan, 1924, 1930; 

 Dobzhansky, 1946a; MacBride, 1936. 

 1936a; Lysenko, 1946; Zirkle, 1946; and 

 Howells, 1947.) 



The capacity for reproducing the entire 

 organism is often lost in the development 

 of somatic tissue (as earlv as the eight-cell 

 stage in the flv Miastor) . It may not be 

 lost in some tissues or parts of organisms 

 even in certain adults such as vegetatively 

 reproducing plants and budding metazoans 

 (Hiidra and Planaria, Hargitt, 1944). 



The soma and the germ plasm may be 

 even farther separated in some cases than 

 they are in the specialized individual 

 multicellular organism. Darwin cited the 

 example of the "neuter insects" to refute 



Lamarck's theory, and this still seems to 

 be the best instance of adaptive evolution 

 in which the possibility of a Lamarckian 

 influence is excluded. 



Characters acquired by a sterile caste of 

 a social insect, for example by a soldier 

 termite (Figs, 146 and 149), that would 

 make it more efficient in its defensive func- 

 tion, could not be passed on to the off- 

 spring. The reproductive castes (Fig. 148) 

 that pass along the hereditary constitution 

 have not acquired the adaptive characters 

 of such sterile castes. There is ample indi- 

 cation of the elaboration and development 

 of adaptive characters during the phy- 

 logeny of these sterile castes, features 

 that could not have been characteristic 

 of the reproductive castes before the ori- 

 gin of sterility. Also, there are many 

 cases in which the sterile castes are in- 

 capable of reproduction even under unu- 

 sual conditions (soldier caste in the Ter- 

 mitidae). As pointed out by Darwin, 

 such adaptive evolution of sterile castes 

 could be explained through the nat- 

 ural selection of the colony rather than of 

 the individual (pp. 684, 687, 692, 693; 

 Weisman, 1893; Emerson, 1938, 1939, 

 1947. For an opposing view, see Eldridge, 

 1925). 



ENVIRONMENTALLY INDUCED 

 MUTATION 



The causes of mutation, both of the 

 gene and of the chromosomal sys*^em (the 

 genome), lie largely in the province of the 

 geneticist, and the ecologist becomes partic- 

 ularly concerned only when the natural en- 

 vironment induces mutations. 



In laboratory studies, x-rays, neutrons, 

 radioactivity, cosmic rays, ultraviolet rays, 

 heat and chemicals (colchicine, sangui- 

 narine, copper sulfate, mustard gas [Auer- 



