^J^Ecohgy and Evolution 



THE ORGANISM AND THE ENVIRONMENT 



Throughout the ages living things have 

 been forced to change as they survived the 

 everchanging conditions of natural selection. 

 Each kind of organism, in fact, has been 

 molded and remolded according to its capac- 

 ity to live and reproduce at such times and 

 in such places as it happened to exist during 

 its evolutionary history. Today, therefore, 

 existing species tend to be well adapted to 

 their present environment. Each displays a 

 variety of adaptations of form, function, and 

 behavior. These adaptations fit each species 

 to survive and multiply not only under the 

 conditions of the environment as a whole but 

 also — and this is even more important — 

 under the special conditions of its own par- 

 ticular natural habitat. 



A systematic study of the manifold interac- 

 tions between organisms and the environ- 

 ment constitutes the science of ecology. Or- 

 ganisms, of course, have been tailored by 

 natural selection to fit some particular en- 

 vironment; but it is equally true that the 

 environment is subject to profound change 

 by virtue of the organisms that live within it. 

 Obviously ecology impinges upon every as- 



pect of biology, and the following summary 

 must therefore be brief and fragmentary. 



BIOTIC AND ABIOTIC ASPECTS OF THE 

 ENVIRONMENT 



Living things are exposed to two kinds of 

 environmental influences: (1) contacts with 

 other organisms (the biotic environment); 

 and (2) contacts with the nonliving elements 

 of nature (the abiotic environment). The 

 biotic and abiotic parts of the environment 

 always work together in shaping the evolu- 

 tionary destiny of the species, but it is easier 

 to discuss them separately. 



The Biotic Environment. Directly or in- 

 directly the animals of a given region are de- 

 pendent upon the green plants of the area, 

 since green plants provide the major source 

 of all organic food. But above and beyond 

 this basic fact, there are many other biotic 

 relationships that enmesh the species in a 

 common web of destiny. Indeed, the equilib- 

 rium that exists is so delicately balanced 

 that any significant change in the population 

 of one species inevitably has an influence on 

 all neighboring species. 



Interdependence of Species. Sometimes it is 



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