Natural Selection; Origin of Species - 543 



that among colon bacilli (Escherichia coli) 

 a mutation that makes the bacterium resist- 

 ant to the sterilizing effects of streptomycin 

 occurs, on the average, in 1 out of every 

 1,000,000 individuals. In an ordinary en- 

 vironment such resistant mutants have no 

 survival advantage and consequently they re- 

 main in circulation only as an insignificant 

 minority (Fig. 28-4). When, however, strepto- 

 mycin is introduced into the environment, 

 the resistant mutant becomes dominant — to 

 the virtual exclusion of the original nonre- 

 sistant form (Fig. 28-4). In fact, the appear- 

 ance of mutant forms of various bacteria that 



Fig. 28-4. Selection of a mutant form by a change of 

 environment. Mutant forms, resistant to streptomycin, 

 are very rare (1/1,000,000) in an ordinary culture 

 (upper diagram) of colon bacilli. In a new environ- 

 ment with streptomycin added (shaded area of lower 

 diagram), the mutant form soon predominates. (After 

 Th. Dobzhansky.) 



are resistant to one or another of the anti- 

 biotic compounds has become a real problem 

 in modern medical practice. 



ADAPTATION 



Under natural selection each species must 

 attain a fitness to survive and reproduce, 01 

 otherwise it faces extinction. Consequently 

 it is not surprising to find many qualities ol 

 "fitness," or adaptations, in all kinds of liv- 

 ing things. 



The most obvious adaptations have to do 

 with the external features of the organism, 

 taken in relation to its particular environ- 

 ment. Among animals, for example, the 

 coloration tends to be inconspicuous in rela- 

 tion to the background of a particular habi- 

 tat; or the mouth parts are shaped for 

 handling a certain kind of food; or the 

 appendages are fitted for locomotion on land, 

 in the sea, or in the air — and so forth. 

 Among plants, the roots, stems, leaves, and 

 other organs are variously modified accord- 

 ing to conditions in different localities. Many 

 of these obvious external adaptations are 

 important in relation to survival, but in- 

 ternal adaptations, which are not so obvious, 

 are probably even more important. 



Each organism consists of an intricate sys- 

 tem of interacting components, ranging from 

 ultramicroscopic units such as enzymes and 

 substrates, up to the macroscopic organs of 

 the body. Internal adaptation embraces the 

 sum total of these parts and their interac- 

 tions, and internal adaptation assumes a 

 paramount, though frequently hidden, im- 

 portance in survival. If a mutation unbal- 

 ances any of the delicate internal functions, 

 the organism cannot survive at all, regard- 

 less of the particular environment. There- 

 fore, the only mutations that can possibly 

 lead to a greater degree of external adapta- 

 tion are those that also have a neutral or 

 advantageous effect upon internal adapta- 

 tion. Likewise, some mutations may be per- 

 petuated if they have a favorable effect upon 

 the internal adaptation of the species, even 



