PROBLEM 4 How Does the Environmejit Affect the 



Characters of an Organis7n? 



Variations caused by genetic make-up. 



Occasionally variations within a species 

 are caused by mutations and chromo- 

 somal changes. These are not of frequent 

 occurrence because they are not part of 

 the regular machinery by which gametes 

 are formed and fertilization and develop- 

 ment take place. 



iMuch more common are the variations 

 produced by normal sexual reproduc- 

 tion. Since, in reduction division, the 

 chromosomes with their genes assort in 

 various ways and since, in fertilization, 

 genes from two individuals are brought 

 together there is a constant reshuffling of 

 genes in every generation. The new com- 

 binations cause endless variation among 

 the individuals of the same species. Ex- 

 amples of such variations in human be- 

 ings are: color of skin, hair, and eyes; 

 length of nails and of fingers and toes; 

 height; shape of nose and chin. 



The environment causes variations. 

 New gene combinations only partly ex- 

 plain variations that occur. You are af- 

 fected by your food, by the amount of 

 exercise and sleep you get, by the kind of 

 air you breathe, as well as by many other 

 factors of your environment, such as in- 

 sects, bacteria, clothes, cosmetics, radios, 

 theaters, books, school, friends, family, 

 and many others. These factors cause 

 changes in you. Disease germs may 



change your blood or other tissues; cos- 

 metics and sunlight may change parts of 

 your skin; practicing the violin may 

 cause changes in your nerve and muscle 

 cells; every other factor in the environ- 

 ment may produce changes in the tissues 

 and organs of your body or in your be- 

 havior. If you do Exercise i, you may 

 discover some interesting facts about en- 

 vironment. 



All animals are changed by the envi- 

 ronment. Every plant, too, is affected by 

 the soil it lives in, by the amount of 

 water it gets, by the amount of light, by 

 the force of the wind, by the degree of 

 temperature, and by the kinds of living 

 things that surround it. Simple experi- 

 ments like the one described in Exercise 

 2 will show this. 



Since organisms are changed by the 

 environment, they differ among them- 

 selves. They vary. Even if there were 

 no genetic causes of variations, differ- 

 ences in the environment would cause 

 enormous variation among members of 

 the same species. 



Careful measurement of variations. 

 Biologists have long known that, upon 

 close examination, no two dandelions are 

 exactly alike; no two black bears; no two 

 grasshoppers. You can convince yourself 

 of this by doing Exercise 3. It should not 

 surprise you to find no two living things 



