Natural Selection 101 



Other examples indicate that intensive selection may result in 

 establishing the homozygous condition of an advantageous trait with- 

 out reducing the general population level. In certain regional popula- 

 tions of mosquitoes and house flies exposed to intensive control 

 programs, single-allele factors for insecticide resistance have risen 

 from an extremely low initial level to a practically homozygous 

 condition without causing a perceptible drop in field populations 

 (Brown, 1960). These instances of the establishment of insect 

 resistance are striking examples of rapid evolution at work. They 

 provide evidence supporting Carson's (1959) view that heterozy- 

 gosity lost through intense selection for one trait may be offset by 

 heterozygosity associated with other traits. 



Occasionally freakish mutations arise which would seem to be 

 at a great selective disadvantage because of their inability to func- 

 tion mechanically. For example, in a population of the foxglove 

 aphid Myzus persicae on Presque Isle, Maine, several individuals 

 of a mutant having wings on only one side appeared in 1956 and 

 1957. These can fly but only in a curious spiral pattern (Wave, 

 1958). Because these aphids normally migrate between hosts, it is 

 difficult to see how this mutant type can survive many years in 

 nature unless it is carried as a recessive trait which does not appear 

 phenotypically in the heterozygote. Possibly, wind currents may 

 be the chief agents both of propulsion and direction in the migratory 

 flights of aphids. If this is the case, these lopsided aphids can at 

 least become airborne and may not be at as much of a mechanical 

 disadvantage as it would seem. 



Selection by Environmental Factors 



The environment has a most complex selective action on the genetic 

 makeup of a population. From the standpoint of the organism the 

 environment is composed of many factors : food, water, and shelter; 

 physical conditions such as temperature, humidity, pH, wind ve- 

 locity, water current, and light; parasites and predators, usually 

 of many kinds; and competitors for food, shelter, mates, and other 

 requirements for successful growth and reproduction. In contrast, 

 the organisms of a population may possess collectively great varia- 

 tion in genetic makeup, representing differences for which any 

 factor of the environment may have a selective effect. 



The following cases are relatively simple examples of the selective 

 action of environmental factors. 



