Other species pairs which consist of a well-armored marine form and a smaller , unarmored freshwater form are being studied in ponds and lakes in south-central Alaska that were once marine habitats such as those uplifted during the 1964 Alaska earthquake . The evolutionary dynamics of these species pairs are providing a model for the processes of speciation which has taken place in less than 20 years in at least one lake . In 1982 , a chemical eradication program intended to make room for trout and salmon at Loberg Lake , Alaska , killed the resident freshwater populations of sticklebacks . Oceanic sticklebacks introduced through nearby Cook Inlet recolonized the lake . In just 12 years beginning in 1990 , the frequency of the oceanic form dropped steadily , from 100 % to 11 % , while a variety with fewer plates increased to 75 % of the population , with various intermediate forms making up another small fraction . This rapid evolution is thought to be possible through genetic variations that confer competitive advantages for survival in fresh water when conditions shift rapidly from salt to fresh water . However , the actual molecular basis of this evolution still remains unknown
1964 Alaska earthquake
1964 Alaska earthquake
Three-spined stickleback