The water flea (Daphnia lumholtzi) is a cladoceran native to Eastern Africa, Australia and the Asian subcontinent of India and was first reported in 1991 in an Eastern Texas.
Dagram Frisch and Lawrence Weider of the University of Oklahoma are examining the ecological genetics of D. lumholtzi in Lake Texoma, Texas, USA. As no spatial and or temporal survey of the genetic variation of the specie was ever conducted in this lake, they took a combined approach of field surveys and controlled lab experiments to examine the seasonal variation of in the Lake Texcoma population.
The population genetic structure of the D. lumholtzi was observed on 22 dates for a three year period along with temperature and salinity gradient. A two-allele polymorphism at the PGI locius was discovered. Frisch and Weider concluded that the rapid expansion and micro evolutionary dynamics of D. lumholtzi throughout North America might be a result of the genetic and environmental interactions. 
Sources: Freshwater Biology 2010, 55:1327-1336 DOI: 10.1111/j.1365-2427.2009.02356.x