Taconic Long Evans rats originated with Drs. Long and Evans in 1915 by a cross of several Wistar Institute white females with a wild grey male. Rederived in 1975 by Simonsen Laboratories from stock obtained from the University of California at Berkeley in 1949. Derived by Taconic in August 1998. Like all Taconic outbred rats, a monogamous mating system is used to maximize the heterozygosity of the stock.