Despeciation
From Infogalactic: the planetary knowledge core
Despeciation is the loss of a unique species of animal due to its combining with another previously distinct species.[1] It is the opposite of Speciation and is much more rare. It is similar to extinction in that there is a loss of a unique species but without the associated loss of a biological lineage.
Example of three-spined sticklebacks
For example, Taylor et al.'s genetic analysis of three-spined sticklebacks across six lakes in southwestern British Columbia found two distinct species in 1977 and 1988 but only one combined species in data from 1997, 2000, and 2002.[2] They concluded that external factors had imperiled the living conditions of the two species, thus eliminating the evolutionary specializations that had kept them unique.