Professor Geoffrey Alan Parker (born 24 May 1944) is a Derby professor of biology at the University of Liverpool.He has a particular interest in behavioural ecology and evolutionary biology, and is most noted for introducing the concept of sperm competition in 1970. Much of his work from the 1970s onwards has related to the application of game theory (theory of games) to various biological problems, using the evolutionarily stable strategy (ESS) approach pioneered by John Maynard Smith and George Price.

 With R. R. Baker and V. G. F. Smith in 1972, he proposed a leading theory for the evolution of anisogamy and two sexes, and in 1979 made the first theoretical analysis of sexual conflict in evolution. He has also investigated the evolution of competitive mate searching, animal distributions, animal fighting, coercion, intrafamilial conflict, complex life cycles, and several other topics.In 2008, he won the Darwin Medal for his lifetime contribution to the foundations and development of behavioural ecology, in particular for understanding evolutionary adaptations and their consequences for natural populations.