! Speciation!is!the!process!of!a!single!lineage!splitting!into!two!or!more!daughter! lineages!over!time.!!The!evolution!of!barriers!to!gene!exchange!mark!the!point!in!splitting! where!diverging!lineages!represent!unique!entities!with!independent!evolutionary! trajectories.!!By!identifying!the!genes!that!contribute!to!barriers!between!recently!diverged! species!we!can!begin!to!understand!the!evolutionary!forces!that!led!to!their!divergence,! and!ultimately,!speciation.!!This!dissertation!focuses!on!two!recently!diverged!species!of! field!cricket,!Gryllus'firmus!and!G.'pennsylvanicus,!which!hybridize!in!zone!stretching!from! Connecticut!to!Virginia.!!We![1]!characterize!the!nature!of!understudied!barriers!that!occur! after!mating,!but!before!fertilization,![2]!define!the!role!of!proteins!secreted!from!the!male! accessory!glands!in!those!barriers,!and![3]!describe!patterns!of!variation!and!introgression! within!the!hybrid!zone!for!genes!expressed!in!the!male!accessory!gland.!!Chapter!1!finds! that!there!are!no!postXmating!prezygotic!barriers!between!G.'pennsylvanicus!females!and!G.' firmus!males,!but!that!the!reciprocal!crosses!are!isolated!by!barriers!that!occur!after!sperm! are!released!from!storage,!but!before!fertilization.!!Chapter!2!finds!that!seminal!fluid! proteins!secreted!from!the!male!accessory!glands!induce!a!shortXterm!eggXlaying!response,! but!alone!cannot!explain!the!normal!induction!of!eggXlaying!or!unsuccessful!fertilization!in! ! heterospecific!crosses.!!Chapter!3!lays!the!foundation!for!Chapters!4!and!5!by!describing! patterns!of!admixture!and!the!influence!of!environmental!variables!on!species!distributions! in!a!previously!uncharacterized!region!of!the!hybrid!zone!in!Pennsylvania.!!Chapter!4! compares!patterns!of!introgression!in!Pennsylvania!for!genes!known!to!encode!seminal! fluid!proteins!and!other!genes!expressed!in!the!male!accessory!gland.!!We!find!no!evidence! that!seminal!fluid!proteins!contribute!to!barriers!operating!within!the!hybrid!zone,!but! identify!a!number!of!other!candidate!barrier!genes.!!Chapter!5!compares!the!patterns!we! see!in!Pennsylvania!with!an!independent!transect!of!the!hybrid!zone!in!Connecticut.!!We! find!the!same!genes!under!selection!in!both!regions!of!the!hybrid!zone,!suggesting!that! these!genomic!regions!contribute!to!maintaining!species!boundaries.! !