In this thesis I investigated the genetic basis of several behaviours to answer questions surrounding the evolution and mechanistic basis of behaviour. Firstly, I took a single-gene approach to investigate the influence of fruitless (fru) on the courtship behaviour of Drosophila. fru is an alternatively-spliced transcription factor that is necessary for the production of male sexual behaviours, and has also been implicated in producing species-specific differences in courtship song. I investigated the patterns of selection acting on fru at the sequence level and found that positive selection was restricted to the alternatively spliced exons of fru. From this I hypothesised that the positively selected changes in fru would contribute to species-specific differences in courtship song. To test this I examined how isoform-specific fru loss-of-function mutants influence courtship song, and generated “species-swapped” flies whereby regions of fru that showed evidence for positive selection were transferred from four species of Drosophila, into D. melanogaster. Contrary to prediction, I found flies that lacked isoforms containing positively selected regions did not show any differences in courtship song. Unfortunately “species-swapped” flies were not generated in time to examine phenotypes and neuroanatomy as intended. Next, I examined the genetic basis of cold acclimation in two species of Drosophila using a transcriptomic approach. I found that the genes differentially expressed in response to cold acclimation were largely different in each of the species; however, the biological processes they were involved in were broadly similar. Finally, I investigated the transcriptomic changes associated with parental care in the burying beetle Nicrophorus vespilloides to determine if males and females alter the genes they express when parenting alone versus with a partner. I found that males greatly reduced their transcriptional response when parenting with a partner, suggesting they reduce the care they provide when present with a female.