Because of historical random mutation and genetic drift, any trait within a population includes a distribution of phenotypic variants. Further recombination and mutation in the genes underpinning that trait can occasionally lead to a phenotypic variant outside the normal distribution. Provided that this change is heritable, there is then the possibility of evolutionary modication. is chapter discusses how such changes can occur, with the particular focus being on how genomic mutation leads to novel anatomical features that are beyond normal variation. e rst part discusses the ways in which an ospring’s phenotype is observed to dier from that of its parents. e second part examines the possible ways in which the phenotype can be changed by mutation and epigenetic mechanisms. e focus of this chapter is on the eect of mutation on individual genes and it introduces the next chapter, which examines the eect on the phenotype of mutations that alter networks.