A gene is considered essential when its loss of function results in a deleterious phenotype, hugely reducing the organism's viability or fitness. However, the link between the essentiality of a gene and its degree of polymorphism is unclear. In this review, we show that there is a place for a certain degree of variability, even for essential genes. We first study the role of infectious diseases in the prevalence of genetic disorders among humans: balancing selection has selected harmful variants due (...) to the selective pressure of pathogens because the heterozygous carrier can resist them. Then we show that the environment can induce adaptation of species by rapidly evolving genes. We also study the role of positive selection on speciation, particularly upon genes of the immune, reproductive and nervous systems. Finally, we highlight the role of regulatory sequences in changes in morphology between species and adaptation to the environment within species. (shrink)