1. The insects represent around 75% of the world's fauna and as such provide especially good examples of the evolutionary process in action, aided by their often rapid generation time and high rate of reproduction.
2. Here, I review some of the main mechanisms of mutational, ecological, and evolutionary change in insects. All those described, whether allo- para- or sympatric, involve changes in the genome or in behaviour that may ultimately isolate newly changed individuals from the parental population/s. They include: loss of sexuality by various means, including (potentially) mutation of the gene/s controlling sexuality; karyotypic changes, both in terms of the number of chromosomes, translocation, polyploidy, and hybridisation; host shifts as pre- and post-zygotic isolating mechanisms, and in asexuals such as aphids, ‘divergence hitchhiking’ around key quantitative trait loci (QTL), and in moths, selection acting at a few linkage groups; enzyme-based adaptive changes; sex and contact pheromone-based isolating mechanisms; phenotypic plastic changes; and epigenetic changes. These last such changes are brought about by stress inducers, and may be transgenerational in their effects.
3. Of the above mechanisms, probably chromosomal changes and host shifts represent the commonest mechanisms.