The unraveling of evolutionary history, phylogenetics, is a stimulating and contentious area of biology, particularly for the insects. Although the various groups (taxa), especially the orders, are fairly well defined, the phylogenetic relationships among insect taxa are a matter of much conjecture, even at the level of orders. For example, the order Strepsiptera is a discrete group that is recognized easily by having the fore wings modified as balancing organs, yet the identity of its close relatives is not obvious. Stoneflies (Plecoptera) and mayflies (Ephemeroptera) somewhat resemble each other, but this resemblance is superficial and misleading as an indication of relationship. The stoneflies are more closely related to the orthopteroids (cockroaches, termites, mantids, earwigs, grasshoppers, crickets, and their allies) than to mayflies. Resemblance may not indicate evolutionary relationships. Similarity may derive from being related, but equally it can arise through homoplasy, meaning convergent or parallel evolution of structures either by chance or by selection for similar functions.
Only similarity as a result of common ancestry (homology) provides information regarding phylogeny. Two criteria for homology are:
- similarity in outward appearance, development, composition, and position of features (characters);
- conjunction — two homologous features (characters) cannot occur simultaneously in the same organism.
A test for homology is congruence (correspondence) with other homologies.
In segmented organisms such as insects (section 2.2), features may be repeated on successive segments, for example each thoracic segment has a pair of legs, and the abdominal segments each have a pair of spiracles. Serial homology refers to the correspondence of an identically derived feature of one segment with the feature on another segment (Chapter 2).
Traditionally, morphology (external anatomy) provided most data upon which insect relationships were reconstructed. Some of the ambiguity and lack of clarity regarding insect phylogeny was blamed on inherent deficiencies in the phylogenetic information provided by these morphological characters. After investigations of the utility of chromosomes and then differences in electrophoretic mobility of proteins, molecular sequence data from the mitochondrial and the nuclear genomes have become the most prevalent tools used to solve many unanswered questions, including those concerning higher relationships among insects. However, molecular data are not foolproof; as with all data sources the signal can be obscured by homoplasy. Nevertheless, with appropriate choice of taxa and genes, molecules do help resolve certain phylogenetic questions that morphology has been unable to answer. Another source of useful data for inferring the phylogenies of some insect groups derives from the DNA of their bacterial symbionts. For example, the primary endosymbionts (but not the secondary endosymbionts) of aphids, mealybugs, and psyllids co-speciate with their hosts, and bacterial relationships can be used (with caution) to estimate host relationships. Evidently, the preferred approach to estimating phylogenies is a holistic one, using data from as many sources as possible and retaining an awareness that not all similarities are equally informative in revealing phylogenetic pattern.