Homology is at the foundation of comparative studies in biology at all levels from genes to phenotypes. Homology is similarity because of common descent and ancestry, homoplasy is similarity arrived at via independent evolution. However, given that there is but one tree of life, all organisms, and therefore all features of organisms, share some degree of relationship and similarity one to another. That sharing may be similarity or even identity of structure and the sharing of a most recent common ancestor – as in the homology of the arms of humans and apes – or it may reflect some (often small) degree of similarity, such as that between the wings of insects and the wings of birds, groups whose shared ancestor lies deep within the evolutionary history of the Metazoa. It may reflect sharing of entire developmental pathways, partial sharing, or divergent pathways. This review compares features classified as homologous with the classes of features normally grouped as homoplastic, the latter being convergence, parallelism, reversals, rudiments, vestiges, and atavisms. On the one hand, developmental mechanisms may be conserved, even when a complete structure does not form (rudiments, vestiges), or when a structure appears only in some individuals (atavisms). On the other hand, different developmental mechanisms can produce similar (homologous) features. Joint examination of nearness of relationship and degree of shared development reveals a continuum within an expanded category of homology, extending from homology → reversals → rudiments → vestiges → atavisms → parallelism, with convergence as the only class of homoplasy, an idea that turns out to be surprisingly old. This realignment provides a glimmer of a way to bridge phylogenetic and developmental approaches to homology and homoplasy, a bridge that should provide a key pillar for evolutionary developmental biology (evo-devo). It will not, and in a practical sense cannot, alter how homoplastic features are identified in phylogenetic analyses. But seeing rudiments, reversals, vestiges, atavisms and parallelism as closer to homology than to homoplasy should guide us toward searching for the common elements underlying the formation of the phenotype (what some have called the deep homology of genetic and/or cellular mechanisms), rather than discussing features in terms of shared or independent evolution.

Two reversal theory-based studies investigated dancers' psychological experience at final rehearsal, and at three performances in an annual competition. In Study 1, 23 Japanese modern dance specialists completed the Tension and Effort Stress Inventory immediately before and after performance. Pre-study expectations that dancers would experience higher levels of unpleasant emotions (including performance anxiety or stage fright) and stress pre-competition than pre-final rehearsal were not confirmed. Dancers experienced significantly higher levels of unpleasant emotions before final rehearsal than before competition. Also contrary to expectations, competing was found not to be more stressful and not to require greater efforts to cope, than rehearsing. Study 2 set out to investigate dancers' basic motives for dancing and to show whether the anxiety-to-excitement reversal phenomena could be discerned during dance performance. Using the same experimental procedure as Study 1, a similar group of 15 dancers completed a dance performance experience questionnaire. The results indicated that, for the majority of dancers, the primary reason for dancing was either the paratelic, telic, arousal-seeking, or alloic-sympathy meta-motivational categories, and that many dancers experienced anxiety to excitement reversals.