Resemblance between animal taxa may be due to convergence rather than to recent common ancestry. Constraints on biological materials and adaptation to particular habits or habitats will produce widespread convergence. How may we distinguish the two causes of resemblance? The relationship between convergence and taxonomy is discussed, demonstrating that the choice of taxonomic method will itself determine the extent to which convergence is perceived. In particular, cladistic analysis based on parsimony will tend to minimise and thus conceal convergence: neither the resulting cladogram nor a consistency index derived from it can be used to assess the prevalence of convergence. With any taxonomic system, there can be no substitute for evaluation of the morphological characters used. Complementary use of molecular characters shows promise: we wait further understanding of constraints in genetic evolution and of the possibilities of convergence at this level also.

These general principles are illustrated with a range of examples from within and between invertebrate phyla: the phylogeny of Cnidaria and Platyhelminthes cannot be traced with certainty, but where the fossil record allows clear rooting, as for the echinoderms and in particular the echinoids, combination of morphological and molecular methods has made much progress. Sub-groups within a phylum, for example opisthobranch molluscs and the dipteran Phoridae, may show an uncontested phylogeny, and here studies have precisely identified convergence and shown that it may be the commoner cause of resemblance. Adaptation to exacting environments shown by terrestial and freshwater nemertines may also result in a predominance of convergent resemblance.

Traditional grouping of phyla breaks down on re-examination of supposedly key characters, such as segmentation, body cavities, germ layers and symmetry, each of which must have had multiple origins: nor are developmental stages (especially not larvae) a reliable guide to relationships. Demarcation of phyla may be difficult, as with arthropods, and location of phyla is even more difficult, due to their early and rapid radiation. Over-simplified definition of characters has bedevilled invertebrate classification and the use of molecular data has not yet resolved the major controversies.

The question ‘How common is convergence?’ remains unanswered and may be unanswerable. Our examples indicate that even the minimum detectable levels of convergence are often high, and we conclude that at all levels convergence has been greatly underestimated.

Serotonergic neurons are present in all phyla that possess nervous systems. In most of these phyla, serotonin modulates important behaviours, including feeding, sexual and aggressive behaviour. Serotonin exerts its effects by acting in three basic modes: as a classical neurotransmitter, as a neuromodulator, or as a neurohormone. In a number of invertebrate species, the neural circuitry underlying the effects of serotonin has been well characterized, whereas in vertebrates, the mechanisms by which serotonin affects behaviour are currently less fully understood. The following review examines the role played by serotonin in the generation and modulation of behaviour in successively more complex species, ranging from coelenterates to humans.

The evolution of dominance by the selection of modifiers of the phenotypes of deleterious mutations was proposed as a hypothesis by R. A. Fisher in 1928. It has been strongly criticized ever since, is regarded by many as having been made irrelevant by metabolic control theory, and most recently has been claimed to have been ‘falsified’ by H. A. Orr. Is it indeed not only obsolete but wrong? Its history is reviewed and its present status evaluated. We conclude (1) that it has a role as the explanation of the dominance found in many cases of selection through visual predation and (2) that the selection mechanism long claimed to be ineffective (the increase in frequency of a single modifier) will be effective under certain special conditions that may be different from those Fisher proposed.

The aim of this review is to summarize newly available information on lemur social systems, to contrast it with the social organization of other primates and to relate it to existing models of primate social evolution. Because of their evolutionary history, the primates of Madagascar constitute a natural experiment in social evolution. During millions of years of isolation, they converged with other primates only in the most fundamental way in the evolution of solitary, pair-living and group-living species, but deviate in several respects within these basic categories of social organization. Solitary lemurs remain poorly studied, but their social organization appears to be broadly similar to that of other solitary primates, even though the unexpected lack of sexual dimorphism may indicate that similar types of social organization can give rise to different mating systems. The determinants of a solitary lifestyle remain elusive. Pair-living lemurs show striking convergences with other monogamous primates in several behavioural traits, but also deviate in that the majority of species are at least partly nocturnal and do not exhibit direct paternal care of dependent young. Group-living lemurs have not evolved single-male groups, male-bonded and multi-level societies, and polyandrous groups may also be lacking. Female philopatry is common, but female bonds are generally weakly developed and eviction of females from natal groups is not unusual. Group-living lemurs also differ from anthropoids in that their groups have even adult sex ratios, smaller average size and may split up on a seasonal basis. Feeding competition, predation risk and reproductive competition can not fully explain these unusual aspects of lemur social organization. It has therefore been suggested that the social consequences of the risk of infanticide and of recent changes in activity may be ultimately responsible for these idiosyncracies of group-living lemurs, an explanation largely supported by the available evidence. Thus, social factors and fundamental life-history traits, in addition to ecological factors, contribute importantly to variation in social systems among lemurs, and possibly other primates. However, neither the diversity of lemur social systems, nor the evolutionary forces and mechanisms operating in these and other primates are yet fully understood.