The first approach, called character mapping, begins by constructing a phylogenetic tree (that is, a depiction of the presumed relationship of a species of interest to its closest living relatives). Phylogeny refers to the evolutionary history of one or a group of interrelated species. Hypotheses regarding phylogenetic relationships often are based on similarities among existing species in morphological traits and DNA sequences. Once the phylogenetic tree is established, character states, or behaviours (such as parental care), of extant species are attached, or “mapped,” to it. Sites on the tree called ancestral nodes are drawn where changes in the behaviour of interest apparently occurs. This is accomplished by minimizing the number of character state transitions, or changes, necessary to account for all the diversity seen among the related species today. In other words, the shortest evolutionary path taken by any character from its origin to the present is considered to be the “most parsimonious” (that is, requiring the fewest changes) and, therefore, the most probable. Assuming that the behaviours of extant species have remained the same since the last speciation event in their lineage and that the shortest evolutionary path is indeed most likely, a hypothesis can be formulated about the relative timing of the origins of various behaviours and their subsequent loss or evolutionary modification. These assumptions are most valid for complex behaviours whose evolution required many improbable changes rather than highly variable (plastic) behaviours. Moreover, it is more reasonable to suppose that a complex behaviour that is shared by two or more species was present in a common ancestor than that it evolved multiple times independently.

Phylogenetic reconstructions and character mapping have been used to infer the historical trajectories of male secondary sexual characteristics and female mating preferences in several taxa, such as Central American frogs (Physalaemus) and swordtail fishes (Xiphophorus). In the frogs, electrophysiological studies of present-day species indicate that females have identical auditory preferences regardless of the acoustic characteristics of the mating calls of the males. The most parsimonious hypothesis, therefore, is that female preferences evolved first (that is, they are ancestral or older), and that male calls evolved secondarily in some species to take advantage of these preexisting preferences. In the swordtail fishes, females in species with and without swords prefer males with artificial swords attached to their caudal fins over unsworded males. The hypothesis that ancestral females possessed the preference for a swordlike structure is more parsimonious than that the preference for swords evolved multiple times independently in the lineage of each existing species.

One general problem with the character mapping approach is that the most parsimonious evolutionary pathway may not be the most likely. Evolutionary change is seldom unidirectional, so small changes in characters in one direction or the other may have occurred multiple times over the evolutionary history of a species group. A more specific problem with inferring the evolutionary history of sexually selected characters using character mapping is that it is often difficult to determine exactly what aspects of a male trait females prefer. With reference to swordtail fishes, it is unclear whether females have specific preferences for a trait (such as the sword) not possessed by the males or whether females are attracted to any tail modifications that are indicative of male viability or fertility in general (such as relatively large, brightly coloured, healthy, and vigorous males). In other words, do swordtail females really prefer sworded males per se or are they attracted to any males capable of growing brightly coloured and exaggerated tails? Recent evidence suggests the latter.

Phylogenetic grading

A second approach to inferring evolutionary history may be referred to as “phylogenetic grading.” The approach involves making detailed comparisons among extant species with respect to a particular type of behaviour and then arraying the various forms of this behaviour from least to most complex. Assuming that complexity increases over evolutionary time, simple or more “primitive” forms of a behaviour are considered ancestral. Species that exist today with a simpler form of the behaviour are not presumed to have experienced the selection pressures that propelled the evolution of more complex forms of the behaviour in other species. For example, Austrian zoologist Karl von Frisch, who decoded the “dance language” of honeybees (Apis), reportedly said:

We cannot believe that the bee dance of the European bees has come from heaven as it is and, since the Indian honeybees and the stingless bees there live in a more primitive social organization, we should expect some phylogenetically primitive stages of the bee dance.

According to this view, stingless bees (Melipona) might not even possess a dance language, since they live in small, less-organized colonies (that is, they are lower on the phylogenetic grade of social complexity than honeybees). Recent studies of stingless bees, however, indicate that successful foragers do in fact communicate distance, direction, height, and smell of food sources to their colony mates. In other words, stingless bees can do everything that the more “advanced” honeybees do—and more, because honeybees do not indicate food-source height. Stingless bees have a communication system that is different from, but certainly not more primitive than, the communication system of honeybees.

The phylogenetic grade approach probably appeals to investigators because of the human tendency to admire the technological advances that have occurred in human societies. So-called advanced species with complex behaviours and social structures, however, are really no better adapted than so-called primitive species, and complexity is no guarantee of long-term success. Many species with complex behaviours are extinct (such as the dinosaurs), and in some extant phylogenetic groups (such as bowerbirds [family Ptilonorhynchidae]) there are species living today whose ancestors probably engaged in much more complex bower-building activities. In other words, living species with simple behaviour patterns are sometimes descendant from ancestral species with more complex behaviours, and vice versa. Consequently, it is inappropriate to view the behaviour of living species as the rungs of a ladder of complexity progressing back to simpler ancestral behaviours. Natural selection does not inexorably build complexity but rather promotes only the complexity necessary at any given time for survival and reproductive success.