Heterochrony Revisited

The recognition that heterochronic processes play a fundamental role in social adaptations is increasingly recognized in birds and mammals (see references in Gariepy et al., 2001; Lawton and Lawton, 1986) but to date changes in developmental timing have not received the attention they deserve in studies of social insect evolution. Hete-rochrony is pervasive in termite evolution, and most aspects of isopteran biology can be examined within that framework (Nalepa and Bandi, 2000). The evolution of the initial stages of termite eusociality from subsocial ancestors described above is predicated on a behavioral het-erochrony, an alteration in the timing of the expression of parental care (Nalepa, 1988b, 1994). Recently, behavioral heterochrony has been recognized as a key mechanism in hymenopteran social evolution as well (Linksvayer and Wade, 2005). Behavioral heterochronies often precede physiological changes, with the latter playing a subsequent supportive role (e.g., Gariepy et al., 2001); behavior changes first, developmental consequences follow. Development in the first termite workers was suspended as a result of the initial behavioral heterochrony in an ancestor, and selection was then free to shape a suite of interrelated juvenile characters, including allogrooming, kin recognition, coprophagy, and aggregation behavior. It has been noted that paedomorphic taxa frequently develop heightened social complexity, because the reduced aggression associated with juvenile appearance and demeanor enhances social interactions (e.g., Lawton and Lawton, 1986). After alloparental care became established in an ancestor, termite evolution escalated as the social environment, rather than the external environment, became the primary source of stimuli in shaping developmental trajectories (Nalepa and Bandi, 2000, Fig. 4). Major events were the rise of the soldier caste, the poly-phyletic onset of an obligately sterile worker caste excluded from the imaginal pathway (Roisin, 1994, 2000), and the loss of gut flagellates at molt, making group living mandatory. The evolution of permanently sterile castes is outside the scope of this chapter. We do, however, note two conditions among extant young cockroaches that provide substructure for the genesis of polyphenism and division of labor. First, the potential for caste evolution would be stronger in an ancestor with a juvenile physiology, because young cockroaches are subject to the most powerful group effects. Social conditions during the early instars of Diploptera punctata, for example, can irreversibly fix future developmental trajectories (Holbrook and Schal, 1998). Second, evidence is increasing that the process of forming aggregations in cockroaches is a self-organized behavior (Deneubourg et al., 2002; Garnier et al., 2005; Jeanson et al., 2005). In eusocial in sects, self-organization has been shaped by natural selection to produce task specialization, and plays a role in building behavior, decision making, synchronization of activities, and trail formation (Page and Mitchell, 1998; Camazine et al., 2001).

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