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offers a key to understanding how morphological evolution is linked to molecular ontogenetic processes.

Multicellularity itself is a complex trait, yet the phenomenon has arisen independently on numerous occasions. Each evolutionary transition from unicellularity to multicellularity likely proceeds through a succession of stages: initial aggregation of cells, increased cooperation within the group, the evolution of policing mechanisms against cheater cells, increases in group size, and the spatial and functional specialization of cell types. The process is remarkable because it entails, in effect, the emergence of reproductive altruism, wherein most cells forego personal reproduction in favor of working on the colony’s behalf, a situation that undoubtedly necessitates high within-colony kinship (Maynard Smith and Szathmáry, 1995). In Chapter 7, Rick Michod discusses these topics with special reference to living volvocine green algae, which collectively display several stages along the unicellularity to multicellularity continuum. Michod contends that multicellularity is not irreducibly complex in an evolutionary sense, but rather can be understood in terms of evolutionary trade-offs and fitness advantages that can attend various intermediate stages in the evolutionary transitions between one kind of individual and another.

Eusociality is perhaps the epitome of complex social behavior and apparent reproductive selflessness. In eusocial colonies, such as those of many hymenopteran insects, individuals show striking reproductive divisions of labor, with sterile workers striving to maintain and defend a colony whose offspring are produced by the reproductive elites. Eusociality has long intrigued biologists. A key insight came from Hamilton (1964a,b) who proposed that the evolution of extreme reproductive altruism by workers was facilitated by the altered genetic relationships among various colony members stemming from haplodiploid sex determination. In Chapter 8, Joan Strassmann and David Queller review current thought about the evolution of eusociality, including the important point that kin selection predicts a degree of cross-purpose and conflict (as well as extensive cooperation and common purpose) in eusocial insect colonies. They conclude that kin-selection theory, by making specific testable predictions about behavioral phenomena in eusocial colonies, nicely exemplifies the power of scientific explanation for complex biological phenomena.

Genomic evolution was traditionally thought to proceed independently in different lineages, but a growing body of literature has revealed numerous exceptions. For example, horizontal gene transfer events have proved to be rather common in various prokaryotic groups, sometimes affording the recipient with novel metabolic capabilities. Another evolutionary route by which lineages may acquire functional innovations involves the establishment of stable (and sometimes heritable) symbiotic associations. In Chapter 9, Nancy Moran interprets various symbioses



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