FROM INDIVIDUAL ONTOGENY TO SYMBIOSIS: A HIERARCHY OF COMPLEXITY
Biological complexity is displayed at many hierarchical levels, from molecular and cellular operations within an organism to species’ interactions in ecological communities. At any level, biological entities are enmeshed in interactive networks that typically involve potential conflicts as well as collaborations. For example, a multicellular organism can be viewed as a social collective of cells whose genes must not only collaborate to generate a viable individual but also compete for inclusion in gametes that will form the next generation. Chapters in Part III deal with some of the complex interactions that characterize biological systems at the levels of ontogeny, multicellularity, eusociality, and symbiosis.
During ontogeny, suites of genes (and the RNA and protein molecules they encode) direct the molecular dances of development that produce a functional multicellular organism. The ontogenetic choreographies themselves evolve, as evidenced by the great diversity of body plans and other phenotypes in different organismal lineages. What kinds of genetic mechanisms underlie ontogenetic shifts and the emergence of novel morphologies? Most researchers suspect that evolutionary changes in gene regulation are especially important, and that such alterations often involve the cooption of preexisting genes and proteins into new functions. In Chapter 6, Benjamin Prud’homme, Nicolas Gompel, and Sean Carroll illustrate how such cooptions can occur via shifts in the deployment of cis-regulatory elements and their associated transcription factors. They argue that this specific kind of architectural change in regulatory networks
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
among microorganisms, and between microorganisms and their multicellular hosts, as important (and previously underappreciated) evolutionary sources of phenotypic novelty. Using compelling examples from insects and other organisms, Moran shows how obligate symbiosis can yield complex evolutionary outcomes, ranging from the emergence of specialized cell types and organs to various developmental mechanisms that regulate the intergenerational continuance of the symbiotic association.