Evolutionary transitions

In the last 20 years, a new problem, the evolutionary origin of levels of organization, or evolutionary transition, has been added to those celebrated since Darwin (Buss, 1987). In their recent book, The Major Transitions in Evolution, Maynard Smith and Szathmary (1995, p. 6) identified what they considered to be eight major originations of new levels of organization (Table 11.1). For most of these, they claimed a common feature . .. entities that were capable of independent replication before the transition... 

The problem of evolutionary transition is to formulate a coherent theory that can explain these transitions and guide evaluation of empirical evidence for each. Part of this work involves describing units of evolution adequate to explain the evolutionary origin of new levels and not merely evolution at levels (Griesemer, 2000c). The key insight into the units problem afforded by consideration of evolutionary transition is that units of evolution themselves have an evolutionary history. Differently put, there is a temporal or processual dimension to the units problem as well as spatial and functional dimensions. Because the spatial and functional perspectives on units mentioned above were not articulated with the evolutionary transition problem in mind, they are not clearly suited to its theoretical solution. In particular, if a perspective assumes the existence of levels of organization or embeds assumptions about these products of evolution in their analysis of units, then it has assumed what is to be shown by a theory of evolutionary transition. 

In this essay, I argue for a new perspective on units of evolutionary transition. I analyze the process of reproduction, which leads to a conception of units of evolution as reproducers. These units resolve to more familiar ideas of replicators or interactors at levels of spatial organization when explicit spatial and functional models are imposed on abstract reproducers. I also sketch a heuristically promising program of reductionistic research that flows from the new perspective. 

Notice that the common feature of many evolutionary transitions identified by Maynard Smith and Szathmary involves a change in the status of replicators. Once the replication of a class of entities becomes dependent on a larger whole, they do not become independent replicators again. So new replicators at the emergent level must exist and replicate independently in order for there to be a still higher level transition. Given the nature of replicators as analyzed by both Dawkins and Hull, there is a problem with this characterization of the common feature only one or possibly two evolutionary transitions are at all likely to have occurred in the history of life. 

I offer a different and complementary perspective on units which accommodates developmental processes explicitly and which articulates the intimate relationship between units of hereditary transmission and developmental expression. I argue that a process perspective on the temporal dimension of the transition problem, focusing on the propagation of developmental capacities, is a helpful addition to the spatial and functional perspectives. Reproduction is the process that, in general, forms the basis for evolution at a level and also for evolutionary transition to new levels. Processes of inheritance and replication can be understood as special cases of reproduction. In order to formulate a view of how processes of development and hereditary propagation are intertwined in reproduction, let us consider development further. 

I suggest that we take the acquisition of the total set of species-typical traits (however that is to be measured empirically) to be a maximum specification of the process of development. Anything more exhaustive than species-typical might entail that new species could not evolve. As a minimum bound, I suggest the following evolutionary specification - development is the acquisition of the capacity to reproduce. It will become clear in a moment why I bracket the process of development in this way. First, though, let us consider the sorts of questions about development at multiple levels of evolutionary transition that must be addressed if we are to understand units of evolutionary transition in terms of the propagation of developmental capacities. 

Questions of development at multiple levels of evolutionary transition... 

In order to analyze the process of reproduction, two more basic concepts are needed progeneration and development. Development was described briefly above. Here, I will adopt the evolutionary minimum concept of development as the acquisition of the capacity to reproduce. The evolutionary minimum concept is very general, although limited to the context of evolutionary processes. More precise notions of development could be substituted that specify mechanisms by which the capacity to reproduce can be realized, although it remains to be seen whether more precise, non-evolutionary concepts can be general enough to apply to all levels of evolutionary transition. 

Griesemer, J. R. (2000c), The units of evolutionary transition , Selection, 1, 67-80. 

Martin, W. and Russell, M.J. (2003). On the origin of cells a hypothesis for the evolutionary transition from abiotic chemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells. Philos. Trans. R. Soc. London, B 358, 27-85... 

Forterre, P. 2006. The origin of viruses and their possible roles in major evolutionary transitions. Virus Research, 117(1) 5-16. 

The evolutionary transition from RNA to DNA is recapitulated in the biosynthesis of DNA in modem organisms. In all cases, the building blocks used in the synthesis of DNA are synthesized from the corresponding building blocks of RNA by the action of enzymes termed ribonucleotide reductases. These enzymes convert ribonucleotides (a base and phosphate groups linked to a ribose sugar) into deoxyribonucleotides (a base and phosphates linked to deoxyribose sugar). 

Stanley S. M. (1976) Fossil data and the Precambrian-Cambrian evolutionary transition. Am. J. Sci. 276, 56-76. 

See A. Lazcano et al. The evolutionary transition from RNA to DNA in early cells. Journal of Molecular Evolution, 27 (1988), 283-90. 

Szathmdry, E. and Maynard Smith, J. (1995). The major evolutionary transitions. Nature, 374, 227-32. 

Martin W., Russell M.J. (2003) On the origin of cells an hypothesis for the evolutionary transitions from abiotic... 

The evolutionary transition from water to land has resulted in an expansion of the chemoreceptor genes, most likely in response to the multitude of airborne odorants (Bargmann 2006). Organisms that frequently change between aquatic and terrestrial environments (e.g., amphibians) appear to have chemosensory systems for perception of both water-soluble as well as volatile odorants (Freitag et al. 1995). Soluble and volatile chemicals can also be perceived by aquatic and terrestrial crustaceans, respectively (e.g., Hansson et al., Chap. 8). However, at least in terrestrial peracarids, taste reception of odorants appears to be mediated by liquids (Seelinger 1983 Holdich 1984), just as food-smelling of terrestrial mammals under water is mediated by air bubbles (Catania 2006). 

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