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11Plant Individuality and Multilevel Selection Theory Ellen Clarke Individuality in plants seems as obscure and ambiguous as in animals it appears clear and simple. —Gray (1849), in White (1979, 113) Gray’s statement may seem an exaggeration to the modern reader. Although philosophers of biology have become accustomed to worrying over whether genes or species are real units of selection, it is generally taken as uncontroversial that organisms, at least, are individuals . Even multilevel selection theorists, who may acknowledge the challenges presented by things such as outlaw genes or eusocial insect colonies, don’t tend to include plants among their list of entities that warrant serious philosophical concern. Yet in the nineteenth century such fears were commonplace among biological thinkers. Even before Charles Darwin was discussing the possibility of group selection, his grandfather Erasmus was discussing some of the peculiarities that can prompt confusion over the status of plants. He considered plant buds to be like babies growing on their parent stem. Many writers then subscribed to the view that plants and trees are not individuals at all, but rather metapopulations , or collections of unit parts. The eruption of green shoots and leaves each spring is not mere growth after a dormant period, but the birth of a new generation. A tree is therefore a family or swarm of individual plants. (Erasmus Darwin 1800, quoted in White 1979, 109) This chapter explores the motivation behind such views and derives some consequences for multilevel selection theory. The first section explores the problem of individuating plants and the suggestion that individuality should be settled using genetic homogeneity. The second section argues that, in some lineages, high somatic mutation rates might actually be favored, and selection processes acting on these mutations could actually be adaptive for the higher-level individual. The third section concludes that genetic heterogeneity, and the intraorganismal selection it can give rise to, does not always undermine a higher level of selection. Individuals can, given certain conditions, have competition among their parts. The lesson I draw is that multilevel selection theorists are wrong to assume genetic heterogeneity necessarily results in evolutionary conflict that must be suppressed in order for 228 Ellen Clarke higher-level individuals to persist as units of selection. Under particular circumstances (circumstances satisfied by many plants) competition at a lower level can be beneficial for a higher-level individual. Plants and Individuality Modularity—The Plant as a Metapopulation All vascular plants, including ferns, conifers, and flowering plants, grow by the accumulation or iteration of smaller constructional units. When a coconut palm grows, it does so by producing a new leaf at its crown. As every new leaf appears, an older leaf below will die and fall away, leaving its stem to contribute to the trunk. You can clearly see cross-sectional marks all along its trunk where these units have been repeated. Other plants iterate more than one unit at once. An oak tree develops by growing new shoot units in a forked or branching pattern. Clonal plants such as bracken or aspen iterate whole plants, by growing them from the ends of underground runners.All such growth patterns can be called modular. Modular growth is open ended and does not progress toward any fixed adult form, in contrast to development in so-called unitary organisms, which is determinate. A modular organism grows by the repetition of some unit or module. These modules are self-reproducing, which is part of what prompted Erasmus Darwin and others to say that a modular organism should be viewed as a collection of individuals—reproductive ability is often thought to be the kind of property that only individuals possess. Yet there has to be more to it than that, because even humans are composed of smaller parts that are capable of reproducing themselves—cells. We will see later that it is very important to be clear on what kind of reproduction an entity must be capable of, if we are to use that as an indicator of individuality. A couple of distinctions need to be made. First, clonal growth of the sort described is vegetative. Another type of cloning is parthenogenesis, which occurs even in vertebrate lineages and in which an organism self-fertilizes one of its gametes. For the purposes of this chapter, cloning by parthenogenesis or selfing is less interesting because it involves a single-celled stage, precluding many of the interesting consequences of multicelled propagation that I will be looking at later. Specifically, reproduction by means of a...

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