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with differential proliferation

Size of exaptive pool High

Generally high

Source: Adapted from Gould (2002, 717-19).

Source: Adapted from Gould (2002, 717-19).

the growth of a child where a debate concerns an alleged distinction between microgrowth and macrogrowth. In monitoring child growth, measurements of height, weight, and so forth are measured every month or year, and significant developmental events (for examples, signs of puberty) noted. These are descriptors of macrogrowth and do not say anything about microgrowth that occurs on an hourly or daily basis. Plainly, as Dawkins states, macrogrowth is the sum of lots of little episodes of microrgrowth. However, does this reasoning apply to macroevolution? Perhaps the coarse temporal resolution of the fossil record deludes palaeontologists in to thinking that macro changes sometimes occur without intervening micro changes. That is Dawkins's point. However, this argument applies only if the development of an individual is a strict parallel for the evolution of a species. There is a danger of confusing ontogeny with phylogeny, to both of which the term evolution is applicable but in different senses (cf. Mayr 1970). Evolution can mean the unfolding, or growth and development, of an individual organism; this process of ontogeny involves homeorhe-sis (Waddington 1957). Homeorhesis is a set of processes leading to the development of an individual organism, from egg to adult. It operates in conjunction with homeostasis, the processes that maintain an individual in a steady state. In a grander sense, evolution means phylogenetic evolution - the derivation of all life forms from a single common ancestor. Arguably, there is a crucial difference between these two ideas. Development (homeorhesis) produces a new organism that is almost identical to its progenitors (or identical in the case of asexual reproduction). Phylogenetic evolution creates organisms that have never before existed, and that may be more complex than their progenitors. A process of com-plexification occurs in both cases. With development, complexification leads to a familiar, pre-existing organism. With phylogenetic evolution, complexification leads to a novel organism, occasionally at some higher level of organization. This reasoning cast doubt over the idea of macroevolution as an accumulation of microevolution. Nonetheless, recent work suggests that it may be possible to explain microevolution and macroevolution using a single model (see p. 107).

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