Tempo and Mode in Evolution Genetics and Paleontology 50 Years After Simpson (1995) / Chapter Skim
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Morphological Evolution Through Complex Domains of Fitness
Morphological Evolution Through Complex Domains of Fitness
Pages 145-166
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From page 145... ...
, conceives of evolution as a "local search" for "adaptive peaks" by progressively fitter mutants. This image of a walk over a fitness-landscape forcefully draws attention to the relation between the number and location of fitness peaks, on the one hand, and the number and magnitude of phenotypic transformations among neighboring variants required to increase fitness, on the other (Kauffman, 1993; Maynard Smith, 1970~.
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From page 146... ...
Nonetheless, the image of the fitness-landscape continues to inspire questions about the tempo and mode of evolution for example, what is the relation between the number of fitness peaks and the number of functional tasks that an organism must simultaneously perform to grow, survive, and reproduce? Although there is no a priori reason to assume that the number and location of phenotypic optima depend upon the number of tasks an organism must perform, there are good reasons to believe that manifold functional obligations author "course-grained" landscapes with many phenotypic optima.
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From page 147... ...
Indeed, one is left with the impression that the walks of plants, in general, and those of early tracheophytes, in particular, feature phenotypic transformations sufficient to achieve many, perhaps most, of the morphological optima widely scattered over their fitness-landscapes. An added advantage to dealing with plants is that their fitness calibrates closely with the operation of physical laws and processes governing the exchange of mass and energy between the plant body and the external environment, which have remained constant over evolutionary time (Gates, 1965; Brent, 1973; Alberch, 1980, 1981, 1989; Oster et al., 1980; Gill et al., 1981; Odell et al., 1981; Nobel, 1983; Sultan, 1987; Van Tienderen, 1990; Niklas, 1992; Schmid, 1993~.
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From page 149... ...
Simulations of this sort are brought to closure when each phenotypic maximum or optimum within the morphospace is reached by a walk, after which the number and magnitude of the phenotypic transformations in a walk, as well as the number of phenotypic maxima or optima within different fitness-landscapes, are computed and compared. Clearly, to be useful, this heuristic protocol requires nonarbitrary definitions for "morphology," "function," and "ancestor." It also must be cast in terms of a real evolutionary episode against which simulated walks and predicted phenotypic maxima or optima can be compared and contrasted with the actual morphological trends established by the fossil record.
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From page 150... ...
Although this makes the graphic display of simulated walks somewhat difficult, the situation is greatly simplified by initiating simulations of walks in the isometric domain of the morphospace (i.e., P
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From page 151... ...
(C) A walk through a light interception landscape.
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From page 152... ...
In terms of mechanical stability, the maximum bending stresses Max that develop in a cylindrical plant axis may be computed from the formula 32M Max = + 7Td3 ' (2a) where M is the bending moment, which has units of force times length, and + and - denote tensile and compressive stresses, respectively (Niklas, 1992~.
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From page 153... ...
Assuming that each of these three functional obligations contributes equally and independently to fitness, the most parsimonious mathematical expression for the total fitness F of a phenotype is the geometric mean of E and R divided by M- i.e., F = f(E)
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From page 154... ...
Each walk is brought to closure when it obtains all the phenotypic maxima within a single-function landscape or all the phenotypic optima within multi-function landscape. The mean diameter D and the SE of D for all the steps in a walk quantify the mean variation in the phenotypic transformations required to achieve all maxima or optima within a particular fitness-landscape.
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From page 155... ...
(D) A walk through a light interception, mechanical stability, and reproductive success landscape (see Table 1~.
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From page 156... ...
N n D (+ SE) VF Single-task walks Light interception, E 116 3 5.81 + 0.152 0.568 Mechanical stability, M 81 3 5.49 + 0.186 0.335 Reproductive success, R 56 1 4.37 + 0.130 0.117 Y + SE 84.3 + 17.4 2.33 + 1.15 5.22 + 0.44 0.34 + 0.13 Multi-task walks E-M-R 49 7 14.4 + 0.58 3.65 E-R 41 5 15.1 + 1.02 3.53 E-M 40 5 21.6 + 1.26 10.1 M-R 13 1 13.7 + 1.63 0.84 Y + SE 35.8 + 7.85 4.50 + 1.26 16.2 + 1.82 4.52 + 2.26 AN, number of steps in walk; n, number of phenotypic maxima or optima in landscape; D, mean diameter of steps in walk; VF, volume fraction of morphospace occupied by walk.
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From page 157... ...
differs from one landscape to another because fitness is defined in different terms in each landscape. Because phenotypic maxima and optima occupy fitness peaks, their absolute fitnesses define a landscape's elevation and, therefore, the gradient of the phenotypic transformations attending a walk.
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From page 158... ...
Thus, the fitness of phenotypic optima apparently falls closer to the mean fitness of all the phenotypes within a landscape as the functional complexity of the phenotypes under selection increases. As noted, this result is consistent with engineering theory that indicates that the performance levels of artifacts designed to perform individual functional tasks are higher than those of artifacts designed to simultaneously perform two or more of the same tasks.
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From page 159... ...
Second, although only three were considered, the functional obligations elected to define and quantify fitness are biologically realistic for the majority of past and present terrestrial plant species. Third, although the environment, living and nonliving, is in constant flux, the particular episode of plant evolution Focused upon here most likely was dominated by the operation of Physical laws and processes.
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From page 160... ...
Second, if the hypothetical relations between fitness topologies and the dynamics of walks forecast by computer simulations have any relevancy, then they must, at the very least, mimic trends evinced in the fossil record of plants. There can be little doubt that the phenotypic optima reached by simulated walks, particularly those over the tripartite fitness-landscape, are morphologically complex (Figure 4D)
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From page 161... ...
In very broad terms, the appearance of ancient tracheophyte lineages evincing parallel or convergent phenotypic evolution is a feature mimicked by simulated walks. In all but two fitness-landscapes, walks repeatedly branched to obtain many phenotypic maxima or optima, most of which have a tree-like appearance (i.e., unequally branched with a main vertical "stem")
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From page 162... ...
However tantalizing the similarities between simulated walks and the broad morphological trends seen in early land-plant evolution, they cannot be taken as prima facie evidence that the mathematical and statistical properties of simulated walks reflect reality, nor can they be taken as evidence that the early evolution of vascular plant shape was governed by the biological obligations to intercept sunlight, remain mechanically stable, or to disperse large numbers of spores over great distances. However important these biological tasks may be to plant growth, survival, and reproductive success, the correspondence between simulated and empirically determined morphological trends for early tracheophytes may be simply fortuitous.
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From page 163... ...
The results of these simulations, whose credibility is discussed in the context of early vascular land-plant evolution, suggest that both the number and the accessibility of phenotypic optima increase as the number of functional obligations contributing to total fitness increases (i.e., as the complexity of optimal phenotypes in
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From page 164... ...
(1975) Early vascular land plants: proof and conjecture.
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From page 165... ...
(1981) Diversity and major events in the evolution of land plants.
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