Heterochrony in Evolution: A Multidisciplinary Approach by Stephen Jay Gould (auth.), Michael L. McKinney (eds.) PDF

By Stephen Jay Gould (auth.), Michael L. McKinney (eds.)

... an grownup poet is just someone in a country of arrested development-in short, a type of moron. simply as we all, in utero, go through a level during which we're tadpoles, ... so we all go through a nation, in our nonage, once we are poets. A formative years of seventeen who's now not a poet is just a donkey: his improvement has been arrested even anterior to that of the tadpole. yet a guy of 50 who nonetheless writes poetry is both an unlucky who hasn't ever built, intellectually, past his adolescents, or a unsleeping buffoon who pretends to be whatever he isn't-something some distance more youthful and juicier than he really is. -H. 1. Mencken, excessive and Ghostly issues, Prejudices: Fourth sequence (1924) the place might evolution be, with out this factor, heterochrony? -M. L. McKinney (1987) one of many joys of operating in a renascent box is that it's truly attainable to take care of with the literature. So it truly is with combined feelings that we heterochronists (even larval types like myself) view the hot "veritable explosion of curiosity in heterochrony" (in Gould's phrases during this volume). at the confident facet, it's ob­ viously useful and fascinating to increase and extend the inquiry; yet one regrets that already we're commencing to speak earlier, lose song of, or even forget about one another as we carve out person interests.

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Millimeters, degrees, counts, character state codes) (Sneath and Sokal, 1973). As with raw data, log-transformed mixed-mode data introduce scale effects that influence subsequent principal components. , 1981). If the number of traits is small, however, this approach may be unfeasible. Moreover, separate analyses of different types of data expand rather than simplify our descriptions of ontogeny. Transformation of the data to z-scores eliminates problems of mixed-mode variables. Variances are standardized by dividing data values by standard deviations (Sokal and Rohlf, 1981).

17 18 18 19 20 20 22 25 25 28 29 32 1. Introduction Heterochrony is evolution via change in timing (and/or rate) of development. However, this oft-repeated definition threatens to dull by repetition the important fact that virtually all evolution involves such changes somewhere in the chain of developmental events. Whether size, shape, or behavior, phylogenetic change almost invariably springs from a change of rate or timing in the ontogeny of descendant individuals. [Since development is a series of highly contingent, interwoven processes, it is much simpler to change the rate or timing of ontogenetic processes rather than accommodate the exponentially cascading effects from changing the processes themselves.

Thus, genes affecting hormonal production (or reception) can affect rate and timing of growth. In Ambystomo a drastic change can be effected by a simple allelic substitution in one gene (Tompkins, 1978). , height in humans, the duration and rate of growth are clearly polygenically controlled for the most part, but can be greatly affected by single-gene mutations affecting growth hormones (Shea, this volume). This is also the case in other animals (Prothero and Sereno, 1982). Further discussion is found in Chapter 16, but the main point is that the degree of rate and time change can vary from mild to extreme, depending perhaps in part on whether the change is under polygenic or single-gene control.

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