Kinds of Novel Changes
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© 1997
David H.A. Fitch
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Describing changes involved in the evolution of novel features

I.  Different levels at which novel features have evolved
(Darwinian evolution requires that all "novel" features arise by modification of pre-existing ancestral features)

A.  Molecular genetic level (see the pages on Molecular Evolution for more information)
1.  There is generally high conservation in basic components and "modules" of biochemical and developmental pathways
     a.  e.g., human bcl-2 can replace ced-3 function in C. elegans
     b.  e.g., Drosophila Antp can replace mab-5 function in C. elegans
2.  Structural changes to proteins or enzymes
3.  Changes in gene regulation (level or pattern)

B.  Morphological level (on which we will focus here)
1.  Even very distinctive features have been modified from ancestral features
     e.g., middle ear bones of eutherian mammals are modified from jaw elements
2.  Differences in "Bauplan"
     a.  Pose the greatest difficulties because:
          (1) Hard to imagine intermediates
          (2) Hard to imagine adaptive significances
          (3) But "hard to imagine" does not preclude their existence
     b.  Data on transitions is forthcoming from:
          (1) Paleontology of Cambrian and Precambrian forms (to reveal possible forms that could be posed as intermediates)
          (2) Developmental genetics (to reveal changes in developmental pathways that may be significant for determining body patterning)
     c.  Are differences in Bauplan "special" differences related to some special event(s) or factor(s) in the Cambrian?  Or could such differences be a result of the same kinds of changes that occurred later (and that resulted in more moderate modifications); i.e., phyletic differences are so great because they represent accumulations of changes over larger periods of time than differences between more recently diverged taxa?
3.  Increases or reductions in size
     a.  e.g., reduction (or loss) of wings in some insects
     b.  e.g., reduction (or fusion) of digits in several vertebrates
4.  Changes in serially homologous organs
     a.  Numbers (e.g., increases in body segments in millipedes and ovules in lilies?)
     b.  Positions (e.g., cervical vs. thoracic vertebrae)
     c.  Differentiation (e.g., diversification of leaves, arthropod appendages)
5.  Evolutionary changes in shape as described by evolutionary changes in the timing or rates of developmental events

II.  Analysis of change in shape:  Isometry, allometry, heterochrony

A.  ISOMETRY
1.  The relationship between the rates of developmental growth along two different "axes" (i.e., different measurements, such as width and weight) can be described as y = bxa, where change along the y axis is related to change along the x axis by some order, a, the "coefficient of allometry" (adumbrated by Darwin's principle of the "Correlation of Growth")
2.  If a = 1, then x and y are isometrically related, and shape does not change with growth

B.  ALLOMETRY
1.  If a does not equal 1, then x and y are allometrically related, and shape will change with growth
2.  Levels at which allometry can be recognized
     a.  Intraspecific
          (1) Between individuals at different developmental stages (ontogeny)
          (2) Between individuals of similar developmental stage (e.g., caste differences)
     b.  Interspecific (probably results from conservation of the allometry)

C.  HETEROCHRONY
1.  Evolutionary change in the timing or rates of developmental events
2.  Two types of results (classically applied to heterochronic effects involving particular features of gross somatic development, often with respect to the timing of reproductive maturation)
     a.  PERAMORPHOSIS
     b.  PAEDOMORPHOSIS
3.  Modes of heterochronic change
     a.  HYPERMORPHOSIS (with RECAPITULATION) vs. PROGENESIS (no change in the coefficient of allometry, but in the extent of growth or timing of reproductive maturation) (if features are isometric, changes in extent of growth or timing of reproductive maturation will not result in shape change, only in size change)
     b.  ACCELERATION vs. RETARDATION (NEOTENY) (allometric coefficient changes, but the timing of reproductive maturation may not change)
     c.  PRE- vs. POSTDISPLACEMENT (no change in allometry, but in onset)

D.  Changes in shape are often heritable and thus of evolutionary significance
     a.  Genetic correlations (e.g., by pleiotropy or linkage disequilibrium) between the different "axes" would allow genetic variation to occur along the allometric curve
     b.  Heritable variation along the allometric curve would be subject to selection
     c.  Heritable variation may also exist with respect to a, the coefficient of allometry, also resulting in heterochronic shape changes (which would also be subject to selection)

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[Kinds of Novelties] [Mechanisms for Changes]

[Origins of Evolutionary Novelties]

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