Mechanisms for Novelties
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© 1997
David H.A. Fitch
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Lecture notes

Mechanisms involved in the origin of novel features

I.  "Ultimate" mechanisms for evolutionary innovation (e.g., selection, constraint):  How does selection work with the developmental genetic machinery to evoke these changes?  Are these changes adaptations; if so, to what?  Are there constraints imposed by the developmental genetic system on possible variation; if so, how?

A.  Selection
1.  If transitions are postulated, each step must have been adaptive, or at least not deleterious (a necessary correlate of natural selection)
2.  Selection can be indicated, for example, by convergences as revealed by phylogenetic analysis

B.  "Constraints" (i.e., limitations on variations before selection results; note that this definition excludes "internal selection" against variants that deleteriously disrupt development)
1.   "Preadaptations" are the ancestral features that were modified in the evolution of an adaptation (anticipation of future need is not implied), and may thus be viewed as "constraints"
2.  Genomic historical contingency limits the field of possible genetic changes
3.  Hierarchical regulatory structures (e.g., genetic correlation) may limit or allow particular kinds of (coordinate) changes
4.  Constraints can be indicated, for example, by correlated character changes as revealed by phylogenetic analysis

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II.  "Proximate" mechanisms for evolutionary innovation (e.g., changes in genes specifying components of developmental pathways):  What kinds of and how many changes are involved (big, epistatic and few, or small, additive and many; the "Macromutationalist" vs. "Neodarwinist" controversy)?  Do major innovations and gross changes to body organization involve mechanisms that are different from those involved in modifications on a smaller scale?

A.  First, we can dispose of some overly simplistic mechanisms
1.  Organismal complexity is not simply a result of increased genome size
2.  Changes in form are not simply due to changes in chromosomal organization

B.  Because developmental pathways are often highly integrated, some developmental mechanisms or intermediate stages may be highly conserved
1.  Integration in this sense means that:
     a.  Particular mechanisms (e.g., signal transduction "modules") may be reused for different purposes at different developmental times in different regions, implying high conservation is required to maintain the integrity of the system
     b.  There may be many mechanisms that converge at a particular stage that is important for continued development (e.g., when gastrulation forms 3 "germ layers" in triploblastic animals)
2.  Integration is indicated by:
     a.  The conservation of "critical stages" (e.g., notochord formation in vertebrates) that may be partly responsible for our ability to recognize homologies
     b.  The existence of ATAVISTIC mutants and experimental results that demonstrate the maintenance of developmental integrity ("cryptic potential") even if they have not been phenotypically expressed for millions of years
3.  Integration and redundancy may allow a form to be retained despite changes in some of the developmental pathways (CANALIZATION)
     a.  Homology is "being the same in the face of change"; B. K. Hall, 1995)
     b.  Mechanistic change without change in form could provide preadaptations for subsequent changes in form (e.g., change from nonautonomous to autonomous determination prior to a heterotopic change)

C.  Hierarchical organization of developmental pathways allows the coordinated evolution of complex forms
1.  If organisms were completely mosaic (e.g., an ultra-additive assumption), a large number of genetic changes would have to occur coordinately (more improbably the more loci are required)
2.  "Mechanical" models suggest that simple changes could produce complex results, assuming simple (e.g., single locus) control of suites of cellular and developmental processes (e.g., D'Arcy Thompson, 1917; Raup, 1962)
3.  Developmental genetics has identified many such genes involved in the control of development
     a.  Patterning along the anteroposterior (AP) axis by "homeobox" genes
     b.  Determination of cell "fate" by "nonautonomous" mechanisms (i.e., signaling)
     c.  Determination of cell "fate" by "autonomous" mechanisms (i.e., cell lineage)
     d.  Differentiation and morphogenesis

D.  Conclusion:
Although integrated developmental systems may be constrained (making parallel and reversal homoplasies more likely), their hierarchical organization makes possible the evolution and diversification of complex systems
Futuyma, 1986:  "Thus, the major problem of macroevolution, the evolution of complex traits that cannot function unless their several parts evolve in concert, may be partly solved by the hierarchical, integrated nature of development."

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III.  Case study:  The evolution of form in the nematode male tail

A.  The male tail model allows the study of morphological evolution at the level of developmental mechanisms and genes
1.  There IS morphological diversity in nematode male tails!
     a.  They vary in positions (and sometimes numbers) of "ray" sensilla
     b.  They vary in the shape of the tail tip
2.  Many aspects of nematode development and cellular anatomy are known
     a.  The entire cell lineage is known for at least 2 species
     b.  The positions and fates for each cell is known for both sexes of Caenorhabditis elegans
3.  Caenorhabditis elegans is a "genetic model organism"; e.g.:
     a.  Short life cycle allows many generations to be followed
     b.  Hermaphrodites allow selfing
     c.  Complete genome is almost sequenced
     d.  Many mutants are kept for many genes at known positions (many of which are "developmental control" genes)
     e.  Caenorhabditis elegans can be "transformed" by injecting hermphrodite parents with exogenous DNA

B.  A phylogeny can be inferred using molecular tools
1.  18S rDNA is phylogenetically informative (not too conserved, not too rapidly changing)
2.  This molecular tool provides an independent measure of phylogeny from the morphology that will be studied

C.  We can propose hypotheses of homology using developmental criteria
1.  A homology hypothesis is required to trace changes
2.  Pattern of cell origins is always the same:  special and complex
3.  Complete conservation of this pattern suggests symplesiomorphy

D.  An "archetype" allows comparisons among species and the identification of characters and character states
1.  Particular cells (or cell clusters that form particular structures, like the "rays") can be designated as characters
2.  Positions or fates of cells can be designated as the states of these characters

E.  By superimposing the male tail character states on the phylogenetic tree, directions of particular changes can be inferred (with the assumptions of parsimony)
1.  Some homoplasious changes correlate with changes in copulatory behavior, implying selection may play a role in determining morphology (not too surprising!)
2.  Some cell fate changes occur together (i.e., are significantly correlated) that have no apparent morphological effect, suggesting coordinate regulation of the fates of those cells and canalization of part of male tail development
3.  Some changes are extraordinarily similar to single-gene mutational changes made in the lab (in Caenorhabditis elegans), suggesting that evolutionary changes could have occurred in these genes or in the same developmental pathway controlled by these genes:
     a.  Certain changes in ray position and identity, as regulated by a "homeobox" gene involved in AP patterning
     b.  Certain losses in ray cell lineages, as regulated by a gene involved in determining neuroblast cell fate
     c.  Certain changes in the morphogenetic process of tail tip development, as regulated by a gene we hope to clone soon

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  1. Design an experimental test of the hypothesis that a reduced function change in a candidate gene was involved as the primary evolutionary step in the loss of a particular ray in the nematode male tail.

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(Sorry, none yet.  But keep tuned!)

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 Kinds of
Novelties  Mechanisms
for Changes
[Kinds of Novelties] [Mechanisms for Changes]

[Origins of Evolutionary Novelties]

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