Examples of Adaptations
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© 1997
David H.A. Fitch
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Lecture notes

Adaptation:  Some examples demonstrating imperfection (and thus historicity)

These examples are taken from Richard Dawkins' The Blind Watchmaker, which is, of course, required reading, and from George C. Williams' Natural Selection:  Domains. Levels and Challenges, 1992 (Oxford Univ. Press, New York; ISBN 0-19-506932-3), which is optional reading, on reserve in Bobst Library.

I.  The mammalian male urogenital system

A.  During the evolution of mammals, the testes moved from the originally internal position to a specialized structure outside the body, the scrotum
1.  The migration of the testes also occurs developmentally
2.  Although the sperm duct passes ventral to the ureter, the testis migrates dorsal to the ureter, requiring a lengthening, not a shortening, of the sperm duct
3.  A more efficient design would be a migration of the testis on the same side of the ureter as the sperm duct.

B.  These features imply that testis migration was processive, not designed in a single creative step
1.  Once a pathway of successive adaptations begins, reversals may be competitively disadvantageous (e.g., to make this feature more efficient, the testis would have to move inward, back around the ureter; but sperm would not survive under these conditions)
2.  Ancestry apparently constrains the range of variation available to natural selection (e.g., because of the accumulation of successive, ancestral steps toward the current state, reversals are highly improbable = "Dollo's law")

II.  The vertebrate eye

A.  The retina of the vertebrate eye (in contrast to some other eyes, such as those of some molluscs) is "inside out" relative to an efficient design
1.  The vascular tree shadows the photoreceptor cells
2.  Because it is on the "wrong side", the optic nerve has to go through a hole in the retina, resulting in a blind spot

B.  These features imply that vertebrate retina evolution was processive, not designed in a single creative step
1.  From its origin in a flat, light-sensitive layer in the dorsal side of the anterior nerve system, development of the vertebrate nerve system (making a tube from a planar surface) resulted in the inverted orientation of the future retina
2.  This retinal "inversion" is not a result of some underappreciated function, because some independently evolved eyes have "noninverted" retinas.
3.  Once a pathway of successive adaptations begins, reversals may be competitively disadvantageous (e.g., to completely reorganize the vertebrate retina, a redesign of neural tube development might be required (depending on the complexity of the developmental pathways involved, which is likely to be substantial), likely to be too disruptive to other vertebrate features)
4.  Again, ancestry apparently constrains the range of variation available to natural selection

III.  These examples demonstrate the processive nature of selection in the evolution of adaptations and that each step must be advantageous

A.  Hypothetical ancestral intermediates to eyes could have been advantageous

B.  Extant "intermediates" are good adaptations

C.  The scenarios are easier to envision if each step involves slight modifications that are compounded over vast stretches of time

D.  They are especially well explained if each step in the adaptive sequence bestows a competitive advantage

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Exercises

  1. How do "imperfections" (with respect to a priori  engineering and design principles) of adaptations suggest that natural selection (with descent) was responsible for producing such adaptations?  Why would conformity with a priori design principles not provide good support for either creative design nor natural selection?
     
  2. Which is more important:  that an adaptation perform its function really well, or that an adaptation perform slightly better than another variation of that adaptation occurring in the same population?  Why?
     
  3. What good is "half an eye" or "half a wing", or even a small fraction of an eye?

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 Examples of
Adaptations  Defining
Adaptation  Levels of
Selection  Optimal
Models  Tradeoffs  Sexual
Selection
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[Examples of Adaptations] [Defining Adaptation] [Levels of Selection] [Optimal Models] [Tradeoffs] [Sexual Selection]

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