Homology and Adaptive Modification of Vertebrate Limbs For by mwv14394


									                     Homology and Adaptive Modification of Vertebrate Limbs
                                 For November 24 discussion

I. Homologies in the Forelimbs of Vertebrates
The forelimbs of the four-legged (tetrapod) vertebrates perform an astonishing variety of functions,
including digging (moles, armadillos, anteaters), flight (birds, bats, pterosaurs), sustained running
(horses), and swimming (penguins, whales), among others. Despite this variety of functions, the
forelimbs of these animals are remarkably similar in their basic skeletal components. This
similarity of structure provides strong evidence of common evolutionary history, and is thus
considered to be a clear example of homology.

Homology refers to the possession by two or more species of a trait derived, with or without
modification, from a common evolutionary ancestor. Homology can be contrasted with Analogy,
the possession by two or more species of traits that are superficially similar (convergent) but not
derived from a common
evolutionary ancestor. Thus, a
trait found in both species A
and species B is homologous if
the trait is also present in A
and B's most recent common
ancestor, and in all
intervening ancestors.
However, a trait found in both
species A and species B is
analogous if that trait is absent
in either A and B's most recent
common ancestor or any
intervening ancestor.

For example, the wings of
birds and bats are analogous.
The most recent common
ancestor of these two groups
(primitive reptiles) were non-
winged. However, the principal
bones that make up the wings
(humerus, ulna, radius, etc.) are
the same in both birds and bats.
What's more, these same bony
elements were also found in the
forelimbs of primitive reptiles,
and in fact were derived from
the bones of the fleshy fins of
Crossopterygians, the lobe-
finned fishes (see below). Thus,
the bones that make up the
wings of birds and bats are
Biology 221 -- Vertebrate Limbs                                                                     2

       crossopterygian (diagram)
       labyrinthodont (diagram)
       mudpuppy (salamander) skeleton
       frog skeleton
       turtle skeleton
       bird skeleton (chicken or pigeon)
       cat skeleton
       human skeleton

1. Examine the diagram of the labyrinthodont forelimb. Labyrinthodonts were primitive
amphibians that lived during the Permian period, about 250 million years ago. Identify the basic
components of the Labyrinthodont forelimb: humerus, radius, ulna, carpals, metacarpals, and
phalanges. Now compare its forelimb the forelimb of its ancestor, one of the lobe-finned fishes
(crossopterygians) of the Devonian Period. What homologous structures can you identify?

2. Crossopterygians flourished during the Devonian period, about 400 million years ago. In
addition to having fleshy (and bony) lobe-fins, they also had nostrils and at least rudimentary
lungs. Geological evidence suggests that Devonian seas were often shallow and dried up
periodically in many areas. Do you think that these structures would have been favored by natural
selection during the Devonian? Why?
Biology 221 -- Vertebrate Limbs                                                                         3

3. Compare the forelimb structure for each of the organisms listed in the chart below with that of a
primitive tetrapod (labyrinthodont). Which forelimb features appear to be ancestral (i.e., similar to
the forelimbs of labyrinthodonts) and which features are derived (i.e., modified from the ancestral
condition)? Examples of derived characters would be: loss of one or more digits, fusion of bones
that are separate in labyrinthodonts, elongation or other changes in bone shape, etc. List all the
derived characters that you can identify.

Organism                                         Derived Characters






Biology 221 -- Vertebrate Limbs   4
Biology 221 -- Vertebrate Limbs                                                                        5

II. Adaptive Modification of Vertebrate Limbs
Although tetrapod limbs display a great deal of homology among both contemporary and fossil
species, it is also clear that they have been modified by natural selection in response to
environmental challenges imposed by certain habitats and ways of life. Here, you will identify
specific morphological modifications of the vertebrate forelimb that appear to be adaptations to
particular ways of life. As a frame of reference, it may be helpful to recall the structure of an
ancestral unspecialized forelimb, such as that of the labyrinthodont amphibians (see above).

       Forelimbs for Digging (diagrams, plus mole skeleton)
       Forelimbs for Flying (diagrams)
       Forelimbs for Swimming (diagrams)
       Forelimbs for Sustained Running (diagrams)

Examine the diagrams of forelimbs (or hindlimbs) for each of the representatives of the different
ways of life. Identify as many modifications as you can that you think are related to the
specialized function of the limb. Pay particular attention to relative sizes of the component bones;
shapes of bones; articulation of joints; potential sites for muscle attachment; extra bumps and
protuberances; reduction in parts. Also note where different modifications serve the same overall

Function       Organism              Limb modifications





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