Feburary 6. Functional constraints

FUNCTIONAL MORPHOLOGY

• but in addition to morphological changes that result from changes in the rate of growth, many changes involve the creation of new features that are not found in the ontogenetic progression
• a given morphology will become more abundant in a population if it favors increased survival of progeny-this is the essence of natural selection-changes in the behavior or ecology of organisms can often be accompanied by changes in form
• the advent of faster, more convenient computers in the 1960s and 70s led to a wave of theoretical work on morphology-it became possible to generate all potential variations on a particular shape, e.g. a planispirally coiled shell, and then compare the potential shapes to what was actually observed in the fossil record
• Dave Raup did this with planispirally coiled cephalopods-he varied two parameters: 1) the distance of the aperture from the axis (D), which amounts to an index of the amount of overlap between whorls (the progression from involute to evolute) and 2) the expansion rate (W), which measures the rate of increase in the diameter of the living chamber
• the curved line has the equation W = 1/D-shells to the right of the line have successive whorls that are not touching at all-shells to the left of the line have progressively more overlap-the contoured area represents the shapes of 405 genera of Paleozoic and Mesozoic ammonoids-90% of all ammonoids are within the outermost contour-most ammonoids have a W = 2 and D = 0.35 (the center of the bull's eye)
• no ammonoids have open spiral shells-functional reasons: 1) shells with whorls touching are stronger and 2) the dead space between the whorls would create drag for a nektonic organism and waste energy-_Spirula_, the paper nautilus, has an open spiral shell, but the shell is internal, so structural and fluid dynamic caveats do not apply-the chambered nautilus has a higher W value than any ammonoid did, but this is similar to other nautiloids-the reasons for this are not known
• it is tempting to always connect form with function, but this is both theoretical unnecessary and sometimes results in an unprovable 'just-so' story

CAVEATS FOR CONNECTING FORM WITH FUNCTION

1. STRUCTURAL CONSTRAINTS-some features of an organism are not there as a functional response to the environment, but are merely necessary engineering features that exist to hold the organism together
2. EVOLUTIONARY HERITAGE-every new adaptive feature has to be fashioned out of what the organism inherits from its ancestor
3. PLEIOTROPY-single genes have multiple effects- "big toe" exists because thumb responded to natural selection-a big toe is actually a hinderance to bipedalism
4. ZERO SELECTIVE ADVANTAGE-some features are not acted upon by natural selection, but attain their form because features around them change-e.g. the human chin
5. IMPERFECTLY DESIGNED FEATURES-because of reason (2) a species may have to 'jury-rig' a feature from imperfect materials-e.g. the panda's thumb
6. FUNCTION WITHOUT STRUCTURE or ENIGMATIC STRUCTURES-some aspects of an organism's ecology may simply not demand an structural changes or the structural changes may not be divined as adaptations to their true purpose merely by looking at them-e.g. the water ouzel or 'dipper'