EES 227: Paleobiology

Spring 2004

return to syllabus

Lecture 11: March 2

Reading:

Web resources for this lecture:

Lecture Notes: Functional Morphology Topics:

  1. Organism and environment
  2. Form and function: adaptationist views
  3. Theoretical Morphology
  4. Diversity-Disparity

1. Organism and environment:

Before Darwin and Pasteur (mid-19th century): continuity between environment and organism. Examples: life could be generated from non-life; Lamarckian evolution. After Darwin: organism separate from environment. After Mendel, Weisman: inheritance not influenced by environment (see lecture 6). Some scientists argue that the separation organism/environment has been seen as too strict. Search for life on Mars shows how difficult it is to demonstrate presence/absence of life in the absence of knowledge of how it would affect its environment:

  1. Environments do not exist in the absence of organisms, but are constructed by organisms from parts of the external world.
  2. The environment is constantly being remade by organisms: we can not live without changing the environment.
  3. Organisms determine the statistical nature of the environment ('averaging over time'): fluctuations in environments matter only as transformed by the organism
  4. Organisms actually change the basic nature of signals from the external world: physical nature of environment as relevant to organism is made by organism itself. Example: gravity. Not relevant to very small organisms. Brownian motion: not relevant to large organisms.

2. Form and function (design in nature): law of correlation of parts. Organisms do NOT consist of unrelated parts (Cuvier: animals can be reconstructed from few pieces because they predict rest of organism). Some scientists (e.g., Gould, Lewontin) have argued that emphasis has been too much on parts (of whole). Maybe not every part of an organism can be seen as adaptive for something (e.g., why do we have an appendix?); maybe some structure are NOT optimal but inherited from ancestors (e.g., human back bone and walking upright). It has been argued that a strict adaptationist view results not in testable hypothesis, but in 'Just so stories'.

Specific problems with 'strict adaptationist' views:

  • Structural constraints: cells in close packing (insects eyes, honeycomb) tend to get hexagonal because that is an efficient way of close packing. Insects can not grow as large as mammals because of their exoskeletal structure.
  • Heritage and history: organisms do not just exist, they have ancestors and MUST work within the constraints of their heritage: vertebrates have 4 limbs which evolved in water-dwelling ancestors, so these 4 limbs did not evolve 'for' stability in walking on land.
  • Pleiotropy: single genes may have multiple effects. If one effect is very advantageous, the other effects of the same gene will just be there because of that advantageous effect (example: toe and thumb).
  • Neutrality: some features may not be truely features by themselves, e.g.  our chin (remained 'behind' when jaws shrunk); no function, no selection.
  • Things that are not optimal; jerry-rigged. Panda's digestion: gut too short for efficient digestion of plants; need to eat very much (ancestor omnivore-carnivore).
  • Structure and function: not always easy to to see from structure what its function is.
  • Structures may have more than one function.

This discussion is similar to that in which evolution is equated to 'change in gene frequency' by some geneticists, which is seen as too reductionist by others (see lecture 6).

3. Theoretical Morphology: the case of Molluscs, and which theoretical Molluscs do not (maybe can not) exist.

  • The Molluscan shell (bivalve, snail, ammonite) can be mathemathically described in a rather simple way, using the equation for a logarithmic or equiangular spiral, which just 3 variable: expansion rate, distance of opening tube to center of coiling, and translation rate (whether the shells 'moves out of one plane' (ammonites have a translation of 0, snails of >0).
  • Not (by far) all theoretical possibilities occur in nature; e.g., 'open coiled shells' are rare (to non-existent).  This possible 'space' in the three-dimensional figure (of three parameters) where no known organisms 'plot' is called empty morphospace. Probably at last some of this empty morphospace exists because of structural reasons: strength, smooth move through water, etc.. But why only such a small part of theoretical variability existent in nature?
  • Hypothesis of 'adaptive landscape': peaks in a landscape seen as expressing 'optimum' in combination of various allelles of genes. With changes in environment, the 'adaptive landscape' changes, and species find themselves 'off the peaks'. They must adapt (move back to peaks) or become extinct.
  • Organisms could move to these hypothetical peaks through strict adaptationism, but also because of 'genetic drift' (random process through which specific mutations become fixed in genotype).
  • One can see specific parts of morphospace (possibly multidimensional morphospace) as reflecting specific ecological niches. One can then compare various recent and modern assemblages to see whether specific nuches were filled (or not) in fossil ecosystems, and trace differences in ancient and modern ecosystems.

4. Diversity-Disparity Debate

Basic arthropod

  • Construction of a body from repeated segments
  • Most primitive arthropod: all segments identical with double appendage
  • Evolution: pattern of fusion and differentiation of segments
  • Specialization of appendages ('legs')

Arthropod leg:

  • Primitive 'leg': two branches (biramous)
  • One: walk, hand food to mouth,break up food in pieces (jaws), 'feel' objects (antennae)
  • Two: gill, taking up oxygen from water; evolve into wing
  • During evolution leg looses a branch, modified.

Subdivision of Arthropods:

  1. Uniramia: insects, millipedes, centipedes: gill branches lost, breathe through thin tubes in body (trachea).
  2. Chelicerata: spiders, mites, scorpions, horseshoe crabs, extinct eurypterids. Appendages on front of body (jaws, legs) from leg branch; on rear from gill branch.
  3. Crustacea: crabs, lobsters, shrimp, barnacles, copepods, ostracods, isopods (pill bugs). Five pairs of appendages on head; first 2 before mouth, one branch; last 3 behind mouth, used for feeding. Further back: double appendages.
  4. Trilobita: extinct;1 pair appendages before mouth, 3 after; appendages on body double.

So: some combinations of features do not exist in the present world (and that's why we can distinguish the 4 groups in the subdivision). But why is this?

  • 'Gould's explanation': presently, diversity (number of species) may be high, but disparity (number of fundamentally different bodyplans) is relatively low. Other building plans did exist, but became extinct; these bodyplans were thus at least in part 'outside' the present possible morpho-space. Example: Anomalocaris, Opabinia.
  • 'Conway Morris' explanation': presently, diversity is high and disparity is similar to that in the past. The 'unknown body types' are expected to have existed, because they are intermediates between presently existing groups. Example: Halkieria, 'worm' covered with 'scales' (sklerites), but with two shells on back and front: in between 'worm-like organism' and 'brachiopod-like organism'.

Implications of this discussion:

  • According to Gould's ideas, the present biosphere reflects only a very small portion of the large 'disparity' which was present after the Cambrian explosion (see lecture 10). Therefore, large-scale evolution as seen as fully unpredictable: the small part that survived did so mainly because of 'being lucky' during mass extinctions. Mass extinctions kill indiscriminately (see lecture 8), thus do not select for 'better' adaptations. We can not predict whether in any world an intelligent organism would evolve, and if it evolved, from which group it would do so; evolution is almost 'set back to zero' during each mass extinction. Long term evolution patterns are mainly determined by mas extinctions, and they are completely contingent upon random events (such as a meteorite hitting or not).
  • According to Conway-Morris, the present biosphere reflects pretty much the total in morphological diversity present since the Cambrian explosion. We can, indeed, not predict the outcome of evolution precisely (e.g., which organism would become intelligent), but engineering principles direct the overall process, and over time more complex organisms will evolve (e. g., only an organisms of about our size could establish a culture in our world with its physical constraints, i.e., not an organism with external skeleton; arthropods were most diverse in the Cambrian, now and in between). Long term evolution patterns are mainly determined by interaction between organisms and competition, not by indiscriminate kill-off.

My present (2004) opinion: the recent increasing databases on the organisms from the Burgess Shale as well as older faunas (such as the Cheng-Jiang faunas in China, the Sirius Passet fauna in Greenland) buried in a similar setting indicate that Conway Morris' interpretation is more probable, at least as to the understanding of arthropod diversity. It is not clear, however, whether in  long-term  evolution mass extinctions or competition between organisms is the dominant process.

One can test diversity-disparity for specific fossil groups, and check whether diversity of disparity increase at the same time, or whether one increases first. Gould's model requires disparity to increase first (not in agreement with estimates of disparity of various arthropod groups). But HOW can one quantify disparity? (see above discussion). Several researchers have tried to quantify shape (multivariate analysis), but this is not so easy: how does one pick characters to quantify? Very different morphologies may not really reflect total difference in 'building plan', but adaptation to different mode of life.