Lecture 20 Optimality and Symmorphosis

Lecture 20 Optimality and Symmorphosis

Four General Approaches to Studying Evolution: • Comparisons of Species (or populations) = "The Comparative Method“Shows what has happened in past evolution • Biology of Natural PopulationsShows present evolution in action • 3. Selection ExperimentsShows, experimentally, what might happen during future evolution • 4. Comparison of Real Organisms with Predictions of Theoretical ModelsShows how close selection can get to producing optimal solutions

Some Examples of Optimality Models (formal or informal) Applied to Biology:

Bird Wing Shapes: Lots of interest, lots of data (including informal observations), lots of models due to out interest in human flying machines. Nice resource:http://people.eku.edu/ritchisong/554notes3.html http://i.bnet.com/blogs/bird-plane.jpg http://www2.unil.ch/biomapper/opengl/Bird_wing_types.gif

Bird Wing Shapes: • As many as 7 Basic Wing Shapes and Arrangements have been recognized, and they seem to represent evolutionary adaptations for different flying styles: • short, broad, cupped wings for rapid takeoff and short-distance flight; • shorter and broader wings with slotted primary feathers for soaring flight; • flat moderately long, narrow, triangular wings for high-speed flight; • large, distinctly arched wings for flapping flight; • long, narrow, flat pointed wings for gliding flight; • pointed, swept-back wings for hovering or motionless flight • http://www.paulnoll.com/Oregon/Birds/flight-shape.html

Four Common Wing Shapes in Birds Active Soaring (Calif. Gull) Passive Soaring (Bald Eagle) Elliptical - good forshort bursts of high speed (Common Raven) High-speed (Forster's Tern) http://www.birds.cornell.edu/education/kids/books/wingshapes

Optimality Models (formal or informal) can be useful in biology, but in general …

Organisms are not optimal!

Why Organisms are not Optimal: 1. Organisms are not "designed;" natural selection is not engineering 2. Biological materials have limitations 3. Energetic efficiency is not necessarily what selection maximizes 4. Environments often change too rapidly 5. Selection cannot anticipate 6. Genetic drift operates in all populations 7. Behavior evolves too rapidly 8. Sexual selection counters natural selection

Although engineers have final goals and purposes, natural selection does not. Moreover, engineers often design for a single purpose, whereas organisms must do many things, not just one. Jared Diamond has used the analogy of elevators to consider what he likes to call "safety factors." You can show that freight elevators have lower safety factors -- smaller cables -- than do passenger elevators.

Safety factors of some engineered structures Cable of fast passenger elevator 11.9 Cable of slow passenger elevator 7.6 Cable of slow freight elevator 6.7 Wooden building 6.0 Cable of powered dumbwaiter 4.8 Steel building or bridge 2.0 Diamond, J. M. 1993. Evolutionary physiology. Pages 89-111 in C. A. R. Boyd and D. Noble, eds. The logic of life: the challenge of integrative physiology. Oxford University Press, Oxford.

This analogy is spurious because although elevators may only carry one thing, such as freight, and be designed for that purpose alone, organisms are different. Human "elevators" not only carry freight, they also carry people (including during pregnancy). In addition, they feed themselves, grow, repair themselves, avoid becoming food, mate, and raise offspring.

What if functional demands on a given structure conflict? Can it be "optimally" designed in any meaningful sense? Think about skin … Should skin be designed optimally for gas exchange, temperature regulation, regulation of vitamin D, exteroception, osmoregulation, crypsis, as a barrier to infection, protection from physical assaults, or rapid healing from wounds? Lindstedt, S. L., and J. H. Jones. 1987. Symmorphosis: The concept of optimal design. Pages 289-309 in M. E. Feder, A. F. Bennett,W. W. Burggren, and R. B. Huey, eds. New directions in ecological physiology. Cambridge Univ. Press, New York.

Think about the lung … Should the lung be designed optimally for uptake of O2, elimination of CO2, temperature regulation, vocalization, resistance to infection, resistance to particulates, or rapid healing?

Another way to view excess capacities: "Why should phenotypes be overdesigned? Statements that such overdesign represents a 'factor of safety' (cf. Kummer, 1959) hardly explain its origin. In any case they implytechnological planningor prescience and shouldprobably be discouraged." (Gans, 1979, p. 227) Gans, C. 1979. Momentarily excessive construction as the basis for protoadaptation. Evolution 33:227-233.

"... for most individual organisms the structural and physiological capacities are likely to be excessive for the needs of any particular moment. Obviously, natural selection does not 'look' just at an organism's momentary utilization of each aspect of the phenotype, but at the requirements imposed on all phenotypic aspects of an individual throughout its life span."(Gans, 1979, p. 227)

Although engineers can start from scratch, natural selection cannot. Selection works with pre-existing materials, whatever a species happened to inherit from its ancestors. And these may not be the best possible materials for a particular function. Thus, relatively severe "constraints" are placed on living systems. If an engineer wants to build a shell out of titanium, to maximize strength while minimizing mass, they can.

But selection cannot do that with, say, tortoises, because a mutation conferring the ability to produce titanium apparently has never occurred during the course of life on this planet. Similarly, as compared with natural materials, engineers can make artificial joints and heart valves that last far longer than tissues and are not susceptible to disease.

Often for simplicity, most optimality models are phrased in terms of energy as the common currency, and the "goal" of selection is seen as minimizing energy costs, maximizing energy gain or maximizing energetic efficiency (gain/cost). But we have little empirical evidence that this is what selection actually tends to do.

Moreover, what matters in evolutionary models is relative fitness -- how good one individual is as compared with others in its population -- not absolute fitness measured against some external scale. To escape from a predator, you only have to run faster than the guy next to you … Thus, selection generally leads to adequacy or sufficiency, not necessarily optimality.

"In spite of occasional statements to the contrary, there can be little argument that natural selection is unlikely to be a mechanism for generating perfection in individual animals. ... that the structure of an animal allows [it] to perform particular actions, highly advantageous under a particular set of circumstances, does not require perfect matching, but only adequacy ..."(Gans, 1983, pp. 101-102) Gans, C. 1983. On the fallacy of perfection. Pages 101-112 in R. R. Fay and G. Gourevitch, eds. Perspectives on modern auditory research. Amphora Press, Groton, Connecticut.

Selection cannot change organisms very rapidly, for 3 reasons: 1. if selection is too strong, then population size will be reduced such that extinction by demographic stochasticity is likely; 2. the narrow-sense heritability of traits is usually far less than 1.00; 3. genetic correlations with other traits, e.g., caused by pleiotropy, will "constrain" the organismal response to selection. Quantitative genetics clearly indicates that evolution often will fail to keep pace with the rate of environmental change.

Even if selection could generally track typical rates of environmental change,it cannot anticipate major environmental changes, such as asteroids hitting the earth, severe droughts, "100-year floods" or even the invasion of a population by some new pathogenic organism, such as HIV (which causes AIDS) in the human population. Thus, what is "optimal" now may soon not be. Today's adaptation is tomorrow's constraint: specialization is often an evolutionary dead end.

Random genetic drift operates in all populations that are of less than infinite size -- which means all populations. Both theoretical and empirical studies suggest that random drift is often strong enough to counter selection. Thus, drift alone should ensure that average values for populations or species are rarely if ever at the optimum dictated by selection.

You can think of the interplay between selection and drift with the following analogy. Imagine the population's mean phenotype as a billiard ball on a pool table with a lumpy surface. Selection is a pool cue that aims the population's mean phenotype at a particular pocket, but drift is the lumpy surface which makes it go off course. But, if the cue hits the ball with enough force, it may get there anyway.

Genetic drift is a key element of Sewall Wright's shifting balance theory of evolution, in which drift is argued to often push populations temporarily in the direction of lower fitness. Another analogy is an unbalanced bowling ball. The bowler may throw it straight down the alley towards the pins, but the unbalance, as genetic drift, will often make it not go straight to the center pin.

Under the action of natural selection alone, populations often might get stuck on local optima (never make it to the highest peak). Drift can allow them to "explore" valleys on the adaptive landscape. http://evolutionarysystemsbiology.org/intro/index.html

http://www.sdbonline.org/fly/lewheldquirk/1.2.jpg

Natural& Sexual Selection Act On Behavior Constrain Morphology, Physiology, Biochemistry Deter- mine Organismal Performance Abilities

Cinclus mexicanus The Dipper: example of an organism in which a behavior, diving to forage, seems to have evolved more rapidly than underlying morphological and physological traits that might enhance the ability to dive. Figure 3 (page 257) from D. J. Futuyma. 1986. Evolutionary biology. 2nd. Ed. Sinauer Associates, Sunderland, Massachusetts.

"In the case of the water ouzel, the acutest observer by examining its dead body would never have suspected its subaquatic habits. ... In such cases, and many others could be given, habits have changed without a corresponding change of structure." Charles Darwin, in The Origin of Species, Ch. 6

… hard to argue that the dipper is optimally designed for its current behavior. Maybe a penguin is close to optimally adapted for underwater foraging (at the cost of greatly reduced abilities for terrestrial locomotion), but even penguins still reproduce and lays eggs on land, and have not reevolved gills.

Quotes about behavior as an evolutionary pacemaker: Ernst Mayr & others (1904-2005)

"A shift into a new adaptive zone is, almost without exception, initiated by a change in behavior … other adaptations to the new niche, particularly the structural ones, are acquired secondarily (Mayr 1958, 1960). With habitat and food selection - behavioral phenomena - playing a major role in the shift into new adaptive zones,

the importance of behavior ... is self-evident. ... Most shifts into new ecological niches are, at first, unaccompanied by structural modifications (Robson and Richards 1936)."(Mayr, 1963, p. 604) Ernst Mayr in 1994, after receiving an honorary degree at the University of Konstanz. Mayr, E. 1963. Animal species and evolution. The Belknap Press of Harvard Univ. Press, Cambridge, Mass. 797 pp.

"Many if not most acquisitions of new structures in the course of evolution can be ascribed to selection forces exerted by newly acquired behaviors (Mayr, 1960). Behavior, thus, plays an important role as the pacemaker of evolutionary change. Most adaptive radiations were apparently caused by behavioral shifts." (Mayr, 1982, p. 612) Mayr, E. 1982. Systematics and the origin of species. Columbia Univ. Press, New York. 334 pp. Columbia Classics in Evolution Series.

"... when a major environmental shift occurs, natural selection should initially favor compensatory changes in behavior ..." (Huey and Bennett, 1987, p. 1098) "These results indicate that rather striking differences in ecology and behavior may be accompanied by modest differences in physiology." (Taigen and Pough, 1985, p. 991) "Indeed, behavior is in the vanguard of evolution."(Plomin, 1990, p. 183) "It has often been said that behavior is one of the most labile traits in animal evolution. Whether this is so remains to be demonstrated..." (Bush, 1986, p. 1) Bush, G. L. 1986. Evolutionary behavior genetics. Pages 1-5 in M. D. Huettel, ed. Evolutionary genetics of invertebrate behavior, progress and prospects. Plenum Press, New York. 335 pages. Huey, R. B., and A. F. Bennett. 1987. Phylogenetic studies of coadaptation: preferred temperatures versus optimal performance temperatures of lizards. Evolution 41:1098-1115. Plomin, R. 1990. The role of inheritance in behavior. Science 248:183-188. Taigen, T. L., and F. H. Pough. 1985. Metabolic correlates of anuran behavior. American Zoologist 25:987-997.

Extinct"IrishElk" Sexual selection by male-male competition for access to females, or by female choice of particular males, can cause the evolution of bizarre structures and behaviors.

Among these species of tragopan's, males vary, but females are similar and presumably adapted for crypsis, etc. The females may be relatively close to the "optimum" dictated by natural selection. However, the males of these closely related species are thought to have diverged and speciated non-adaptively by sexual selection. Some of their external traits may even be disadvantageous from the standpoint of natural selection. Note the amazing colors on the next slide, which may be relatively easy for a potential predator to see!

Note the amazing colors of the males (but not the female), which may be relatively easy for a potential predator to see!

Sexual selection can even work in direct opposition to natural selection, because it is a largely independent process.

In summary, we may not generally expect organisms to be optimal. Nevertheless, optimality models can be useful tools for understanding the "design"* of biological systems, as they provide a sort of measuring stick or frame of reference. * Note that use of the word "design" here does NOT imply any sort of "intelligent design," which is a form of creationism. "Design" in the present context is simply a common and convenient shorthand for "how organisms work." Biologists understand that organisms came to be they way they are, and work the way they do, by a number of evolutionary processes, including natural selection, sexual selection, and random genetic drift. As noted above, they were not "designed" by anything. Scientists attempt to formulate testable hypotheses about how organisms work and how they have evolved. Intelligent design and creationism are not scientific theories and do not attempt to formulate testable hypotheses. They are based on faith and are generally outside the scope of scientific discourse or the scientific method.

Optimality models can indicate the best that organisms could be, given some explicit assumption of a design criterion and specified constraints; in other words, one end of a continuum (a frame of reference) Even if we don't believe organisms are likely to be optimal, it is useful to know what an optimal organism would be like.

Lecture 20 Optimality and Symmorphosis

Lecture 20 Optimality and Symmorphosis

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