In recent years there has been a growing attempt to use mathematical methods borrowed from engineering and economics in interpreting the diversity of life. It is assumed that evolution has occurred by natural selection, and hence that complex structures and behaviors are to be interpreted in terms of the contribution they make to the survival and reproduction of their possessors-that is, to Darwinian fitness. There is nothing particularly new in this logic, which is also the basis of functional anatomy, and indeed of much physiology and molecular biology. It was followed by Darwin himself in his studies of climbing and insectivorous plants, of fertilization mechanisms and devices to ensure cross-pollination. What is new is the use of mathematical techniques such as control theory, dynamic programming, and the theory of games to generate a priori hypotheses, and the application of the method to behaviors and life history strategies. This change in method has led to the criticism (e.g. 54, 55) that the basic hypothesis of adaptation is untestable and therefore unscientific, and that the whole program of functional explanation through optimization has become a test of ingenuity rather than an enquiry into truth. Related to this is the criticism that there is no theoretical justification for any maximization principles in biology, and therefore that optimiza­ tion is no substitute for an adequate genetic model. My aim in this ' review is not to summarize the most important conclusions reached by optimization methods, but to discuss the methodology of the program and the criticisms that have been made of it. In doing so, I have taken as my starting point two articles by Lewontin (54, 55). I disagree with some of the views he expresses, but I believe that the development of evolution theory could benefit if workers in optimization paid serious attention to his criticisms. I first outline the basic structure of optimization arguments, illustrating this with three examples, namely the sex ratio, the locomotion of mammals, and foraging behavior. I then discuss the possibility that some variation may be selectively neutral, and some structures maladaptive. I summarize and comment on criticisms made by Lewontin. The most damaging undoubtedly is the difficulty of testing the

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Optimization Theory in Evolution

Magnetotactic bacteria were discovered almost 30 years ago, and for many years and many different reasons, the number of researchers working in this field was few and progress was slow. Recently, however, thanks to the isolation of new strains and the development of new techniques for manipulating these strains, researchers from several laboratories have made significant progress in elucidating the molecular, biochemical, chemical and genetic bases of magnetosome formation and understanding how these unique intracellular organelles function. We focus here on this progress.

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Magnetosome formation in prokaryotes

1 Internal Factors.- A. Hunger: Expression through Overt behavior.- I. Predatory Schedules.- 1. Patterns of Satiation.- 2. Feast and Famine.- II. Hunger and Diel Rhythms.- III. The Ramification of Hunger Effects.- 1. Capture-eliciting Prey Stimuli.- 2. Search behavior.- IV. The Motivation Underlying Feeding Responses.- 1. Hunger Thresholds of Feeding Response Components.- 2. The Complexity of Predatory Motivation.- V. The Diversity of Foraging Tactics.- VI. Feeding Components Affected and not Affected by Hunger.- B. The Control of Feeding Responses by Factors Other than Hunger.- I. The Readiness to Hunt.- II. Prey Storing.- III. Providing Food for Dependent Family Members.- C. The Problem of Specific Hungers.- I. Switching of Prey.- II. The Prey-density Predation Curve.- III. Swamping the Appetite of Predators.- D. Daily and Annual Rhythms in Predator-Prey Interactions.- I. Daily Rhythm of Predation.- II. Daily Activity Patterns of the Prey.- III. Annual Rhythm of Predation.- 2 Searching for Prey.- A. Path of Searching and Scanning Movements.- B. Area-concentrated Search.- I. Short-term Area Concentration.- 1. Living Scattered and Area-concentrated Search.- 2. The Nature of the Path Changes.- 3. Search Behavior after the Disappearance of Prey.- II. Long-term Area Concentration.- III. One-prey : One-place Association.- C. Object-concentrated Search.- I. Existence and Properties of "Searching Image".- 1. Ecological Evidence.- 2. Experimental Evidence.- II. Social Facilitation of Searching Image Formation.- III. Searching Image and "Training Bias".- IV. Searching Image and Profitability of Hunting.- 1. Ecological Evidence for Profitability of Hunting.- 2. Experimental Evidence for Profitability of Hunting.- V. Prey-specific Expectation.- VI. Ecological Implications of Searching Image.- 3 Prey Recognition.- A. The Stimulus-specificity of Prey Capture.- I. Capture-eliciting Prey Stimuli.- II. Capture-inhibiting Prey Stimuli.- B. One-prey : One-response Relationships.- C. The Assessment of the Circumstances of a Hunt.- D. Prey Recognition by Prey-related Signals.- E. Prey Stimulus Summation.- F. Novelty Versus Familiarity.- I. The Rejection of Novel Prey.- II. Familiarization with Prey and Its Consequences.- G. The Multi-channel Hypothesis of Prey Recognition.- 4 Prey Selection.- A. Preying upon the Weak and the Sick.- B. Preying upon the Odd and the Conspicuous.- C. The Mechanics of Prey Selection.- D. Evolutionary Implications.- 5 Hunting for Prey.- A. Modes of Hunting.- I. Hunting by Speculation.- II. Stalking and Ambushing.- 1. Stalking.- 2. Ambushing.- III. Prey Attack under Disguise.- IV. Pursuit of the Prey.- 1. Changes of Velocity of Attack (Pursuit).- 2. Interception of the Flight Path.- 3. Counteradaptations of the Prey.- V. Exhausting Dangerous Prey.- VI. Insinuation.- VII. Scavenging and Cleptoparasitism.- 1. Modes and Extent.- 2. Cleptoparasitism and Competition.- 3. Counter-measures of the Robbed.- VIII. Tool-use.- IX. Mutilation.- B. The Diversity of Hunting Methods.- I. Prey-specific Methods.- II. Situation-specific Methods.- III. Mechanisms and Causes of Predatory Versatility.- 1. General.- 2. Individual Predatory Repertories.- 3. The Persistence of Individual Traits.- 4. Predatory Specialization and Structural Modification.- 5. Predatory Versatility in Relation to Prey Availability.- C. Behavioral Aspects of Hunting Success.- I. A Comparison of Hunting Success across Predator Species.- II. Variables Influencing Hunting Success within Predator Species.- III. Aspects of Communal Hunting.- 1. Modes and Properties of Communal Hunting.- 2. Factors Conducive to Communal Hunting.- 3. Benefits of Communal Hunting.- References.- Scientific Names of Animals and Plants.

The Ethology of Predation

The cyanobacterium Prochlorococcus is the dominant oxygenic phototroph in the tropical and subtropical regions of the world's oceans. It can grow at a range of depths over which light intensities can vary by up to 4 orders of magnitude. This broad depth distribution has been hypothesized to stem from the coexistence of genetically different populations adapted for growth at high- and low-light intensities. Here we report direct evidence supporting this hypothesis, which has been generated by isolating and analysing distinct co-occurring populations of Prochlorococcus at two locations in the North Atlantic. Co-isolates from the same water sample have very different light-dependent physiologies, one growing maximally at light intensities at which the other is completely photoinhibited. Despite this ecotypic differentiation, the co-isolates have 97% similarity in their 16S ribosomal RNA sequences, demonstrating that molecular microdiversity, commonly observed in microbial systems, can be due to the coexistence of closely related, physiologically distinct populations. The coexistence and distribution of multiple ecotypes permits the survival of the population as a whole over a broader range of environmental conditions than would be possible for a homogeneous population.

Physiology and molecular phylogeny of coexisting Prochlorococcus ecotypes

This text details animal orientation with the help of information from the geomagnetic field. It reviews the magnetic effects on spatial behaviour in the various groups of the animal kingdom from platyhelminths to vertebrates, with an emphasis on birds as the best studied group. The discussion covers "compass" and "non-compass" effects and different types of responses, for example, alignments, compass orientation and orientation by the spatial distribution of magnetic parameters, which should aid the reader's understanding of the various ways magnetic information may be used.

Magnetic orientation in animals

1. Previous experiments have demonstrated that ( a ) the shark Scyliorhinus canicula and the ray Raja clavata are extremely sensitive to weak electric fields; ( b ) their electrical sensitivity is due to the ampullae of Lorenzini; ( c ) the sharks and rays can be stimulated by the bioelectric fields emanating from the flatfish Pleuronectes platessa .

2. When hungry, Scyliorhinus and Raja perform well-aimed feeding responses to flatfish, even if the prey have covered themselves with sand. The object of the present study was to determine whether the sharks and rays use the bioelectric fields of the flatfish to detect the position of their prey.

3. To analyse the feeding responses of the sharks and rays, a flatfish was put into an agar chamber. The predators responded to the so screened prey from the same distance, and tried to feed on it in the same way as if there were no agar at all. As the flatfish in the agar chamber was completely hidden from view, the sharks and rays were thus shown not to need visual contact to locate the prey.

4. If the agar chamber was filled with cut-up pieces of whiting, the sharks and rays did not respond to the food, although the odour of whiting juice normally attracts them strongly. Therefore, the sharks and rays did not detect the position of the agarscreened flatfish by smell.

5. The feeding responses to the flatfish could be entirely abolished by covering the agar chamber with a very thin sheet of plastic. The mechanical attenuation offered by the plastic film was too weak to explain its dramatic inhibitory effect, and, thus, a purely mechanical detection of the agar-screened flatfish without plastic film was also ruled out.

6. As the responses to the agar-screened flatfish were not merely due to visual, chemical, or mechanical stimuli, it was tentatively concluded that the sharks and rays perceived the prey electrically. This conclusion was fully in agreement with the results of the experiments, for the agar chamber did not appreciably distort the bioelectric fields of the flatfish, and the electrical impedance of the plastic film was extremely high.

7. Further, the bioelectric field of a flatfish was simulated with a pair of electrodes, buried in the sand. Now, the sharks and rays displayed exactly the same feeding responses to the electrodes as they did previously to the real prey. This crucial experiment confirmed the electrical hypothesis in a very direct way.

8. The experiments described demonstrate clearly that the shark Scyliorhinus canicula and the ray Raja clavata make a biologically significant use of their electrical sensitivity. Therefore, we now are justified in accrediting the animals with an electric sense and in designating the ampullae of Lorenzini as electroreceptors .

9. When the sharks and rays were offered a piece of whiting in the vicinity of two electrodes simulating a flatfish, they were attracted by the odour of the food but usually performed their well-aimed responses to the electrodes. Thus, at short range, the electric fields act as a much stronger directive force than do the visual and chemical stimuli. Only direct mechanical contact dominates over the electrical stimuli.

10. Theoretically, the sharks and rays can detect the electric fields resulting from ceanic and tidal currents. Whether they make use of the available information for orientation in the open sea is not yet known. Furthermore, the observations and measurements described indicate that, in studying shark attacks, the electric fields of the prey and the electric sense of the predators should be taken into account.

Present address: Department of Neurosciences, School of Medicine, University of California, San DiegoLa Jolla, California 92037.

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The Electric Sense of Sharks and Rays

Sharks, skates, and rays receive electrical information about the positions of their prey, the drift of ocean currents, and their magnetic compass headings. At sea, dogfish and blue sharks were observed to execute apparent feeding responses to dipole electric fields designed to mimic prey. In training experiments, stingrays showed the ability to orient relative to uniform electric fields similar to those produced by ocean currents. Voltage gradients of only 5 nanovolts per centimeter would elicit either behavior.

https://science.sciencemag.org/content/sci/218/4575/916.full.pdf

Electric and magnetic field detection in elasmobranch fishes

Fishes have an impressive complement of hydrodynamic and acoustic sensors, commonly referred to as the lateral-line and inner-ear sense organs The basic receptor elements are the hair cells, which detect the minute displacements imparted to their apical ciliary bundles (Fig 41a) The directional sensitivity of the individual receptor cells is indicated by the asymmetric position of the single kinocilium relative to the several rows of stereocilia Morphologically, the hair cells of the various sensory clusters are strikingly uniform Their diversity in function is determined mainly by the peripheral structures coupling the ciliary bundles to the physical world that the animals inhabit

Hydrodynamic and Acoustic Field Detection

Acoustic telemetry was used to follow 22 blue sharks,Prionace glauca (Linnaeus), over the continental shelf and slope in the region between George's Bank and Cape Hatteras between 1979 and 1986. The sharks frequently made vertical excursions between the surface and depths of several hundred meters. The oscillations, which were repeated every few hours, were largest in the daytime and were smaller in amplitude and confined to depths near the thermocline at night. This behavior was prominent in trials from August through March, but was not seen from June through July. Diving is discussed in terms of a hunting tactic and behavioral thermoregulation. Most of the sharks moved in a southeasterly direction from the release point and many maintained a constant course day and night for several days. The sharks may orient to the earth's magnetic field, or to the ocean's electric fields, allowing them to swim on a constant heading in the absence of celestial cues. These possibilities are discussed in the appendix.

Movements of blue sharks (Prionace glauca) in depth and course

Several species of aquatic bacteria which orient in the Earth's magnetic field and swim along magnetic field lines in a preferred direction (magnetotaxis) have been observed in marine and freshwater sediments of the Northern Hemisphere1,2. Their orientation is due to one or more intracytoplasmic chains of single-domain magnetite particles3. These linearly arranged particles impart a net magnetic dipole moment to the bacterium, parallel to the axis of motility. Northern Hemisphere magnetotactic bacteria with unidirectional motility swim consistently in the direction of the magnetic field, that is, to the geomagnetic North1,2,4. This implies that their magnetic dipole is systematically orientated with the North-seeking pole forward. The magnetic polarity can be reversed by single, magnetic pulses of high field strength (1–2 µs, 300–600 G), and these bacteria then swim along magnetic field lines to the South5. Due to the inclination of the Earth's magnetic field, magnetotactic bacteria which swim to the North in the Northern Hemisphere are directed downward at an angle increasing with latitude. It has been suggested that this downward-directed motion confers a biological advantage by guiding the bacteria, when dislodged, back to the sediments1. On the basis of this hypothesis, magnetotactic bacteria of the Southern Hemisphere would be expected to swim to the South to reach the bottom. We report here several morphological types of magnetotactic bacteria present in sediments of the Southern Hemisphere. These bacteria indeed swim consistently to the South, hence downward along the Earth's inclined magnetic field lines, as hypothesized. As revealed by electron microscopy, they contain internal chains of electron-opaque particles similar to those observed in magnetotactic bacteria from the Northern Hemisphere. Like their Northern Hemisphere counterparts, their magnetic polarity can be permanently reversed and they cannot be demagnetized. We also report on Northern Hemisphere magnetotactic bacteria incubated in Southern Hemisphere magnetic conditions, confirming the biological relevance of downward directed motility.

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Ad. J. Kalmijn

Papers

The Electric Sense of Sharks and Rays

Electric and magnetic field detection in elasmobranch fishes

Hydrodynamic and Acoustic Field Detection

Movements of blue sharks (Prionace glauca) in depth and course

South-seeking magnetotactic bacteria in the Southern Hemisphere