[Paleopsych] SW: Physical Performance and Darwinian Fitness

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Evolution: Physical Performance and Darwinian Fitness

    The following points are made by J-F. Le Galliard et al (Nature 2004
    1) Strong evidence for a genetic basis of variation in physical
    performance has accumulated(1,2). Considering one of the basic tenets
    of evolutionary physiology -- that physical performance and darwinian
    fitness are tightly linked(3) -- one may expect phenotypes with
    exceptional physiological capacities to be promoted by natural
    selection. Why then does physical performance remain considerably
    variable in human and other animal populations(1,2,4)?
    2) Sporting events would be exceedingly boring were there no variation
    in human performance; fortunately, this is not the case. For example,
    the distribution of finish times at international marathons has a
    large variance and a long tail(1), due to a variety of factors
    affecting the performance of individual runners(5). Although genetic
    variation in locomotor performance has been documented in human and
    other animal populations(1,2), questions remain as to how genetic and
    non-genetic factors would interact with each other and what effect
    selection has on the resulting individual variation(1).
    3) The authors addressed these two questions using ground-dwelling
    lizards, a popular model system for studies of locomotor
    performance(2,4). The focus of the authors is on the endurance
    capacity as assayed in the laboratory. In lizards, endurance shows
    considerable interindividual variation that reflects differences in
    tight muscle mass, heart mass, and aerobic metabolism The study
    species is the common lizard (Lacerta vivipara Jacquin 1787) for which
    locomotor performance and life-history traits have been routinely
    studied. The authors took advantage of the populations established at
    the Ecological Research Station of Foljuif (Nemours, France) in the
    semi-natural conditions of outdoor enclosures to measure the
    heritability of initial endurance and the age-specific strength of
    natural selection on this trait.
    4) In summary: The analysis by the authors of locomotor performance in
    the common lizard (Lacerta vivipara) demonstrates that initial
    endurance (running time to exhaustion measured at birth) is indeed
    highly heritable, but natural selection in favor of this trait can be
    unexpectedly weak. A manipulation of dietary conditions unravels a
    proximate mechanism explaining this pattern. Fully fed individuals
    experience a marked reversal of performance within only one month
    after birth: juveniles with low endurance catch up, whereas
    individuals with high endurance lose their advantage. In contrast,
    dietary restriction allows highly endurant neonates to retain their
    locomotor superiority as they age. Thus, the expression of a genetic
    predisposition to high physical performance strongly depends on the
    environment experienced early in life.
    References (abridged):
    1. Rupert, J. L. The search for genotypes that underlie human
    performance phenotypes. Comp. Biochem. Physiol. A 136, 191-203 (2003)
    2. Garland, T. J. & Losos, J. in Ecological Morphology: Integrative
    Organismal Biology (eds Wainwright, P. C. & Reilly, S. M.) 240-302
    (Univ. Chicago Press, Chicago, 1994)
    3. Arnold, S. J. Morphology, performance and fitness. Am. Zool. 23,
    347-361 (1983)
    4. Bennett, A. F. & Huey, R. B. in Oxford Surveys in Evolutionary
    Biology (eds Futuyma, D. J. & Antonovics, J.) 251-284, (1990)
    5. Bouchard, C., Malina, R. M. & PÚrusse, L. Human Kinetics 408
    (Champaign, Illinois, 1997)
    Nature http://www.nature.com/nature
    Related Material:
    The following points are made by Gary J. Balady (New Engl. J. Med.
    2002 346:852):
    1) In 1859, Charles Darwin published his theory of evolution as an
    incessant struggle among individuals with different degrees of fitness
    within a species. Although at that time, his explanations created
    remarkable controversy, they were to revolutionize the course of
    science. Darwin's writings reflected conclusions drawn from years of
    study and observation. Now, nearly 150 years later, in the era of
    evidence-based medicine and rigorous scientific method, when fitness
    is quantitatively measured and study subjects are followed for years,
    the data supporting the concept of survival of the fittest are strong
    and compelling. During the past 15 years, many long-term epidemiologic
    studies have shown an unequivocal and robust relation of fitness,
    physical activity, and exercise to reduced mortality overall, to
    reduced mortality from cardiovascular causes, and to reduced
    cardiovascular risk.
    2) Cardiorespiratory fitness, or physical fitness, is a set of
    attributes that enables a person to perform physical activity. It is
    determined, in part, by habitual physical activity and is also
    influenced by several other factors, including age, sex, heredity, and
    medical status. Physical fitness is best assessed by a measure of
    maximal or peak oxygen uptake (volume of oxygen consumed, measured in
    milliliters of oxygen per kilogram of body weight per minute), which
    is viewed as an index of energy expenditure.
    3) It is now becoming clear that exercise modulates many biologic
    mechanisms to confer cardioprotection. Exercise improves the lipid
    profile and glucose tolerance, reduces obesity, and lowers blood
    pressure. However, modification of atherosclerotic risk factors does
    not fully explain the benefits that have been observed. Positive
    effects of exercise on vascular function, autonomic tone, blood
    coagulation, and inflammation are likely to contribute to improved
    cardiovascular health and survival.
    New Engl. J. Med. http://www.nejm.org
    Related Material:
    Notes by ScienceWeek:
    Underlying the various beneficial effects of physical exercise on the
    health of the human body are a constellation of physical, biochemical,
    and physiological factors that have been intensively studied for more
    than a century. At present, maximal oxygen consumption is the primary
    measure of exercise capacity, and mechanisms related to the delivery
    of oxygen to the muscles are considered to be the main factors
    determining exercise capacity.
    The following points are made by N.L. Jones and K.J. Killian (New
    England J. Med. 2000 343:633):
    1) A fit 25-year-old man can generate 650 watts while bicycling for a
    few seconds and can maintain a power of 400 watts while bicycling for
    1 minute, 230 watts while bicycling for 10 minutes, and 175 watts
    while bicycling for 30 minutes. He is able to reach 275 watts in a
    progressive incremental (increasing at 16.7 watts per minute) test to
    capacity; this power represents peak exercise and is the power at
    which maximal oxygen consumption (3.3 liters per minute) is measured.
    To put these figures in perspective, brisk walking represents an
    output of approximately 50 watts of power and an oxygen intake of 0.8
    liters per minute.
    2) Exercise depends on the oxidation of carbohydrate and fat for the
    regeneration of adenosine triphosphate required to sustain muscular
    contraction. Ventilation, gas exchange, and the circulation are
    adjusted to meet the requirements for delivery of oxygen and removal
    of carbon dioxide. As the intensity of exercise increases, the
    concentration of intramuscular creatine phosphate decreases, and the
    concentrations of intramuscular adenosine diphosphate, adenosine
    monophosphate, and inorganic phosphate increase. Increases in the
    intramuscular lactate concentration and decreases in the intramuscular
    potassium concentration contribute to a marked decline in muscle pH to
    below 6.5.
    3) With prolonged submaximal exercise, the changes in intramuscular
    metabolite concentrations are less marked, but intramuscular glycogen
    is progressively depleted. The ability to sustain exercise depends on
    the initial intramuscular glycogen concentration. Fat stores represent
    a huge reservoir of potential energy, but the rate at which fat can be
    oxidized is limited to approximately one-fourth of the rate at which
    glycogen can be oxidized. Thus, even with maximal utilization of fat,
    the ability to maintain exercise is dependent on the oxidation of
    glycogen, which eventually becomes depleted, leading to muscle
    4) In exercise lasting longer than a minute or two, the cardiac output
    and heart rate increase linearly with peripheral oxygen uptake. The
    mean systemic arterial pressure and the vascular resistance in active
    muscle falls, leading to a large increase in blood flow to the
    muscles. Blood is pumped back to the heart by muscular contraction,
    and the cardiac output is determined by the venous return.
    5) Muscle contraction during exercise is initiated by a central
    command from the *motor cortex of the brain that leads to activation
    of *motor neurons, depolarization of *motor end plates, propagation of
    *muscle action potentials, calcium release, formation of
    *cross-bridges, and shortening of *myofibrils. The magnitude of the
    central motor command increases in parallel with the power output, but
    it also increases if the responsiveness of the motor neurons or
    muscles decreases during fatigue. A maximum voluntary command is
    capable of activating virtually 100 percent of the *motor units in a
    fresh muscle (i.e., a muscle that has not been exercised). The
    responsiveness of motor neurons may be decreased by central and
    peripheral factors acting through reflexes in the spinal cord and by
    stimulation of receptors in the muscles.
    6) Among the changes that accompany increasing intensity of exercise
    is a large increase in the intramuscular hydrogen ion concentration
    from 100 nanomoles per liter (pH = 7.0) at rest to 400 nanomoles per
    liter (pH = 6.4) or more at exhaustion, leading to inhibition of
    *excitation-contraction coupling and thus reducing the responsiveness
    of the muscle to stimulation of motor units.
    New Engl. J. Med. http://www.nejm.org
    Notes by ScienceWeek:
    motor cortex of the brain: (cortex) The cerebral cortex is a thin
    surface layering of nerve cells of the brain, the region only several
    millimeters thick but covering all of the brain surface. This is the
    part of the central nervous system most intimately involved with the
    so-called "higher faculties", although the cortex operates in concert
    with other parts of the brain. The structure is primitive in lower
    mammals, and is found progressively more pronounced and with greater
    surface area in primates and man. The motor cortex is the region of
    the cortex involved in voluntary muscle movements.
    motor neurons: In this context, motor neurons are neurons with cell
    bodies in the spinal cord and extensions that leave the spinal cord to
    terminate on muscle fibers. In this report, the general paradigm for
    activation of voluntary muscles is neurons in brain (motor cortex) to
    neurons in spinal cord (motor neurons) to peripheral muscle fibers.
    motor end plates: The junctions between nerve fiber (axon)
    terminations and muscle fibers.
    muscle action potentials: The muscle "action potential" is completely
    analogous to the nerve action potential and consists of a brief
    (approximately 1 millisecond) reversal of polarization that travels
    along the fiber. This electrical change initiates calcium movements
    that begin the contraction process.
    cross-bridges: In general, connections between contractile elements of
    muscle fibers.
    myofibrils: In general, any of the long cylindrical contractile
    elements, 1 to 2 microns in diameter, that constitute the major
    component of muscle fibers.
    motor units: In general, a single motor neuron and all the muscle
    fibers that are innervated by it. (The axons of motor neurons branch
    to make contact with a number of muscle fibers.)
    excitation-contraction coupling: In general, the coupling of an
    excitatory stimulus to the contraction of muscle; at the cellular
    level, the process by which muscle fibers are caused to contract by
    the stimulation of a neuron.

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