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Fitness

Fitness

Fitness is a central concept of evolutionary biology. We will consider individual fitness, followed by fitness as applied to alleles or genotypes.

The direct fitness of an individual is related to the number of offspring that that individual produces. Specifically, it is one-half of the number of offspring produced, because in sexual species, only half of an offspring's genes come from either parent. That proportion, one-half, represents the degree of relatedness, or proportion of genes shared, between parent and offspring.

Indirect fitness derives from shared genes with kin other than the direct offspring of an individual. This might include cousins, nieces, nephews, siblings, and so on. The indirect fitness of an individual is calculated by adding the relations of that individual multiplied by the degree of relatedness. Inclusive fitness represents the sum of direct and indirect fitness.

The concept of indirect fitness was developed by the evolutionary biologist W. D. Hamilton. The idea originated with attempts to explain altruistic behavior in animals. Altruistic behavior is defined as behavior that harms the actor yet benefits the recipient, and includes such actions as alarm calling, which may draw the attention of the predator to the caller.

According to natural selection theory, altruistic behavior should be eliminated from populations because it hampers individual survival and reproduction. However, Hamilton noted that if altruistic behavior benefits the kin of the actor, that behavior can nonetheless be selected for. This is because kin share genes with the actor. Hamilton's Rule dictates when altruistic behavior is beneficial: Altruism is selected for if the cost of a behavior to the actor is less than the benefit to the recipient, multiplied by the recipient's degree of relatedness to the actor. Thus, altruistic acts are more likely if they benefit close kin rather than distant kin, or unrelated individuals.

Kin selection explains a wide variety of altruistic behavior. It also explains the evolution of social systems in which some individuals forego reproduction in order to help parents raise siblings. This is the situation in many pack species, such as wolves. In wolves, packs are often made up of two parents and their offspring from several mating seasons. Only the parents, which are the dominant individuals in the pack, reproduce.

Kin selection also explains more extreme examples of social behavior, such as that found in eusocial insects (species in which there are non-reproductive individuals. The primary groups of eusocial insects are the Hymenoptera (ants and bees) and the termites. Both groups have evolved special genetic systems in order to make kin selection more powerful. The Hymenoptera are characterized by haplodiploidy, a genetic system in which the males are haploid and females are diploid.

One consequence of haplodiploidy is that females (who are the crucial players in the colony) share a greater proportion of genes with their sisters than they would with their own offspring. It therefore benefits females to care for sisters in the colony rather than try to reproduce on their own. Termites are not haplodiploid, but they do go through repeated cycles of inbreeding, which also results in individuals sharing an unusually large proportion of their genes.

Kin selection is more complicated in the real world than Hamilton's Rule suggests because the expected reproductive success of individuals must also be factored in. For example, even though an offspring only shares half its genes with a parent, the parent may protect an offspring more vigorously than expected because reproductive success of the younger offspring may be greater than that of the more aged parent.

So far, this discussion has focused on individual fitness. Fitness can also be defined for alleles or for genotypes rather than for individuals. Allelic or genotypic fitness describes the relative contribution of one allele or geno-type to the next generation as compared to that of possible alternate alleles or genotypes. These forms of fitness are central to population genetics.

Genotypes and alleles with higher fitness are selected for in the next generation, and make up a greater proportion of the total gene pool than other genotypes and alleles. All else being equal, alleles with greater fitness will eliminate and replace alleles of lower fitness. However, the fitness of particular alleles or genotypes may depend on numerous external factors, and changes in the relative fitnesses of alternate alleles/genotypes may help maintain polymorphisms in populations, situations in which a population has multiple alleles for a given locus.

One external factor determining the fitness of alleles and genotypes is the specific environment in which they are found. One well-studied example is that of the sickle-cell anemia allele. This allele is normally disadvantageous because individuals who are homozygous for the allele (that is, carrying two copies of it) have sickle-cell anemia. However, in malaria-prone areas, it has been shown that individuals who are heterozygous (carrying one sickle-cell allele and one normal allele) are more resistant than individuals who have two normal alleles. So, in areas where malaria occurs, the fitness of the sickle-cell allele is higher than in malaria-free areas.

Another external factor determining the fitness of a particular allele or genotype is the alleles an individual possesses for other genes. This is called epistasis.

Yet an additional external factor that may determine the fitness of an allele or genotype is its frequency in the population. This is known as frequency-dependent selection. Frequency-dependent selection is known to operate in mimicry systems, in which there are poisonous individuals as well as non-poisonous individuals of the same species that mimic the appearance of poisonous individuals. The fitness of either type depends on the relative frequencies of poisonous and nonpoisonous individuals in the population.

Jennifer Yeh

Bibliography

Alcock, John. Animal Behavior, 4th ed. Sunderland, MA: Sinauer Associates, 1989.

Futuyma, Douglas J. Evolutionary Biology. Sunderland, MA: Sinauer Associates, 1998.

Ridley, Mark. Evolution. Boston: Basil Blackwell Scientific, 1993.

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Fitness

Fitness


Fitness is a measure of the relative performance or adaptedness of an organism represented by its genotype in a given environment. The term fitness is sometimes also used to describe other biological units, such as the gene or the population. Classically fitness is used to describe differences in survival (viability selection as described by Charles Darwin (18091882) with the phrase "survival of the fittest"), mating success (sexual selection), and reproductive output (fecundity selection) between individuals characterized by their genotypes and measured as their relative contribution to the next generation in terms of the number of offspring a genotype succeeds in producing and rearing to sexual maturity. A genotype that leaves more offspring will thus have a higher fitness.

In the field of classical population genetics theory, evolutionary changes are exemplified by the change in gene frequency at a single gene locus with two alleles, A1 and A2, in a diploid organism. The modes of selection depend on the fitness of the heterozygote A1A2 compared to that of the homozygotes A1A1 and A2A2. If one homozygote (e.g., A1A1 ) has the highest fitness, directional selection will favor that genotype and eventually lead to fixation of allele A1. A famous example of directional selection is the industrial melanism of the peppered moth (Biston bitularia ) in England, where the black or melanic morph increased in frequency after the industrial revolution, then decreased in the 1950s when "smokeless zones" were established and tree trunks became lighter, thus giving the black morph a disadvantage due to increased risk of predation by birds.

If the heterozygote has the highest fitness, stabilizing selection or heterozygote advantage will usually maintain both alleles in the population (an example is variation at the betaglobin gene in humans, where heterozygotes have an advantage in regions with malaria, while one type of homozygotes gets sickle cell disease), while heterozygote disadvantage will lead to disruptive selection favoring both homozygotes. This simple theory was developed for one locus in infinite populations and for constant fitness coefficients by, among others, R. A. Fisher (18901962) and J. S. B. Haldane (18921964) in the 1920s and 1930s. The theory was later modified and expanded to include multiple loci and variable environments, as well as population substructure and finite populations.

The shifting balance theory of Sewall Wright (18891988) describes the fitness landscape of more complex multilocus genotypes, where the fitness of certain genotypes has local peak values, while simple changes in genotype will lead to a fitness decrease. Shifts from one peak to another in that landscape require more complex changes with intermittent genotypes of reduced fitness. In small populations random genetic drift may counteract the selective forces that are driven by fitness differences and push populations from one peak to another.

Darwin considered fitness to be a property of the individual; later biologists sometimes use the term to refer to lower levels of organization, such as, for example, a property of the gene (the idea of the selfish gene is based on this unit) or of higher levels, such as, for example, the population. Socalled group selection is based on higher units, and the concept of inclusive fitness includes contributions of related individuals who share genes. This concept of fitness has been used to explain the evolution of altruistic behaviors, such as warning calls in birds, which may bring the altruistic individual to higher risk but may benefit its genes by improving the chance of survival of relatives.


See also Adaptation; Altruism; Evolution; Selection, Levels of; Selfish Gene; Sociobiology

Bibliography

darwin, charles. on the origin of species by means of natural selection or the preservation of favored races in the struggle for life. london: murray, 1859.


haldane, j. b. s. the causes of evolution. green, new york: longmans, 1932.

fisher, r. a. the genetical theory of natural selection. oxford, uk: clarendon press, 1930.

wright, sewall. "evolution in mendelian populations." genetics 16 (1931): 97159.


volker loeschcke

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fitness

fitness (in genetics) Symbol W. A measure of the relative breeding success of an individual or genotype in a given population at a given time. Individuals that contribute the most offspring to the next generation are the fittest. Fitness therefore reflects how well an organism is adapted to its environment, which determines its survival. See also inclusive fitness; selection coefficient.

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fitness

fitness
1. In ecology, the extent to which an organism is well adapted to its environment. The fitness of an individual animal is a measure of its ability, relative to others, to leave viable offspring.

2. (Darwinian fitness) See ADAPTIVE VALUE.

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fitness

fitness
1. In ecology, the extent to which an organism is well adapted to its environment. The fitness of an individual animal is a measure of its ability, relative to others, to leave viable offspring.

2. (Darwinian fitness) See adaptive value.

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fitness

fitness See ADAPTIVE VALUE.

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fitness

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fitness

fitness

Fitness for what?

When we speak, perhaps with a hint of envy, of a ‘fit’ young man or woman — and even more when we refer, with undisguised admiration, to a ‘fit’ old person — there is little ambiguity as to our meaning: we are referring to fitness to cope with life in general, not only with sport, and certainly not a particular sport. Furthermore the international athlete, in peak of condition, is ‘fit’ for only a limited number of very similar events: the sprinter could not possibly run a marathon, the power lifter could compete with neither kind of runner at their events. The fitness of the racing driver is radically different from that of the dinghy sailor, the gymnast from that of the mountaineer and, perhaps most radically of all, the oarsman from that of the pistol shooter. Furthermore, many highly trained athletes, particularly those conditioned for endurance events, display greater, not less, vulnerability than the average person to many forms of illness.

Clearly then, we must distinguish ‘fitness for life’ from ‘fitness for sport’; and, when considering the latter, must specify which sport.

Fitness for life

This is a condition which we almost all desire, but few of us pursue with vigour. To attain and maintain it requires adequate and balanced nourishment, adequate and varied exercise, adequate but not excessive sleep, avoidance of excess in using social drugs, plentiful stimulation without excessive stress, and psychosocial well-being. The Aristotelian precept, ‘moderation in all things’, remains as good a guide as any to the balances which must be struck. Fitness for work, for leisure and recreational exercise, for family life and parenthood, and even for childbearing itself, and fitness to cope with emergencies — all are optimized in these broad ways. The influences of genetics and of environment are inescapable, so the fitness attained by one person will be very different from that attained by another, but all will approach their individual optima by personal application of the same balanced principles. Even Western and Eastern, secular and religious wisdoms (disregarding the most extreme of the latter) have much more in common than divergence in their guidelines for ‘fitness’, whether or not they would recognize that term; and modern science, while adding a few details on matters like trace nutrients, takes little issue with them about the broader picture.

Endurance fitness

If there is one aspect of specialist, sports-oriented fitness which embodies the greatest part of the lay ideal, it is probably endurance fitness — the ability to continue a demanding physical activity many times longer than the untrained person can. Whether the challenge is a London– Brighton cycle race, an ascent of the Matterhorn, or a Channel swim, the fundamentals of this category of fitness are the same. Each of these activities is trained for in essentially the same way — namely, by covering large mileages several days a week for many months, with few if any periods of exertion that are flat out, either in strength or speed. Each activity is, in turn, necessarily aerobic — an activity performed in balance with oxygen intake — and consequently requires that the heart can pump blood to the working muscles at several times its resting rate throughout the long duration of the exercise; also that the lungs can adequately oxygenate this enhanced blood flow as long as the exercise continues. ‘Cardio-respiratory fitness’ is thus a common feature of all endurance events, though they differ in the skeletal muscles used, and the movement patterns these muscles perform.

When muscles have been endurance-trained they are typically only a little larger than before the training began, months or years before. They become furnished, however, with a much more copious system of blood capillaries. Within the muscle fibres, mitochondria, the organelles involved in oxidative energy provision, may be 2–3 times more numerous than in untrained or differently trained fibres. Connective tissues within the muscle as well as the associated tendons and ligaments are stronger too. The nervous system must also participate in the training, for patterns of movement in the exercise concerned are usually measurably more economical than before the regime began.

Other forms of training

Pure strength training contrasts most markedly with the low-force, multiple-repetition work just described. Though increasing the bulk of the muscles and the maximum loads which they can handle, it adds little or nothing to their endurance. However the more commonly undertaken ‘weight training’, in which less extreme loads are worked against, with several times as many repetitions during the course of each gymnasium session, imparts ‘strength endurance’, a balance between the two extremes which arguably develops the most useful form of fitness for everyday life. Speed training, ‘plyometric’ (resilience) training, and flexibility training are other forms in which it is possible to specialize: in particular, yoga places a degree of emphasis upon flexibility which most other schools of physical educators would consider disproportionate. Nevertheless a programme of muscle stretching and joint flexibility should be part of the regime of every sportsperson seeking to improve not only performance but resistance to injury. Finally, between speed and endurance comes ‘anaerobic endurance’ — the ability to maintain a power output only a few per cent below flat out for several tens of seconds (as in 400 metre running) or to repeat short bursts many times in a period of about 90 min (as in hockey, soccer, and other ‘multiple sprint’ sports).

Specific versus general fitness

It would be widely agreed that the broader-based forms of fitness are of greater value in daily life than the extreme forms, such as pure endurance, pure strength, pure flexibility, or pure speed. Older literature embodied the ideal of breadth in the term ‘general fitness’. However, it is now appreciated that the dominating principle underlying the response of the body to training is its ‘specificity’. A particular exercise elicits the adaptive responses we call ‘training’ only from the specific muscles and other tissues exercised, and enhances only the specific property (endurance, strength, speed, or extensibility) which the exercise challenges. At best only very modest improvements of other properties or at other muscle sites (‘cross-training’) are ever reported, and they cannot be counted upon. A sport requiring many forms of fitness must thus have a training programme including many elements. There is probably only one sense in which ‘general fitness’ can be enhanced by most individual forms of exercise, pursued in isolation: since it is impossible to undertake any exercise without raising both pulse rate and ventilation, every form of exercise provides some cardio-respiratory training, and hence some degree of ‘general fitness’ in respect of these central organs. More thorough-going general fitness can only be attained by an exercise programme which is itself broad-based.

A broad-based programme can, of course, be achieved by regular visits to a well-conducted gymnasium; however, such a clinically purposeful regime is not the only way. Someone who, in a typical 2-week period, goes for a 40-minute run, plays a game of squash, spends an active 30 minutes in the swimming pool, does a couple of hours' heavy gardening, polishes the car energetically, chops wood, vacuum cleans the stairs twice, and scrubs the steps, especially if (s)he precedes at least the first three of these activities with 5–7 minutes of stretching and flexing exercises, will be as fit for life as a neighbour who visits the local gym three times a week. Any difference between them which is non-genetic may well be determined by which of them gets more sleep, or eats less fat.

Women, children, and the elderly

In modern, Western societies, women, children, and the elderly are particularly prone to take insufficient exercise. The Allied Dunbar National Fitness Survey found that, in England during 1990, only one woman in ten, whether aged 20 or 50, took the amount of exercise really recommended for health whereas, among the men, 30% of 20-year-olds and 20% of 50-year-olds did so. Dunbar's standards were admittedly high — among the 20-year-olds, for instance, it hoped to see three games of squash, or equivalent, per week. More recent research has shown that statistically demonstrable improvements in cardiovascular fitness, compared with the effects of taking no exercise at all, can be had from only three 20–30 minute periods per week of moderately vigorous walking. Nevertheless, about a quarter of women in the working age-groups do not even achieve this, which is a much more modest goal than the vibrant fitness sought by Dunbar.

Modern children are distracted by television and computer games and are more likely to be transported to and from school, so that they almost certainly take less exercise than their predecessors before the 1939–45 war (although incontrovertible figures for the past are hard to establish). They should be urged to the maximum amount of physical activity of which they seem capable. No damage will accrue, provided they wear well-fitting trainers, are provided with shock-absorbing landing mats for gymnastics, and don't spend more than 90 minutes, 3 days a week, with specialist, competitive coaches.

Amongst the elderly, a ‘disuse–disability spiral’ operates. Well-meaning younger carers can be the old person's worst enemies. If daily activities fail to maintain independence — the bottle top, the heavy kettle, and worst of all independence at the toilet, being critical markers of diminished capacity — exercise regimes can be of enormous benefit. Often this benefit is proportionately greater than in younger adults, because, through disuse, the elderly have declined further below their genetic capability. Instances of elderly people running marathons are well known, but strength training is at least as effective in the very old as endurance training, and may be even more beneficial.

Neil Spurway

Bibliography

Morris, J. et al. , (1992). Allied Dunbar National Fitness Survey. The Sports Council, London.
Sharkey, B. J. (1990). Physiology of fitness, (3rd edn). Human Kinetics, Champaign, Illinois.
Wilmore, J. H. and and Costill, D. L. (2000). Physiology of sport and exercise. 2nd ed. Human Kinetics, Champaign, Illinois.


See also exercise; health; sport.

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Fitness

Fitness

Fitness is a concept that supports many sports science meanings. In its most general application, fitness describes the current levels of both physical health and physical capabilities present in an athlete. Athletes have an innate understanding of what fitness is through personal experience; physical fitness is the expression that is also used to describe the optimal physical condition or "shape" of an athlete at a given time.

The traditional definition of physical fitness as employed by sport experts until the mid-twentieth century concerned the range of the physical capacity of an athlete; current definitions have evolved to include a greater focus upon the general health of every bodily system that might influence fitness. The foundation question in any determination of physical fitness is the assessment of the athlete's ability to perform athletic activities vigorously, a process that involves a consideration of five distinct benchmarks. These benchmarks include: aerobic or endurance fitness, with regard to the function of both the cardiovascular system and the cardiopulmonary system; muscular endurance, representing the ability to generate sustained muscle output; muscular strength, the maximum available power; flexibility, defined as the range of motion achieved in the movement of the joints, the combined effect of the elasticity of muscles, tendons, and ligaments; and body composition, determined by the percentage of body fat in contrast to bone and muscle structure.

A physically fit athlete may possess greater degrees of fitness in one or more of the five individual fitness headings than another: fitness is a cumulative measure. As an example, a world-class soccer player may not possess formidable muscular strength, but this deficit, which is usually of secondary importance to success in that sport, will be amply compensated in the other four categories, particularly those of endurance and flexibility. Conversely, when an athlete is demonstrably fit in only one of the five areas, it is unlikely that he or she shall possess true comprehensive physical fitness. This phenomenon is often observed in disciplines such as weightlifting, in which the athlete is able to generate remarkable muscular strength, but may possess less measurable fitness in the remaining categories.

Fitness is a concept which also may be understood in contrast to other familiar aspects of human performance. Physical fitness is commonly linked to considerations of life expectancy and longevity. The questions posed by these considerations seek to answer if physically fit persons tend to live longer than sedentary, unfit persons, or if a commitment to lifelong physical fitness actually extends life expectancy, or if such a commitment simply makes a genetically predetermined allotment of years more pleasurable.

There appears to be little question that healthy living practices, combined with a program of physical fitness, will help reduce the risk of early life-ending diseases such as arteriosclerosis and other potentially fatal conditions of the cardiovascular system. Cardiovascular disease is the cause of more death among women in the Western world than all forms of cancer combined. Exercise contributes to the reduction of excess body weight and lessens all of the strains placed on the various physical systems that are caused by obesity. However, elimination of an early cause of ill health or death is not itself the extension of the limit of life expectancy.

Fitness is sometimes sought by adult persons who have lived a demonstrably unhealthy adolescence or young adulthood. In many circumstances, these persons are seeking to reverse the negative lifestyle practices that have impacted their fitness for many years. No matter how devoted to a more healthful lifestyle in later adulthood a person may become, it is unlikely that the poor fitness choices made by an adult earlier in life can be entirely reversed. If the strength and density of physical structures such as the musculoskeletal system were compromised due to a calcium deficiency, or if the cardiovascular system was subjected to excess quantities of plaque-creating cholesterol, the impact of such negative factors may not be eradicated by subsequent health and fitness choices; however, the harmful effects will be lessened over time.

Since the early 1900s, medical science has developed techniques to eliminate numerous previously fatal conditions, particularly in terms of both the prevention of communicable disease, as well as interventions that preserve life. There exists no empirical evidence to confirm that athletes live longer than non-athletes; athletes are more likely to enjoy a healthier, more desirable quality of life. It would appear that the only provable manner of extending one's life from an otherwise expected limit is to maintain strict control of calorie consumption and limit the related negative impacts of excess weight.

see also Cross training; Habitual physical activity; Longevity.

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