Biological Fitness

7 Key excerpts on "Biological Fitness"

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  Human Genetics for the Social Sciences

  • Gregory Carey(Author)
  • 2002(Publication Date)
  • SAGE Publications, Inc

    (Publisher)

  There are three essential components to this definition—(a) differential reproduction, (b) heritable traits, and (c) adaptation to the environment. Charles Darwin noted that most species reproduce at a rate that, if unchecked, would lead to exponential population growth. Such growth is sel-dom realized in nature, however, because many organisms fail to reproduce. Darwin reasoned that if this differential reproduction was associated with adaptation to an environmental niche and if the adaptive traits were transmit-ted to a subsequent generation, then the physical and behavioral traits of a species would change over time in the direction of better adaptation. Genetic variation fuels natural selection, and genetic inheritance trans-mits adaptive traits from one generation to the next. If all the members of a species were genetically identical, then there would be no genetic variation and hence no natural selection. The organisms in this species could still dif-ferentially reproduce as a function of their adaptation, but they would trans-mit the same genes as those who failed to reproduce. Biologists index natural selection by reproductive fitness , often abbrevi-ated as just fitness . Reproductive fitness can be measured in one of two ways. Absolute reproductive fitness may be defined as the raw number of gene copies or raw number of offspring transmitted to the subsequent generation. It may be expressed in terms of individuals (e.g., George has three children), phenotypes (e.g., on average, the red birds produce 3.2 fledglings), or geno-types (e.g., on average, genotype Aa has 2.4 offspring). For sexually repro-ducing diploid 1 species like humans, a convenient way to calculate absolute fitness is to count the number of children and divide by 2. For example, someone with two children would have an absolute fitness of 1.0, indicating that the person has left one copy of his genotype to the next generation.

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  Game-Theoretical Models in Biology

  • Mark Broom, Jan Rychtář(Authors)
  • 2022(Publication Date)
  • Chapman and Hall/CRC

    (Publisher)

  Chapter 10 ) describes five notions of fitness, and we follow his arguments below.

  5.4.1 Fitness 1

  This is the original usage of the term, as described in Spencer (1864 , pp. 444-445) and used by Darwin and Wallace. Without a firm definition, it was the properties which a priori allowed individuals to do well in their environment, and thus made them likely to prosper e.g. for Darwin's finches, large beaks where the food supply was tough nuts or long thin beaks when it was insects.

  5.4.2 Fitness 2

  This is the fitness of a particular class of individuals defined by its genotype, often at a single locus. The fitness is defined as the expected number of offspring that a typical individual of that type will bring up to reproductive age. It is usually expressed as a relative fitness, compared to one particular genotype (see Chapter 12 for an example of this). Thus this fitness can be measured by considering the whole population of individuals and averaging over them. It is said that there is selection for or against a particular genotype at a given locus. As we have seen above, we can consider the evolution of a population of alleles in this way as a game with a symmetric matrix.

  5.4.3 Fitness 3

  This is the lifetime reproductive success of an individual and is the measure we described at the start of this section, and, as stated above, is the most common usage of fitness throughout the book. Whilst fitness 2 is very localised, it is easy to measure since the genotype continually recurs. However, each individual only occurs once. Just because an individual has a large number of offspring that survive to reproductive age, it does not mean that its genes are more likely to survive long term, since its children may be less fit (see the sex ratio problem in Section 4.4

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  Virus as Populations

  Composition, Complexity, Quasispecies, Dynamics, and Biological Implications

  • Esteban Domingo(Author)
  • 2019(Publication Date)
  • Academic Press

    (Publisher)

  The relative reproductive efficiency that we recapitulate with the term “fitness,” correlates with the rate of evolutionary change promoted by natural selection. Fisher’s fundamental theorem of natural selection asserts that the rate of fitness increase of a population equates with the genetic variance of fitness at that time (Fisher, 1930). Fitness measurements go back to the early developments of population genetics, using Drosophila as a model organism (Dobzhansky et al., 1977 ; Spiess, 1977) with observations that were later reexamined with viruses, and provided a molecular interpretation of some fitness variations [reviewed in (Domingo et al. 2019)]. F.J. Ayala showed that mixed Drosophila populations or populations that were artificially diversified by limited radiation displayed higher fitness than the undiversified populations, thus providing experimental support to Fisher’s theorem (Ayala, 1965, 1969). In practical terms, fitness refers to the relative contributions to the next generation made by the different individuals that compose a population. This definition is adequate for viruses because it can be adopted to mean the overall replication capacity of a mutant spectrum, despite possible modulation by intrapopulation interactions of cooperation, complementation, or interference (Chapter 3). Fitness quantifies the ability of a virus population (irrespective of its complexity) to produce infectious progeny, usually compared with a reference viral clone, in a defined environment (Domingo and Holland, 1997 ; Quinones-Mateu and Arts, 2006 ; Martinez-Picado and Martinez, 2008 ; Wargo and Kurath, 2012 ; Domingo et al., 2019). There is no general agreement on how should fitness be measured. Some authors prefer quantifying fitness as the contribution of progeny to the next generation, while others favor the number of progeny in the second or following generations

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  The Study of Behavior

  Organization, Methods, and Principles

  • Jerry A. Hogan(Author)
  • 2017(Publication Date)
  • Cambridge University Press

    (Publisher)

  The function of much behavior is fairly obvious (eating, drinking, sex, escape, care for young), but even with such obvious cases, the possible function of aspects of these behaviors needs investigation. I begin by discussing the concept of fitness and then continue with examples of 274 The Study of Behavior studies of the primary functions of behavior: feeding, defense, and reproduction. I conclude with a discussion of situations in which the primary functions of behavior interfere with each other or are condi- tional on the behavior of other individuals. I will be emphasizing empirical studies and the methods used. The nature of adaptation and selection are considered in Chapter 10. T H E C O N C E P T O F F I T N E S S Fitness is an important concept for evolution, because, without differ- ential fitness of entities in a population, evolution cannot occur! But fitness is a slippery concept. It is a concept like homeostasis and canalization: it is defined by its effects. It is one of those concepts that everyone thinks he/she understands, but then finds it difficult to specify exactly in a particular situation. A typical textbook definition of fitness is the measure of an individual’s success in passing on copies of its genes to future generations. But fitness can be defined with respect to the genotype or phenotype of a trait or an individual or even a group; it can have two or three components (viability, mating success, fecund- ity); it can be considered a property of an individual or of a class of individuals (average value of the group); it can be absolute or relative; it can be direct or indirect or inclusive; and individuals with identical genotypes may have different fitnesses depending on the environ- ments with which they have interacted during development. Also, how many generations need to be considered? The next? Two? Many? Sober (2001) and Orr (2009) discuss most of these issues.

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  Modelling Evolution

  A New Dynamic Account

  • Eugene Earnshaw-Whyte(Author)
  • 2018(Publication Date)
  • Routledge

    (Publisher)

  I hope it is obvious that this ‘shooting skill’ trait is a lot like ‘individual fitness’. We can’t observe it directly, any number we assign to it is never going to be entirely legitimate, and one might question whether it corresponds to any real physical thing. We have to continually bear this in mind: individual fitness is such a central concept in evolution that it is easy to forget how elusive and ephemeral it is.

  Nevertheless, individual fitness seems to play a key role in explaining something that is absolutely true. Because even though we cannot really measure the chances of even a single dandelion seed thriving, we know that some seeds have better chances than others. Biologists regularly find evidence that even tiny measurable differences between living things have meaningful impacts on their survival and reproduction. Birds with slightly longer or shorter beaks, seeds with slightly thicker shells, different colours on snails: all may cause different evolutionary success. So even though we can hardly know what the actual fitness values are, we can still see that the real physical variations between individuals make a real difference to how well they thrive in nature. And by representing these differences in fitness, we can begin to represent evolutionary change by natural selection.

  The fitness of an individual is, as we have seen, a quite elusive and difficult property, and there is no absolute objective value of individual fitness that is out there for us to find, any more than there is an absolute objective value to my chance of hitting a three point shot. It is always relative to circumstances, and so at the very least will depend on what we take the environment, or range of possible environments, to be. It is not the ACTUAL environment experienced by the individual, because some of this will be happenstance. Whatever value of individual fitness we assign will always, therefore, be a constructed value relative to certain interests or assumptions about the relevant environment. It will be an estimate for some assumed range of possible circumstances. If it is derived from empirical data, it will be relative to the circumstances that were prevalent during the time of observation, which could change and might not be representative of ‘typical’ environments, even for that place and time. All we can directly observe is an organism’s actual success in the environment it happened to live in.

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  Virus as Populations

  Composition, Complexity, Dynamics, and Biological Implications

  • Esteban Domingo(Author)
  • 2015(Publication Date)
  • Academic Press

    (Publisher)

  in vivo is determined basically by the complete genome of the pathogen and its expression products in interaction with the physical and biological environments provided by the host cell or organism.

  5.10 Overview and Concluding Remarks

  Fitness is an important parameter to describe the survival potential of a virus (be it a clone, population, mutant, or natural isolate) when confronted with related competitors in a given environment. Fitness, as a measure of relative replication capacity, can influence the progress of viral infections in vivo and, in consequence, the disease manifestations in the infected organisms. It is not surprising that, despite several technical difficulties, efforts are being put in its determination. In this area, application of NGS should provide new prospects for fitness quantification and, equally important, the fine molecular details of dominance of some variant subsets in competition with many others.

  The argument has been made that fitness landscapes, a classic representation of fitness values and variations in the form of mountains and valleys, should be very rugged for viruses as they replicate in their natural environments. One of the reasons is the frequent deleterious effects of individual mutations, evidenced by many of the examples included in different chapters of this book. There is a general debate in biology on the relationship between fitness and virulence, and it is unlikely that this debate is solved solely by studies with viruses. There are several elements that complicate attempts at clarification, the least trivial being that the very exact meaning of both fitness and virulence is itself object of debate.

  One of the consequences of the fitness gain that accompanies a selection event is the presence of memory genomes in viral quasispecies. Many of the implications of memory as part of quasispecies dynamics, and also for the planning at antiviral treatments, have not been fully appreciated.

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  Dynamics Of Complex Systems

  • Yaneer Bar-yam(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  Genome, phenome and fitness 549 It is tempting to consider all variation in phenomic traits as significantly affect-ing fitness so that there are no neutral variations. The more practical aspect of this ap-proach is to understand the existing variation of phenomic properties of a particular population of organisms. The central issue becomes whether the variation in phe-nome represents a diversity that is being acted upon by selection and therefore certain traits will eventually be forced to disappear in favor of others, or whether the varia-tion is neutral with respect to selection and will persist. This limits the scope of the question from the space of all possibilities to the space of extant organisms. Even in this context, the controversy between neutralists and selectionists is not easy to re-solve. The issue is still more complicated since populations of organisms may not act solely to select individual properties but also properties of the whole population. In this case variation may reflect the effects of selection. This will be discussed in Section 6.6.2. From the discussion in this section, we see that there are a wide variety of con-tributions to the fitness of an organism. These factors change in time due to various events that range from change of weather to fluctuations in populations of other or-ganisms. Since we are describing the evolution of organisms due to a fitness that it-self depends on the existence of organisms, we are describing a self-consistent process. Such self-consistency was discussed in Section 1.6 in the simpler context of the Ising model for magnets. In essence, the concept of fitness itself represents a mean field approach. The assumption is that at any time, an average over influences that affect fitness is a meaningful concept, and that evolution takes place in the con-text of this average fitness. This is one of the central assumptions in evolutionary theory, not just in the models we will be discussing.

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