Evolutionary Fitness

9 Key excerpts on "Evolutionary Fitness"

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  Microbial Ecology

  An Evolutionary Approach

  • J Vaun McArthur(Author)
  • 2006(Publication Date)
  • Academic Press

    (Publisher)

  Evolution 15 Futuyma (1998) points out that fitness is most easily conceptualized for an asexual organism in which all individuals reproduce at the same time and then die. There are no overlapping generations. A more extensive discussion of fitness is provided by Futuyma (1998). Evolutionary biology mostly has been restricted to academia. Application of evo-lutionary thinking has been applied to agriculture (both crops and animals) and to some extent in disease control. However, applied evolution (Bull and Wichman, 2001) is being used in a variety of contexts including design of biotechnology protocols that result in new drugs and industrially important enzymes, development of computer technologies, and the avoidance of resistant microbes and pests. Chapter 1 Core Concepts in Studying Ecology and Evolution 16 Figure 1.3 Three different effects of natural selection acting on a population. In each example, the x-axis is the ordering from low to high of some phenotypic trait, and the y-axis is the frequency of the trait. Stabilizing selection is selection that acts on both tails of the distribution and maintains the mean characteristic. In disruptive selection, the mean phenotype is selected against that resulting over time in two separate populations with different mean characteristics. In directional selection, the effect is on one of the tails of the distribution, which results in a shift in the mean characteristic. (From Solbrig OT. Principles and Methods of Plant Biosystematics , 1st edition, ©1970. Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ.) Evolutionary Ecology The study of organismal variability is the purview of evolution. Evolution is change. However, this definition is much too broad. Biological evolution involves the modifi-cation and diversification of organisms over generations. Evolutionary thought and studies have prevailed since Darwin opened the window and exposed a process that seems to explain much of biology.

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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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  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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  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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  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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  Ecology in Action

  • Fred D. Singer(Author)
  • 2016(Publication Date)
  • Cambridge University Press

    (Publisher)

  3.4 How do natural and sexual selection in fl uence fi tness? 71 individuals without (or with reduced quantities of) those traits. Our modern concept of fi tness is a measure of the number of genes an individual contributes to the next generation. Evolu-tionary biologists may estimate fi tness by measuring an individ-ual ’ s reproductive success – the number of genetic offspring an individual produces that survive until reproductive age. In prac-tice this may be dif fi cult to measure in natural populations. Super fi cially, you might assume that a trait that decreases the probability of surviving will always reduce an individual ’ s fi t-ness. But natural selection may favor a trait that decreases the probability of survival if that same trait increases the probability of achieving higher reproductive success. Reproductive success and fi tness Let ’ s consider the redback spider, Latrodectus hasselti , in which males weigh an average of 4.4 mg, while females weigh an average of 256.0 mg. During sperm transfer, the male actually somersaults his body onto the fangs of his mate, effectively committing suicide approximately 65% of the time ( Figure 3.14 ). In cannibalistic matings, females begin to eat the males within a few seconds of beginning copulation – nonetheless, copulation may continue for more than a half hour. Maydianne Andrade ( 1996 ) investigated this phenomenon to see whether male self-sacri fi ce actually increased reproductive success, at the cost of the male ’ s future survival Andrade tested two hypotheses for how self-sacri fi ce might enhance a male ’ s overall reproductive success. One hypothesis, the paternal effort hypothesis , was that male self-sacri fi ce increases male fi tness by providing additional nutrients to the female, who can subsequently lay higher quality or more numerous eggs. The second hypothesis, the mating effort hypothesis , was that male self-sacri fi ce enhances male fertilization success.

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  The Sources of Value

  • Stephen C. Pepper(Author)
  • 2023(Publication Date)
  • University of California Press

    (Publisher)

  The dynamic element of this selective sys- tem is vital energy exhibiting itself as a pattern of inherited traits characterizing an interbreeding population. One of the traits of this dynamic pattern is a dispositional tendency to re- produce itself. This is exhibited in the mechanism of simple fis- Evolution and Survival Value 635 sion, or in the elaboration of this in the process of sexual repro- duction. This dispositional tendency thus generates a reference which may be called the reproductive reference. Within natural selection as a selective system, it functions as a normative ref- erence. That is to say, in the dynamics of this system a parent generation has a disposition to reproduce itself in a succeeding generation. As part of the dynamics of this system, variations in the pat- tern of traits characterizing the interbreeding group are pro- duced. These are subject to selection, partly by genetic factors within the dynamics of reproduction, and partly, and in the end decisively, by their degree of adaptation to the environment of the group. An unadapted variation is immediately or slowly elim- inated and is said to be lacking in survival value; an adapted variation survives and propagates and is said to possess survival value. If a group is in a steady state, well adapted to its environment, its character will change very little from generation to genera- tion. But if through sexual reproduction and mutation, it is capable of closer adaptation to its environment within its range of variations, natural selection will gradually eliminate the less fit and fill out the population with the more fit. If the environ- ment changes radically, the norm of fitness obviously changes accordingly. The dynamics of natural selection within the group still operates as before to preserve the group by passing on to the next generation as much as is adaptable of the pattern of traits of the parent generation.

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  Holistic Darwinism

  Synergy, Cybernetics, and the Bioeconomics of Evolution

  • Peter Corning(Author)
  • 2010(Publication Date)
  • University of Chicago Press

    (Publisher)

  One primary need that was not addressed (above) at the individual level of adaptation is reproduction. Given the fact that individual reproductive output varies widely in any given population, even when the population as a whole may be growing, we believe that this aspect of human adaptation is most appropriately measured at the population level. For a very small pop-ulation with abundant resources, overall population growth is obviously adaptive. But for large human populations, especially those that are press-ing against the limits of their resources, population stability over time is arguably a more adaptive strategy in strict Darwinian terms. This criterion, in turn, implies a bipolar measuring rod; reproduction at the replacement level would be viewed as optimal, and anything either above or below that rate would be less adaptive. (The analogy with Pareto optimality is often invoked in this regard.) A Population Fitness Index It may be that the most inclusive and practicable measure of fitness for any given human population will be found at the aggregate level, where statisti-cal sampling techniques and routine bureaucratic reporting procedures pro-vide a more economical means of acquiring the necessary database. In brief, our Population Fitness Index is based on the degree to which a given pop-ulation falls short of its collective capacity to function normally and engage in productive activity during a given unit of time. Thus, over the period of one year, the maximum number of Darwins available to an entire popula-tion would be equal to the size of the population multiplied by 365. (Births during the year would add units to the total stock, just as deaths would deplete it.) Of course, no population ever realizes its maximum potential productivity. Decrements or losses occur through mortality, morbidity, and a plethora of other restrictions to normal daily activity.

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  Ecological Paradigms Lost

  Routes of Theory Change

  • Beatrix Beisner, Kim Cuddington(Authors)
  • 2005(Publication Date)
  • Academic Press

    (Publisher)

  s , is simply not good enough for many situations.

  13.3.2 Optimization and Game Theory

  An alternative approach for modelling evolution is the use of optimization and game-theoretic models. I treat them together because optimality models can be viewed as a special case of game-theoretic models. Typically, optimality models ignore the details of how the genotype of an organism gives rise to its phenotype and simply seek to characterize the phenotype that yields the highest fitness. Thus, optimality models require the specification of a fitness function, and the underlying assumption is that natural selection proceeds so as to maximize this function (Maynard Smith 1978 , Parker & Maynard Smith 1990 ).

  Optimality thinking and modelling has a long history in evolutionary biology, but the introduction of game-theoretic thinking and modelling to evolutionary biology took this approach to an entirely new level. Optimization models assume that the fitness of an individual depends only on that individual’s phenotype, but it has long been appreciated that an individual’s fitness is determined by the phenotypes of other individuals in the population as well. The introduction of game-theoretic ideas addressed this complexity, and it was motivated largely to model the evolution of social interactions for which optimality models were simply not tenable (Maynard Smith & Price 1973 , Maynard Smith 1982 ). An individual’s fitness as a result of some social interaction depends on the behaviours of all individuals involved; therefore, it no longer even makes sense to ask the question of what is optimal. The optimal behaviour is context specific, depending upon the behaviour of other individuals. As a result, focus moved from optimal phenotypes to evolutionarily stable phenotypes (Maynard Smith 1982 ). An evolutionarily stable strategy (ESS) is one such that if all individuals are using this phenotype, then no single individual can do better by unilaterally altering its phenotype (Maynard Smith 1982 , Bulmer 1994

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