Types of Natural Selection

8 Key excerpts on "Types of Natural Selection"

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  The Emergence and Evolution of Religion

  By Means of Natural Selection

  • Jonathan H. Turner, Alexandra Maryanski, Anders Klostergaard Petersen, Armin W. Geertz, Jonathan Turner(Authors)
  • 2017(Publication Date)
  • Routledge

    (Publisher)

  2 Types of Natural Selection Driving Religious Evolution A Preliminary Review

  The question of questions for mankind—the Problem which underlies all others, and is more deeply interesting than any other—is the ascertainment of the place which Man occupies in nature and of his relations to the universe of things.

  Thomas Huxley (1863: 71)

  Darwinian Natural Selection

  In the Modern Synthesis in biology, the “forces of evolution” include (1) natural selection, (2) mutations, (3) gene flow, and (4) genetic drift, with each affecting the distribution of genes in a population’s “gene pool” (cf. Mayr 2001). Darwinian natural selection is a process that acts on the genetic variation in living populations of organisms whereby individual phenotypes (and hence the underlying genotypes generating these phenotypes) are selected by environments in which life forms seek to survive and reproduce. If the environment changes, selection acts as a sieve by working on phenotypes that reveal features that enable organisms to survive and reproduce, while removing or selecting out those phenotypes (and underlying genes) less compatible with a new environment. Thus, by non-teleological selection, the “fitness” of variations in phenotypes among members of a population is sorted; and from this process the population of life forms evolves (or changes), with evolution defined as “descent with modification.” Table 2.1 highlights the key processes involved in Darwinian selection, while Figure 2.1 lays out Darwinian selection as a process.

  The central force driving natural selection is environmental changes that lead to intensified competition among conspecifics over resources, a competition that favors those phenotypic variations in members of a population that increase fitness or the probability of survival and reproduction. While selection works on individual phenotypes, the emphasis of biology is on the evolution of the population

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  Human Population Genetics

  • John H. Relethford(Author)
  • 2012(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Although the discussions so far have focused on simple models using a single locus with two alleles, a number of extensions can be made for complex models, including selection on quantitative traits, such as height and skin color. A detailed account of the modeling of natural selection on quantitative genetics is beyond the scope of this book, and interested readers are referred to short reviews by Konigsberg (2000) and Mielke et al. (2011), and the classic, more specialized text by Falconer and Mackay (1996). It is useful, however, to consider in a broad sense the general types of selection on quantitative traits relevant to humans.

  One type of selection is stabilizing selection , where those with more extreme phenotypes are selected against, and selection is instead for those with intermediate phenotypes. Because selection is against both extremes, the average phenotype is maintained (stabilized) over time. An example of this type of selection in human populations is for birth weight. Studies of birth weight have shown that babies that are have either very low or very high birth weights have a lower probability of surviving than do those with average birth weight (e.g., Karn and Penrose 1952). The other major type of selection of interest in human populations is directional selection , where one extreme of the range of phenotypes is selected for, producing a shift in average value. On example from human evolution is the noticeable increase in brain size over the past 2 million years. Another example, illustrated in the next chapter, is the change to lighter skin color among humans whose ancestors moved into northern latitudes.

  6.4.5 Natural Selection: Theory and Reality

  Models of natural selection are elegant descriptions to use in what-if scenarios regarding the impact of mode of selection and variation in fitness rates. Such models are very useful in providing a baseline understanding of how selection could work under different sets of conditions. What about the real world? How well do these models apply to actual case studies of selection, particularly in human populations? Application to actual studies of selection in human populations is the focus of the next chapter, but we need to raise a few questions and caveats at this point before moving on to these case studies.

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  Evolutionary Ecology of Plant-Plant Interactions

  An Empirical Modelling Approach

  • Christian Damgaard(Author)
  • 2005(Publication Date)
  • Aarhus University Press

    (Publisher)

  6. Natural selection Mode of selection While most of the genetic variation found in plant populations probably is selectively neutral (Kimura 1983), some genetic variation makes a dif- ference. Some genotypes will code for a phenotype that in a given envi- ronment on average, due to e.g. faster growth or a better defence against parasites, will leave more offspring in the next generation. Natural selection is composed of three principles (Lewontin 1970): 1) Different individuals in a population have different morphologies and physiologies (phenotypic variation). 2) Different phenotypes have different rates of survival and reproduc- tion in different environments (differential fitness). 3) There is a correlation between parents and offspring in the contribu- tion of each to future generations (fitness is heritable). The fitness of a phenotype depends on many different characters that typically are correlated and determined by a combination of geno- typic and environmental factors. The genotypic difference that causes the average increase in the number of descendants may be a single allele (a Mendelian character) or a number of alleles in a favourable combina- tion (a quantitative character), and such alleles will on average increase in frequency due to natural selection (Wright 1969). The consequent in- crease in mean fitness caused by natural selection is equal to the genetic variance in fitness (Fisher 1958, Frank 1997). Since the biological world has so many different life forms and re- productive strategies it is not operational to give a precise and complete definition of fitness (Lewontin 1970). Instead, it is convenient to define fitness relative to the studied organism. Where the ecological success of a plant species at a specific place only depended on fecundity and vi- ability (chapters 2-4), the fitness of an individual sexual plant depends on the number of gametes transmitted to the next generation by both the maternal and the paternal line.

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  The Princeton Guide to Evolution

  • David A. Baum, Douglas J. Futuyma, Hopi E. Hoekstra, Richard E. Lenski, Allen J. Moore, Catherine L. Peichel, Dolph Schluter, Michael C. Whitlock, David A. Baum, Douglas J. Futuyma, Hopi E. Hoekstra, Richard E. Lenski, Allen J. Moore, Catherine L. Peichel, Dolph Schluter, Michael C. Whitlock(Authors)
  • 2013(Publication Date)
  • Princeton University Press

    (Publisher)

  Studies have also found that the strength of directional selection depends on the component of fitness (e.g., survival, fecundity, mating success) involved. For example, directional selection through mating success and fecundity is generally greater, and more consistent in direction over time, than directional selection through survival. This basic pattern holds both among and within studies, as well as for different types of traits and organisms. These results suggest that sexual selection (selection due to differences in mating success) is frequently stronger than viability selection (selection due to differences in survival). In this sense, the struggle for existence may be less intense than the struggle for mates.

  Figure 1. Three different modes of selection can act on a quantitative trait: (A) directional selection, in which extreme phenotypes on one end of the phenotype distribution have the highest fitness and those on the other end have the lowest; (B) stabilizing selection, in which intermediate phenotypes have the highest fitness and extreme phenotypes on either end have the lowest; and (C) disruptive selection, in which extreme phenotypes on both ends of the phenotype distribution have the highest fitness and intermediate phenotypes have the lowest. The graph on the top row shows the distribution of phenotypes in a hypothetical population before selection, the graphs in the middle row show fitness associated with different phenotypes during each of three different modes of selection, and the graphs in the bottom row show the distribution of phenotypes following selection (in each panel, the dashed line shows the distribution of phenotypes before selection).

  If populations of organisms are well adapted to current environments, then we would expect stabilizing selection (figure 1B ) to be common; however, field studies of phenotypic selection in natural populations provide little evidence for significant stabilizing selection in most study systems. In particular, current estimates of quadratic selection gradients suggest that stabilizing selection is no more common than disruptive selection (figure 1C ).

  This surprising result can be explained in several ways. First, there may be trade-offs among various components of fitness, such that (for example) a trait value that increases survival may also decrease mating success or fecundity (see chapter III.12 ). Consequently, net selection on the trait may be less than directional selection via each fitness component. Second, phenotypic and genetic correlations between traits may cause indirect, correlated selection (see chapter III.4

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  Quantitative Genetics

  • Armando Caballero(Author)
  • 2020(Publication Date)
  • Cambridge University Press

    (Publisher)

  Sometimes stabilizing selection can be inferred even though the character is not subject to this type of selection. This occurs when there are mutations with effect on the trait and, in addition, deleterious pleiotropic effects. Such is the case of bristle number in Drosophila, generally considered a neutral trait. If mutations of great effect on the character appear that are also deleterious or even lethal, the extreme individuals that carry them will be removed generating an apparent stabilizing selection (Kearsey and Barnes, 1970). It is possible that selection favours the most extreme individuals of the distribution, eliminating the intermediate ones, what is called diversifying, disruptive or divergent Stabilizing selection Diversifying selection Directional selection Figure 10.2 Types of Natural Selection. The striped portion of the distribution of phenotypic values is favoured by selection. Directional selection produces a shift of the mean in the direction of selection. Stabilizing selection reduces the variance, keeping the mean invariable when the maximum fitness coincides with it. Diversifying selection increases the variation and can produce a bimodal distribution. 10.3 Directional, Stabilizing and Diversifying Selection 239 selection (Figure 10.2). This may occur, for example, when the population is subjected to two (or more) environments that involve the adaptation of different phenotypes to each of them, and implies an increase in the phenotypic variance and a bimodality (or multimodality) of the distribution. The selection may have two components, one directional and another divergent, if the areas selected at each side of the distribution are different. An example of diversifying selection will be given below.

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  The Routledge Handbook of Evolution and Philosophy

  • Richard Joyce(Author)
  • 2017(Publication Date)
  • Routledge

    (Publisher)

  Owing to this struggle for life, any variation, however slight and from whatever cause preceding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection. (1859: 61)

  For Darwin, “this principle” names a wide set of processes whereby valuable variants are generated, maintained, and refined in a population of organisms.

  In contrast to this, modern treatments of evolution are oft en at pains to give far more compact definitions of natural selection. We might be told, for example, that natural selection occurs whenever organisms—or indeed entities of any kind—vary in their “fitness”—roughly speaking, when they vary in their abilities to leave offspring—and whenever these abilities are passed from parents to their babies (e.g., Lewontin 1970). This very general account allows us to ask whether selection might act at several different levels of natural organization—perhaps at the level of the group, or the species, perhaps at the level of the cell or the gene—and it also allows us to ask whether selection might act on entities outside the organic realm—computer viruses, tools, scientific theories. Call this the “inherited variation in fitness” definition.

  We can appreciate one limitation of this definition by imagining a population that obeys all these conditions for the action of natural selection, and which also has very few members. Maybe it is divided into slow and fast runners, babies grow to run at the same speed as their parents, and running speed assists in catching prey. Consistent with this, it might also turn out, let us suppose, that the fastest running predators in this population all happen to die young from infections. These infections are just as likely to affect slow and fast individuals: the fast ones just happen to be unlucky. The result is that the slower individuals dominate. Here, modern biologists will say that “drift ” is at work, in addition to selection.

  So one drawback of our equation of natural selection with “inherited variation in fitness” is that, taken by itself, it does not help us to distinguish natural selection from drift. Modern theorists oft en move on to define selection in a way that allows us to ask which evolutionary “forces” are at work on a population, and which allows us to give a quantified description of how strong those forces are (Sober 1984). In this mode, we need to find a way of understanding what “selection” is that distinguishes it sharply from other “forces” including drift, mutation, and migration. A standard way of doing this is to propose that natural selection is a force that tends to make the fitter variant in a population increase in frequency, and whose strength depends on the fitness differences between the variants in the population. Drift, on the other hand, is then understood as a force whose strength is in an inverse relationship with population size. In small populations it can overwhelm selection. The broad issue of whether evolution should be understood in terms of interacting “forces” has been the subject of lively debate in recent years, with defences from Sober (1984), Stephens (2004), Reisman & Forber (2005), and Sober & Shapiro (2007), and dissent from Walsh et al. (2002), Matthen & Ariew (2002), and Lewens (2010a), among several others.

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  The Works of Charles Darwin: Vol 16: On the Origin of Species

  • Paul H Barrett(Author)
  • 2016(Publication Date)
  • Taylor & Francis

    (Publisher)

  CHAPTER IV NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST  

  Natural Selection – its power compared with man’s selection – its power on characters of trifling importance – its power at all ages and on both sexes – Sexual Selection – On the generality of intercrosses between individuals of the same species – Circumstances favourable and unfavourable to the results of Natural Selection, namely, intercrossing, isolation, number of individuals – Slow action – Extinction caused by Natural Selection – Divergence of character, related to the diversity of inhabitants of any small area, and to naturalization – Action of Natural Selection, through divergence of character, and extinction, on the descendants from a common parent – Explains the grouping of all organic beings – Advance in organizaton – Low forms preserved – Convergence of character – Indefinite multiplication of species – Summary

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  How will the Struggle for Existence, briefly discussed in the last chapter, act in regard to variation? Can the principle of selection, which we have seen is so potent in the hands of man, apply under nature? I think we shall see that it can act most efficiently. Let the endless number of slight variations and individual differences occurring in our domestic productions, and, in a lesser degree, in those under nature, be borne in mind; as well as the strength of the hereditary tendency. Under domestication, it may be truly said that the whole organization becomes in some degree plastic. But the variability, which we almost universally meet with in our domestic productions, is not directly produced, as Hooker and Asa Gray have well remarked, by man; he can neither originate varieties, nor prevent their occurrence; he can only preserve and accumulate such as do occur. Unintentionally he exposes organic beings to new and changing conditions of life, and variability ensues; but similar changes of conditions might and do occur under nature. Let it also be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life; and consequently what infinitely varied diversities of structure might be of use to each being under changing conditions of life. Can it, then, be thought improbable, seeing that variations useful to man have / undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should occur in the course of many successive generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection, or the Survival of the Fittest. Variations neither useful nor injurious would not be affected by Natural Selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions.

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  Population Genetics and Microevolutionary Theory

  • Alan R. Templeton(Author)
  • 2021(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Targets of Selection Above the Level of the Individual Many phenotypes emerge at the level of interactions between two or more individuals. If the same types of interactions are recurrent across generations, they will have continuity over time and can be targets of selection. Targets emerging from interactions among individuals are not difficult to find. For example, of the three major fitness components outlined in Chapter 11, two in general are more appropriately assigned to interacting individuals: mating success and fertility/fecundity. An individual in a dioecious species does not truly have the phenotypes of mating success and fer- tility/fecundity; such phenotypes take on biological reality only in the context of an interaction with another individual. Another common target of selection is intraspecific competition, a phenotype emphasized by Darwin as being important in his theory of natural selection. But competition also takes on biological reality only in the context of interactions among individuals. It is always possible to assign an average marginal phenotype to the individuals engaging in such interactions, but it is more accurate biologically to assign such phenotypes to the interacting individuals. Similarly, indi- viduals who are relatives, particularly in species that have family structures, often interact in com- plex ways that can affect each other’s fitnesses. As we shall see in this section, qualitatively new properties of natural selection emerge when we assign such interaction phenotypes directly to the set of interacting individuals rather than treating each individual as a separate entity without the context of an interaction. Sexual Selection Sexual selection refers to the selection targeting the events that lead up to successful mating or its failure. Many of these events emerge from interactions among individuals and thereby constitute targets of selection above the level of the individual.

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