natural selection

Examples of natural selection in the following topics:

• No Perfect Organism

  • Natural selection cannot create novel, perfect species because it only selects on existing variations in a population.
  • However, natural selection cannot produce the perfect organism.
  • Natural selection can only select on existing variation in the population; it cannot create anything from scratch.
  • Natural selection is also limited because it acts on the phenotypes of individuals, not alleles.
  • Natural selection acts on the net effect of these alleles and corresponding fitness of the phenotype.

• Natural Selection and Adaptive Evolution

  • Natural selection drives adaptive evolution by selecting for and increasing the occurrence of beneficial traits in a population.
  • Natural selection only acts on the population's heritable traits: selecting for beneficial alleles and, thus, increasing their frequency in the population, while selecting against deleterious alleles and, thereby, decreasing their frequency.
  • Natural selection does not act on individual alleles, however, but on entire organisms.
  • Natural selection acts at the level of the individual; it selects for individuals with greater contributions to the gene pool of the next generation, known as an organism's evolutionary fitness (or Darwinian fitness).
  • Through natural selection, a population of finches evolved into three separate species by adapting to several difference selection pressures.

• Charles Darwin and Natural Selection

  • Charles Darwin and Alfred Wallace independently developed the theories of evolution and its main operating principle: natural selection.
  • Darwin called this mechanism natural selection.
  • Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature.
  • Papers by Darwin and Wallace presenting the idea of natural selection were read together in 1858 before the Linnean Society in London.
  • His book outlined his arguments for evolution by natural selection.

• Evolutionary Psychology

  • Evolutionary psychology seeks to understand human behavior as the result of psychological adaptation and natural selection.
  • It seeks to identify which human psychological traits are evolved adaptations—that is, the functional products of natural selection.
  • Evolutionary psychology stems from Charles Darwin's theories of evolution, adaptation, and natural selection.
  • The brain's adaptive mechanisms have been shaped over time by natural and sexual selection.
  • Charles Darwin's theory of natural selection has been highly influential in the field of evolutionary psychology.

• The Galapagos Finches and Natural Selection

  • Darwin called this mechanism of change natural selection.
  • Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature.
  • Demonstrations of evolution by natural selection can be time consuming.
  • Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976 and have provided important demonstrations of the operation of natural selection.
  • This was clear evidence for natural selection of bill size caused by the availability of seeds.

• Defining Population Evolution

  • Genetic variation in a population is determined by mutations, natural selection, genetic drift, genetic hitchhiking, and gene flow.
  • Just as mutations cause new traits in a population, natural selection acts on the frequency of those traits.
  • In natural selection, those individuals with superior traits will be able to produce more offspring.
  • As novel traits and behaviors arise from mutation, natural selection perpetuates the traits that confer a benefit.
  • As mutations create variation, natural selection affects the frequency of that trait in a population.

• Stabilizing, Directional, and Diversifying Selection

  • If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection.
  • For example, in a population of mice that live in the woods, natural selection will tend to favor individuals that best blend in with the forest floor and are less likely to be spotted by predators.
  • Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages.
  • Diversifying selection occurs when extreme values for a trait are favored over the intermediate values.This type of selection often drives speciation.
  • Different types of natural selection can impact the distribution of phenotypes within a population.In (a) stabilizing selection, an average phenotype is favored.In (b) directional selection, a change in the environment shifts the spectrum of phenotypes observed.In (c) diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against.

• Processes and Patterns of Evolution

  • Natural selection can only occur in the presence of genetic variation; environmental conditions determine which traits are selected.
  • Natural selection can only take place if there is variation, or differences, among individuals in a population.
  • The direction of natural selection shifted so that plants with small leaves were selected because those populations were able to conserve water to survive the new environmental conditions.
  • Natural selection acts on individual organisms, which in turn can shape an entire species.
  • Explain why only heritable variation can be acted upon by natural selection

• Frequency-Dependent Selection

  • In frequency-dependent selection, phenotypes that are either common or rare are favored through natural selection.
  • Another type of selection, called frequency-dependent selection, favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection).
  • An interesting example of this type of selection is seen in a unique group of lizards of the Pacific Northwest.
  • In this scenario, orange males will be favored by natural selection when the population is dominated by blue males, blue males will thrive when the population is mostly yellow males, and yellow males will be selected for when orange males are the most populous.
  • Negative frequency-dependent selection serves to increase the population's genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes.

• Theories of Life History

  • The regulation of population growth by these factors can be used to introduce a classical concept in population biology: that of K-selected versus r-selected species.
  • The first variable is K (the carrying capacity of a population; density dependent), and the second variable is r (the intrinsic rate of natural increase in population size, density independent).
  • Populations of K-selected species tend to exist close to their carrying capacity (hence the term K-selected) where intraspecific competition is high.
  • By the time they reach adulthood, they must develop skills to compete for natural resources.
  • In contrast to K-selected species, r-selected species have a large number of small offspring (hence their r designation).