directional selection

Examples of directional selection in the following topics:

• Stabilizing, Directional, and Diversifying Selection

  • Stabilizing, directional, and diversifying selection either decrease, shift, or increase the genetic variance of a population.
  • If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection.
  • When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation.
  • Directional selection occurs when a single phenotype is favored, causing the allele frequency to continuously shift in one direction.
  • 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.

• 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.

• Homo sapiens

  • The forces of natural selection have continued to operate on human populations, with evidence that certain regions of the genome display directional selection in the past 15,000 years.
  • Selection due to disease has been a major force since the development of agriculture and the rise of urbanization.

• Processes and Patterns of Evolution

  • Natural selection can only occur in the presence of genetic variation; environmental conditions determine which traits are selected.
  • The same traits are not always selected because environmental conditions can change.
  • 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.
  • When two species evolve in diverse directions from a common point, it is called divergent evolution.
  • Explain why only heritable variation can be acted upon by natural selection

• Misconceptions of Evolution

  • For example, applying antibiotics to a population of bacteria will, over time, select a population of bacteria that are resistant to antibiotics.
  • In a larger sense, evolution is not goal directed.
  • The terms “evolution” and “natural selection” are often conflated, as the two concepts are closely related.
  • As expounded by Darwin, natural selection is a major driving force of evolution, but it is not the only one.
  • Thus evolution can occur without natural selection.

• Vestigial Structures

  • If there are no selection pressures actively lowering the fitness of the individual, the trait will persist in future generations unless the trait is eliminated through genetic drift or other random events.
  • Although in many cases the vestigial structure is of no direct harm, all structures require extra energy in terms of development, maintenance, and weight and are also a risk in terms of disease (e.g., infection, cancer).
  • This provides some selective pressure for the removal of parts that do not contribute to an organism's fitness, but a structure that is not directly harmful will take longer to be 'phased out' than one that is.
  • Vestigial traits can still be considered adaptations because an adaptation is often defined as a trait that has been favored 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.

• 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.
  • This a common example of disruptive selection.

• 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.
  • For this analysis, population biologists have grouped species into the two large categories, K-selected and r-selected, although they are really two ends of a continuum.
  • K-selected species are those in stable, predictable environments.
  • Populations of K-selected species tend to exist close to their carrying capacity (hence the term K-selected) where intraspecific competition is high.
  • In contrast to K-selected species, r-selected species have a large number of small offspring (hence their r designation).

• Altruism and Populations

  • One explanation for altruistic-type behaviors is found in the genetics of natural selection.
  • Thus, it is of fitness benefit for the worker to maintain the queen without having any direct chance of passing on its genes due to its sterility.
  • The lowering of individual fitness to enhance the reproductive fitness of a relative and, thus, one's inclusive fitness evolves through kin selection.
  • The definition of "pure" altruism, based on human behavior, is an action that benefits another without any direct benefit to oneself.
  • What is clear, though, is that heritable behaviors that improve the chances of passing on one's genes or a portion of one's genes are favored by natural selection and will be retained in future generations as long as those behaviors convey a fitness advantage.