stabilizing selection

(noun)

a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value

Related Terms

Examples of stabilizing 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.
    • As a result of this stabilizing selection, the population's genetic variance will decrease.
    • Stabilizing selection occurs when the population stabilizes on a particular trait value and genetic diversity decreases.
    • 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.

  • The Goals of Economic Policy

    • Policy is generally directed to achieve four major goals: stabilizing markets, promoting economic prosperity, ensuring business development, and promoting employment.
    • For instance, there may be pressure on the government to reduce inflation, reduce unemployment, and reduce interest rates while maintaining currency stability.
    • If all of these are selected as goals for the short term, then policy is likely to be incoherent, because a normal consequence of reducing inflation and maintaining currency stability is increasing unemployment and increasing interest rates.
    • Examples of rule-based policies are fixed exchange rates, interest rate rules, the stability and growth pact and the Golden Rule.

  • Bibliography

    • Use of academic prediction scales for counseling and selecting college entrants.
    • Stability and change in human characteristics.
    • The state of research on selected alterable variables in education.

  • Time Lags and Targets

    • The Fed uses three criteria to select an intermediate target.
    • Thus, the Fed would never select GDP as an intermediate target.
    • For example, the Fed selected interest rates as its intermediate target.
    • Europe and Japan also emphasize price stability and low inflation rates.
    • If the Fed selected nominal GDP as an intermediate target, then the Fed would be focusing on price stability indirectly.

  • Addition Reactions Initiated by Electrophilic Halogen

    • On the other hand, if two chiral centers are formed in the addition the reaction will be diastereomer selective.
    • We can account both for the high stereoselectivity and the lack of rearrangement in these reactions by proposing a stabilizing interaction between the developing carbocation center and the electron rich halogen atom on the adjacent carbon.
    • The stabilization provided by this halogen-carbocation bonding makes rearrangement unlikely, and in a few cases three-membered cyclic halonium cations have been isolated and identified as true intermediates.
    • The positive charge is delocalized over all the atoms of the ring, but should be concentrated at the more substituted carbon (carbocation stability), and this is the site to which the nucleophile will bond.
    • However, in order to prevent solvent nucleophiles from competing with the halide anion, these non-polar solvents are often selected for these reactions.

  • Halogenation

    • Bromination of alkanes occurs by a similar mechanism, but is slower and more selective because a bromine atom is a less reactive hydrogen abstraction agent than a chlorine atom, as reflected by the higher bond energy of H-Cl than H-Br.
    • alkyl radical stability increases in the order: phenyl < primary (1º) < secondary (2º) < tertiary (3º) < allyl ≈ benzyl.
    • Because alkyl radicals are important intermediates in many reactions, this stability relationship will prove to be very useful in future discussions.
    • The enhanced stability of allyl and benzyl radicals may be attributed to resonance stabilization.
    • The argument that the stability of alkyl radicals is simply derived from R-H bond dissociation energies is flawed.

  • Claisen Condensation

    • Note that this stabilization is only possible if the donor has two reactive alpha-hydrogens.
    • As noted above, formation of this doubly-stabilized enolate anion provides a thermodynamic driving force for the condensation.
    • (iv) The stabilized enolate product must be neutralized by aqueous acid in order to obtain the beta-ketoester product.
    • The selective formation of one of the four possible condensation products is due to the reversibility of these reactions and the driving force provided by resonance stabilization of the enolate anion of 2,4-pentanedione (pKa=9).
    • The last equation (#5) presents an interesting example of selectivity.

  • 1:2-Diastereoselection in Reactions with Chiral Aldehydes

    • A and D predict syn-selectivity, while B and C lead to the anti-diastereomer.
    • Coordination of the aldehyde carbonyl with BF3 increases the electrophilicity of its carbon, and may serve to stabilize an orthogonal orientation of the adjacent phenyl group, as in A (and B).
    • Reaction 4 makes use of a malonate nucleophile and, because of its greater stability and reduced reactivity, Lewis acid activation of the aldehyde is required.
    • The 5,6-syn selectivity is slightly better than that for R = CH3 in equation 1, and conforms to the Felkin-Ahn model.
    • The 4,5-diastereoselectivity remains exclusively syn, but the 5,6-selectivity is changed to anti (60% de) as a consequence of chelation (magenta shaded box).

  • The Trait-Theory Approach

    • Understanding the importance of different core personality traits can help organizations select leaders.
    • Understanding the importance of these traits can help organizations select, train, and develop leaders.
    • Hoffman groups intelligence, conscientiousness, openness to experience, and emotional stability into this category.