Examples of directional selection in the following topics:
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- 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.
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- 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.
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- Among the most useful reactions for the synthesis of complex molecules are those that achieve direct selective functionalization of a hydrocarbon moiety.
- Since organometallic reagents function as powerful nucleophiles, selective metal hydrogen exchange, Metalation, would represent a powerful first step to that end.
- The following equation illustrates this Directed ortho Metalation (DoM) reaction, where DMG refers to a directing metalation group and E+ is an electrophile.
- The second diagram above shows the iodination via directed ortho metalation of anisole will be shown.
- Direct electrophilic substitution would normally occur at the meta position, so the action of the amide DMG is particularly noteworthy.
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- 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
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- Note that the selectivity given in the table is for the syn-syn diastereomer versus the other three isomers combined.
- The 1,4-anti-selectivity shown in reactions 6 and 7 is predicted by the transition state model, but the 1,4-syn-selectivity and 1,3-anti selectivity (α':α) in reaction 5 is anomalous.
- Both enolates react with excellent but complementary facial selectivity.
- The exceptional and unusual 1,3-anti selectivity (α':α) shown by the Z-enolate is noteworthy.
- A final example of the remarkable directive influence that neighboring chiral centers may exert on carbon-carbon bond forming reactions is found in the 1,5-diastereoselectivity induced by β-substituents present in methyl ketone donors.
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- Antibiotics are able to selectively target specific types of bacteria without harming the infected host.
- He then proposed the idea that it might be possible to create chemicals that would act as a selective drug that would bind to and kill bacteria without harming the human host.
- The remainder must be tested for their selective toxicities and therapeutic activities, and the best candidates can be examined and possibly modified.
- This involves screening directed towards finding new natural products that inhibit a specific target, such as an enzyme only found in the target pathogen, rather than tests to show general inhibition of a culture.
- In antibacterial production, microorganisms must be isolated, cultured, and tested for growth inhibition of target organisms and for their selective toxicity.
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- Steric hindrance by the pendent isopropyl group directs the reaction to the 2S,3R product.
- Steric hindrance of the Lewis acid with the Z-methyl group changes the facial selectivity of the aldehyde from re to si.
- In seeking to direct such reactions to a single stereoisomer the following features must be controlled.
- As a result, the bulky t-butyl group serves to direct bonding from the re-face of the enolate to the si-face of the carbonyl group in the lithium enolate, but changes the facial selectivity of both reactants in the enol borinate.
- Once again, chelation is prevented, and dipole opposition causes a reversal in facial selectivities, leading to the enantiomeric anti-aldol product.
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- A selectable marker is usually a gene that confers resistance to an antibiotic that would otherwise kill the cells.
- Scientists who do experimental genetics employ artificial selection experiments that permit the survival of organisms with user-defined phenotypes.
- Artificial selection is widely used in the field of microbial genetics, especially molecular cloning.
- Molecular cloning is a set of experimental methods in molecular biology that are used to assemble recombinant DNA molecules and to direct their replication within host organisms.
- Experimental scientists deal with this issue through a step of artificial genetic selection , in which cells that have not taken up DNA are selectively killed, and only those cells that can actively replicate DNA containing the selectable marker gene encoded by the vector are able to survive.
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- To hold the audience's attention, consider their readiness to perceive, the selection of stimuli, and how to maintain current awareness.
- To hold the attention of the audience, a public speaker should consider three important aspects of the process of perception: readiness to perceive, selection of certain stimuli for focus of attention, and state of current awareness.
- Using vocal variety purposely helps the audience know what is important and directs their attention to those elements.
- To understand where he or she wants the audience to direct their attention, the speaker can consider a quick internal summary of an idea.
- Create a narrative that is relevant to the topic and is dramatic for the audience, and use a surprise ending to direct the audience attention to the message.
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- When selecting a topic consider areas in which you have expertise.
- When trying to select a topic for your speech consider any areas in which you are an expert.
- Your fluency will be in direct ratio to two important conditions: your knowledge of what you are going to say, and your being accustomed to telling what you know to an audience.