What is an example of directional selection in bacteria?

The development of antibiotic resistance in a population of pathogenic bacteria is an example of directional selection. 


Antibiotic resistance evolves when bacteria with a mutation that confers resistance survive antibiotic exposure, reproduce and pass the resistant allele on to offspring. Over time, the resistant population increases.

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What is directional selection, and what effect does this have on the normal distribution curve?

Directional selection is a type of natural selection where one extreme phenotype is favoured over other phenotypes, shifting the normal distribution curve in that direction.


An example is antibiotic resistance in bacteria, where the resistant bacteria survive and reproduce.

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What is stabilising selection, and what effect does this have on the normal distribution curve?

Stabilising selection selects for the average phenotype and selects against extreme phenotypes.


Stabilising selection narrows the normal distribution curve.


An example is human birth weight, where infants of average weight have higher survival rates.

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What are the three main types of selection in evolution?

  1. Directional selection
  2. Stabilising selection
  3. Disruptive selection

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What is the role of variation in natural selection?

Variation provides a range of phenotypes in a population, increasing the chance that some individuals will have the alleles that code for the advantageous traits.


Individuals with these advantageous traits survive and reproduce in changing conditions, passing on the beneficial alleles to offspring and leading to natural selection.

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How can directional selection lead to a change in allele frequency and so to evolution?

Directional selection leads to an increase in the frequency of alleles that code for one extreme phenotype, usually when there is a change in selection pressures, or when a new allele arises by mutation.


This leads to that extreme phenotype increasing in frequency in the population, leading to evolution  over time.

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How can stabilising selection lead to a change in allele frequency and so to evolution?

Stabilising selection leads to an increase in the frequency of alleles that code for the average phenotype, and to a reduction in the frequency of alleles at the extremes.


This leads to the average phenotype increasing in frequency in the population, leading to evolution over time.

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What is disruptive selection, and what effect does this have on the normal distribution curve?

Disruptive selection selects for extreme phenotypes and against the intermediate phenotype, often when an environmental factor takes two or more distinct forms.


Disruptive selection shifts the normal distribution curve, so the distribution of phenotypes shows two peaks.


An example is birds that adapt to have either small or large beaks for two food sources, but no intermediate-sized beaks as they are less effective at obtaining either type of food.

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How can disruptive selection lead to changes in allele frequency and so to evolution?

Disruptive selection leads to an increase in the frequency of alleles that code for both extreme phenotypes, and to a reduction in the frequency of alleles that code for the intermediate phenotype.


This leads to both extreme phenotypes increasing in frequency in the population, leading to evolution over time.

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