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How does natural selection lead to adaptation and change in populations over time?

Explain natural selection as a mechanism of evolution and how it leads to adaptation in populations over time (North Carolina Standard Course of Study, Biology, LS.Bio.9).

A standard-level answer on natural selection for the North Carolina Biology EOC: the conditions Darwin identified, how variation and selection produce adaptation, and examples such as antibiotic resistance.

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  1. What this topic is asking
  2. The conditions for natural selection
  3. Adaptation and the source of variation
  4. A worked example: antibiotic resistance
  5. Try this

What this topic is asking

North Carolina LS.Bio.9 asks you to explain natural selection as a mechanism of evolution and how it leads to adaptation. For the Biology EOC you need the conditions Darwin identified (variation, overproduction, a struggle to survive, and differential survival and reproduction), the link to inherited variation from mutation, and clear examples such as antibiotic resistance. Items often give a scenario and ask you to explain the change using selection.

The conditions for natural selection

Darwin's reasoning rests on four observations, all of which must hold:

  • Inherited variation. Individuals in a population differ, and some of those differences are heritable (passed to offspring through genes).
  • Overproduction. Organisms produce more offspring than the environment can support.
  • Struggle to survive. Because resources are limited, there is competition, and not all offspring survive.
  • Differential survival and reproduction. Individuals with traits that suit the environment are more likely to survive and reproduce, so they leave more offspring.

The result, repeated over many generations, is that the favorable traits become more common. This is sometimes summed up as "survival of the fittest," where fitness means reproductive success, not strength.

Adaptation and the source of variation

This is the deep link to the genetics modules: mutation creates new alleles, and natural selection decides which spread. Without the variation from mutation and meiosis, selection would have nothing to act on.

A worked example: antibiotic resistance

Antibiotic resistance is the EOC's favorite real example, because it shows selection on a short timescale.

  1. A population of bacteria contains a few individuals that, by chance mutation, carry an allele for resistance.
  2. An antibiotic is applied. It kills the non-resistant bacteria but the resistant ones survive.
  3. The survivors reproduce, passing on the resistance allele.
  4. Over repeated treatments, the resistant allele becomes common, and the population is now largely resistant.

Notice that the antibiotic did not create resistance; it selected for the resistance that some bacteria already had. The same logic explains pesticide resistance in insects and the peppered-moth color change during industrial pollution.

Try this

Q1. State the four conditions required for natural selection. [4]

  • Cue. Inherited variation; overproduction of offspring; a struggle to survive (limited resources); differential survival and reproduction of the better-suited individuals.

Q2. Explain why an antibiotic does not "create" resistance in bacteria. [2]

  • Cue. The resistance already exists by mutation in a few bacteria; the antibiotic selects for it by killing the non-resistant ones, rather than causing the resistance.

Exam-style practice questions

Practice questions written in the style of NCDPI exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

NC Biology EOC (style)1 marksWhich condition is required for natural selection to occur in a population? (A) All individuals are identical. (B) There is inherited variation and a struggle to survive. (C) The environment never changes. (D) Organisms can choose to adapt.
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A 1-point item on the conditions for natural selection.

The correct answer is B. Natural selection needs inherited variation, more offspring than can survive (a struggle to survive), and differences in survival and reproduction. A removes variation, C is unnecessary, and D is wrong because organisms do not choose to adapt.

Selection needs inherited variation plus a struggle to survive and reproduce.

NC Biology EOC (style)2 marksA population of bacteria is treated repeatedly with an antibiotic, and over time most bacteria become resistant. (a) Explain how natural selection produced the resistant population. (b) State where the resistance variation originally came from.
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A 2-point item on natural selection in action.

(a) 1 point: a few bacteria already had a resistance allele; the antibiotic killed the non-resistant bacteria, so the resistant ones survived and reproduced, passing on resistance until most of the population was resistant.
(b) 1 point: the resistance variation originally arose from a random mutation, the source of new alleles.

Markers reward a survival-and-reproduction explanation and identifying mutation as the source of the variation.

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