Studyledge

Variation and Natural Selection: Question 9

Syllabus 18.3

Structured Core 6 marks

A dog breeder wants to produce a line of dogs with an unusually thick, warm coat, suitable for working in cold climates. Each year, she chooses the dogs in her kennel with the thickest coats and breeds them together, repeating this over several generations.

Wild wolves, the ancestors of domestic dogs, are not bred by any breeder. In a wolf population living in a very cold region, individual wolves vary naturally in coat thickness.

(a) State two ways in which the process used by the dog breeder differs from the process of natural selection that could change coat thickness in the wild wolf population. [2]

(b) Explain how natural selection, rather than a breeder, could cause coat thickness to increase in the wild wolf population over many generations. [4]

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Worked solution

Part (a): Two differences between the breeder’s process and natural selection

  • Who or what does the selecting. The dog breeder is a human who deliberately chooses which dogs to breed together, based on the coat thickness she can see; in the wild wolf population, there is no breeder making a choice, instead the cold environment itself determines which wolves are more likely to survive and reproduce.
  • Whether there is a chosen goal. Selective breeding is directed towards a specific feature that the breeder has decided she wants, a thick, warm coat for working dogs; natural selection has no chosen goal at all, it simply favours whatever features happen to give an individual wolf a survival and reproductive advantage in its particular environment.

Any two correctly explained differences along these lines would be credited, for example also referring to the fact that the breeder’s dogs are bred deliberately at a time she chooses, while wolves breed and are exposed to selection continuously in the wild.

Part (b): Explaining the increase in coat thickness by natural selection

The wild wolf population already shows natural variation in coat thickness, meaning some wolves inherit alleles that give them thicker coats than others, before any change in conditions occurs. In a very cold region, wolves with thinner coats lose more body heat and are more likely to die from the cold, or to become weakened and lose out in competition for food and mates, while wolves with thicker coats are better able to stay warm and are more likely to survive these conditions. Because the thicker-coated wolves are more likely to survive, they are also more likely to reproduce, and when they do, they pass their alleles for thicker coats on to their offspring. As this pattern repeats over many generations, an increasing proportion of the wolf population inherits alleles for thick coats, so the average coat thickness of the population gradually increases, without any breeder choosing which wolves mate.

Final answers

  • (a) A human breeder chooses which dogs breed, aiming at a feature she wants; in natural selection, no person chooses, the cold environment determines which wolves survive and reproduce, with no chosen goal.
  • (b) Existing variation in coat thickness, greater survival of thicker-coated wolves in the cold, and their greater reproduction and passing on of alleles together explain the gradual rise in coat thickness across generations.