Natural selection

  • Created by: portia
  • Created on: 01-08-17 16:13

All organisms have the reproductive potential to increase their populations. Rabbits, for example produce several young in a litter, and each female may produce several litters each year. If all the young rabbits survived to adulthood and reproduced, then the rabbit population would increase rapidly.

Such population explosions are rare in normal circumstances.

  • As a population of rabbits increases, various environmental factors come into play to keep down the rabbits' numbers.
  • These factors may be biotic - caused by other living organisms such as through predation, competition for food, or infection by pathogens
  • or they may be abiotic - caused by non-living components of the environment such as water supply or nutrient levels in the soil

For example,

  • the increasing number of rabbits eat an increasing amount of vegetation, until food is in short supply.
  • the larger population of rabbits may allow the populations of predators such as foxes, soats and weasels to increase.
  • overcrowding may occur, increasing the ease with which diseases such as myxomatosis may spread
  • this disease is caused by a virus that is transmitted by fleas
  • the closer together the rabbits live, the more easily fleas, and therefore viruses, will pass from one rabbit to another

These environmental factors act to reduce the rate of growth of the rabbit population. Of all the rabbits born, many will die from lack of food, or be killed by predators, or die from myxomatosis. Only a small proportion of young will grow to adulthood and reproduce, so population…


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