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5-2-12 cmi 8570-beagle

Does Inbreeding Always Decrease Genetic Variety?

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E.K. from Sweden writes in response to article What! … no potatoes?:

I have a question on genetic variation.

If a population arises from a few individuals for example you start with two rabbits and then let them reproduce. Do they show an accumulating amount of sicknesses till the point they die off?

Or do the sicknesses over time get weeded out and thus a healthy population result. Are there any observations on that?

Why did not the rabbits in Australia get weakened by inbreeding and were instead so successful?

What happens if you inbreed like that to fast producing animals like fruit flies. Thank you for this site which helps me a lot in making my faith stronger.

CMI’s Dr Don Batten responds:

Dear E.,

The effect of inbreeding depends on several things, including:

  1. The number of genetic defects in the population. This is the most important factor. If organisms do not have any genetic defects, then there is no problem with inbreeding. This would have been the case with the children of Adam and Eve, for example (mutations have caused the accumulation of genetic defects since the Fall in Genesis 3). It would also have been the case with all the animals and plants created in Creation Week (Genesis 1).
  2. The population size. This is the factor that you have focused on. This is not important if the creatures do not have many genetic defects, but becomes more important with more defects. However, it is more a case of having a genetically diverse population, where particular deleterious mutations are not present in all individuals, rather than its size.
  3. The number of offspring possible. Creatures with high breeding potential have more chance of getting rid of deleterious mutations.

The rabbits in Australia are an interesting matter. They have certainly thrived, apparently from a relatively small number introduced in 1859. There were rabbits on the First Fleet (1788) but apparently they did not thrive (maybe they were all eaten as food was in rather short supply in the early days of European settlement). The current population mainly came from the later introduction, although it is not certain that these were the only rabbits introduced. Hares had been brought in earlier (1840?) and became feral and foxes were brought in to control the hares in 1855 (and also for hunting). Clearly both hares and foxes have thrived. There was also a small introduction of sparrows (1863), which have also thrived in Australia.

There is quite a list of troublesome imports into Australia (e.g.: Wikipedia: Invasive species in Australia). It is difficult to find the actual numbers introduced, but there must have been quite small numbers of most of them (except perhaps the cane toad) and they have all thrived….

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