IBDP Biology: Main problems with Selective Breeding / Artificial Selection

• Population bottlenecks increase genetic drift - the process of change in allele frequencies.
• They also increase inbreeding due to the reduced pool of possible mates.
• Reduced genetic variation means that the population may not be able to adapt to new selection pressures, such as climatic change or a shift in available resources.

• Hunting reduced their population size to as few as 20 individuals at the end of the 19^th century.
• Their population has since rebounded to over 30,000.
• However, they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

Example: cheetahs

• About 10,000 years ago - because of climate changes - all but one species of the cheetah became extinct.
• With the drastic reduction in their numbers, close relatives were forced to breed.
• Therefore, all cheetahs became closely related.
• Cheetahs exhibit much lower levels of variation than other mammals.
• In most species, related individuals share about 80 percent of the same genes.
• With cheetahs, this figure rises to approximately 99 percent.
• The genetic inbreeding in cheetahs has led to low survivorship (a large number of animals dying), poor sperm quality, and greater susceptibility to disease.
• Cheetahs lack the ability to adjust to sudden changes in the environment, such as disease epidemics, and have unusually high susceptibility to certain viruses.

• A founder effect occurs when a new colony is started by a few members of the original population.

• This small population size means that the colony may have:
  • reduced genetic variation from the original population.
  • a non-random sample of the genes in the original population.
• The new population may be distinctively different, genetically and phenotypically, from the parent population.
• In addition, there is a raised probability of inbreeding, resulting in an unusual number of defects due to recessive genes.
• For example, the Amish populations in the United States, which have grown from a very few founders but have not recruited newcomers.
• They tend to marry within the community
• They and exhibit founder effects phenomena:
• Eg Polydactyly extra fingers and toes are more common in Amish communities than in the US population.
• Founders are likely to be birds and rodents - since they have the best migration abilities.
• In the common case of island speciation, founders may:
  • Have better swimming capabilities
  • Have more fuel reserves and better thermal insulation.
  • Founders may tend to be extremely physically fit - and possess more stamina and strength than most members of the population.

This is the end of this topic

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