6.2.2.2 Selection may lead to bottlenecks

Very strong natural selection, for example as a consequence of an outbreak of a very infectious and lethal disease, will result in a severe decrease in population size in combination with a change in allele frequency. Only the animals with some degree of resistance will have survived the infection, together with the lucky few that did not get infected. Those are the animals that need to build up the population again. Consequently, the allele frequency in future generations will depend on the allele frequency in that generation just after the bottleneck: the large decrease in population size. Some alleles that were carried by animals that were very susceptible to the disease will have been dramatically reduced in frequency or lost completely. An infamous example of a strong bottleneck is the outbreak of rinderpest in Africa in 1890. It swept through the entire continent and killed 80 to 90% of the indigenous cattle, buffalo, eland, giraffe, wildebeest, kudu, and antilopes (Mack, 1970). Because cattle and goats were killed, it had large social consequences and approximately one third of the human population of Ethiopia and two third of the Masaai of Kenya and Tanzania died of starvation. Rinderpest still causes problems at about 10 year intervals, but in 1890 veterinary support was very limited and the spread of the disease could not be stopped.

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Figure 4. Cattle that died of rinderpest in Africa in 1890

Bottlenecks in domestic animal species can also occur because the specific breed lost its original purpose, but has gained a new purpose before the breed died out. Example of this is the Friesian horse that originally was used for work on the farm and lost its purpose with the introduction of the tractor. The breed was severely reduced in size severely before it started to become popular as a breed for leisure sports: both for harness driving and under saddle. Nowadays it is the second largest pure bred horse breed in the Netherlands (the Shetland ponies are the largest breed).

There are other examples of breeds that have lost their popularity, such some of the old Dutch cattle breeds. You can say they are still in the bottleneck because they have reduced in size dramatically, but there are no signs of recovery yet. However, others, such as the Brandrode and the Friesian Red, are showing signs of population growth.