Creating a Bottleneck Effect in an Ostrilope Population in the Sim Goal: In fewer than 30 generations, change the starting ostrilope population into a population of at least 70 individuals with low variation in color traits (4 traits or fewer). Do this without adding a predator.

Do:

Decide on a strategy to reduce the variation of the ostrilope population.
Open the Natural Selection Simulation and open the mode: Color Change Mission.
Try out your strategy by making any changes in either Build or Run.
Use the histograms in Analyze or the Traits Histogram Window in Run to see if your strategy was successful.
Tips:

You can modify abiotic factors in Run at any time by pressing the Abiotic Factors icon in the lower-right corner of the screen.
Do not make any changes to the starting ostrilope population settings.
Return to Build and press REBUILD to reset the Sim to the default settings.

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Bottlenecks and founder effects

Genetic drift can cause big losses of genetic variation for small populations.

Population bottlenecks occur when a population's size is reduced for at least one generation. Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population's genetic variation by a lot, even if the bottleneck doesn't last for very many generations. This is illustrated by the bags of marbles shown below, where, in generation 2, an unusually small draw creates a bottleneck.

Loss of genetic variation as a result of a population bottleneck

Download this graphic from the Image library.

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, because the genetic variation that selection would act on may have already drifted out of the population.

Elephant seal

An example of a bottleneck

Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000 — but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

Founder effects

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.

For example, the Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington's disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency. This effect is easy to recognize in genetic diseases, but of course, the frequencies of all sorts of genes are affected by founder events.

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