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Evolution in Action
Bottleneck effect
Bottleneck effect

Peccary Populations: In small peccary populations, the gain or loss of one individual can make a very big difference to the population gene pool.
Large population size is one stabilizing condition that is not met by peccaries.

Assume for a moment that 100 peccaries could be brought together in a study area. Blood samples are taken from each one, and the analysis of the various alleles for the bristle-length gene is carried out.

When the data are assembled the following numbers are produced:

Genotype BB - frequency = 0.64
Genotype Bb - frequency = 0.32
Genotype bb - frequency = 0.04

In practical terms, what this analysis means is that in a herd of 100 peccaries, only 4 individuals would have the phenotype of short bristles (the bb genotype). In large herds of peccaries, this genotype, although rare, would be represented at the appropriate frequency.

But, peccary herds rarely consist of more than 15 individuals, not 100 or greater. It is therefore very possible that these smaller herds have no members of the bb genotype and thus none show the shorter bristle phenotype.

In this small herd, the frequency of the bb genotype would be reduced to zero, and not be the value of 0.04 seen in the larger herd.

If, by chance, a small herd of peccaries did include one member with the genotype bb, then the frequency of this genotype in this herd would be 0.06 not the original 0.04.

There is quite a difference between the frequency of this genotype as found in a large herd (0.04) and the two smaller herds (0.00 and 0.06).

When a population is small, the presence or absence of a single individual can cause large variations in the frequencies of genes and in the ratios of genotypes. Small populations, therefore, have a tendency to be unstable with regard to their gene frequencies.

Sudden reductions in population size can also alter the resulting gene pools. Any environmental condition in which many individuals are killed and only a small number survive, creates a bottleneck effect.

This acts like a narrow passageway, or bottleneck, through which only a few may pass. For example, when crossing a river in flood, many peccaries may be killed, leaving only one or two survivors. For reasons given above, the gene pool of the survivors will most likely be very different from the gene pool of the original population, and the potential for new gene combinations and new adaptations is reduced.

Figure legend: Bottleneck effect. From an original peccary population with its own gene pool, a few individuals sqeeze though a "bottleneck".

In this case a few peccaries with the genotype bb survive a catastrope. These individuals become the founders (originators) of a new peccary population.

As they reproduce, the new gene pool is very different from that of the original population.

© 2001, Professor John Blamire