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The World of Darwin
Decreased Variation
Sources of Decreased Variation Mutation, recombination, and gene flow all act to increase the amount of variation in the genotypes of a given population. There are also forces at work that act to decrease this variability.

Selection differentially removes certain phenotypes from the population, and thus determines which individuals will contribute their genes to the next generation.

Obviously, many factors influence the chances of any individual's reproductive success, and it is quite rare that any one gene alone will dramatically alter the fitness of an individual.

But, as natural selection removes certain combinations of genes from a population, the number of variants left steadily shrinks.

Normally, plants have variants at 18% of their genes (i.e., they are heterozygous at 18% of their loci). This level of variation is critical for the survival of these species, because if one combination of genes suddenly becomes detrimental (if the climate becomes drier, for example), then there will be lots of other combinations left to try.

When selection reduces this heterozygosity, as humans have done in the case of the corn plant Zea mays, the plant's survival is threatened.

In the 1970s, a corn blight struck and almost devastated the domestic corn crop. These plants have been so highly selected for traits, such as rapid growth and cob size, that they were incredibly homogeneous.

Without the possibility of forming new combinations of genes that could resist the corn blight, the corn plant almost became extinct. (Humans rescued the corn plant by adding back some variant genes from a Mexican grass!).

Genetic Drift:
the element of chance
In all aspects of life, chance often has much to do with the outcome of a particular situation. Losing one individual by accident rather than by natural selection will have little effect in large populations.

In small populations, however, nonrandom events can have quite serious consequences.

Genes lost to a small population, as a result of a chance event, means that certain combinations cannot arise and other combinations will occur very infrequently. These combinations will, therefore, never be tested by natural selection.

A direction may be given to a small population's course of evolution just because a different possibility never saw the light of day, and thus was never tried.

Such genetic drift can have far ranging effects, and small populations can play quite major roles in sudden, unexpected, and eventful changes in evolutionary direction.

If a natural disaster randomly wipes out most of the variants in a population, this creates a bottleneck through which only a few genotypes can squeeze.

The term is derived from the small amount of liquid that can squeeze through the neck of a bottle.

Such bottleneck survivors have a very different set of gene combinations from those found in the original population, and thus the fitness of the population will have changed. Some populations become extinct, but others, with more favorable adaptations, progress and create new populations with alternative fates.

Figure legend: Genetic Drift: In small populations, the accidental loss of one individual has a much greater impact on the gene pool that a similar loss on a large population.

In this example, the ratios of purple fish to yellow fish starts at 2:4 (or 0.5) in the small population and 8:16 (or 0.5) in the large population.

After the loss of one purple fish (by accident) from both populations, this ratio is reduced to 1:4 (or 0.25) for the small population and 7:16 (or 0.4375) for the large population.

The ratio change is much greater for the small population.

© 2001, Professor John Blamire