HOW DOES GENETIC DIVERSITY BECOME REDUCED?
How are gene variants lost?
Genetic drift is the name for a process by which the diversity of alleles in a new generation becomes different from that of the previous generation. Changes in migration, selective pressures, geographical isolation, establishment of a new population from only a few individuals (founder effect) all result in genetic drift. Each of these factors affects which individuals will be part of the new generation and therefore what the sample of alleles will be that determines the future of that species. A population is more susceptible to genetic drift when the number of individuals becomes small.
Sometimes a population can have many members but still undergo loss of genetic diversity. This can happen through geographical isolation. If a new geographical block is imposed (e.g. the new course of a river or the development of a subdivision creating houses, roads, and sidewalks on a previously empty meadow) a population of plants or animals may become separated into two groups. When this happens, the pool of gene variants in the two separated populations may differ from one another.
For example, a flower species may have two forms, and because of a new subdivision, they might become separated to two ends of the development. The first form, a tall, sturdy, but unattractive plant, may become isolated to the north end of the subdivision while the second form, a weaker, prettier plant, is isolated to the south end. Even though the new homeowners may preserve both populations on their various lots, the two types of plants are sufficiently isolated from each other that they can no longer exchange genes. In this situation, each type is exposed to environmental pressures without the ability to crossbreed with the other type to form plants with new, perhaps more advantageous, combinations of genes. The new pressures created by building the development may affect the two types of plants differently. For example, the weaker variety might dwindle away due to the lack of shade caused when nearby trees were cut down for the subdivision. If this form had been able to crossbreed with the tall, sturdy variety, plants with the prettier flower type might have survived. Since they were unable to do so, the gene responsible for the attractive flower might be lost permanently.