A) A textbook definition would be that genetic drift is: a random change in allele frequency caused by a series of chance occurences that cause an allele to become more or less common in a population. In layman's terms, this means that genetic drift happens when luck makes the genetic pool of the population to deviate from what is expected.
B) The cause for this genetic drift is the aformentioned couple. Because amish communities are small and they select partners from their community, having even a couple of carriers of alleles in a community can make the allele freuency much larger than expected; for example, if the community was 100 persons, the percentage would be in the order of 1%, still much larger than the general population. Thus, the cause here is that a small population had a couple of carriers.
C) Sexual reproduction leads to a mixing of alleles from both mother and father and helps diversity. When a population is isolated, the gene pool is fixed and no new genes can come in, reducing diversity. Also some people that have an allele might die, hitting diversity even more. Finally, having a small population creates a strong pressure in some circumstances that leads to elimination of some traits and diversity.
Rainwater is normally free from dangerous substances, which makes it perfect for irrigation schemes. Added significant benefit is that it decreases the requirement for fresh water. It is important particularly in regions with low water levels.
Another benefit might water harvesting have in these areas with land erosion due to runoff are as follows;
Reduces Water Shortages
In temperate areas, the strain on public water supply can become hard, even in advanced countries. Rainwater decreases the uncertainty of water shortages. Using rainwater for non-essential actions such as spraying the lawn means less pressure on the supply of safe drinking water.
Replenishes Groundwater
Extensive rainwater harvesting projects with the water collected into storage containers or tanks are important to assure a sustainable groundwater level.
Reduces soil erosion
Heavy rainfall generates erosion because some of the soil is transferred along with the passage of water. Soil erosion exhausts agricultural land of topsoil and nutrients, provides to sediment build up in waterways and, in advanced cases, can turn land inadequate for agriculture. Harvesting rain checks a great chance of soil erosion before it can originate.
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