Understanding the importance of population size is fundamental to understanding genetic drift (see the earlier section "Situations in which drift is important" for details). But population sizes don't remain constant; they fluctuate over time, sometimes growing larger and sometimes smaller. To understand how genetic drift is operating in nature, it helps to understand the factors that result in a small population. Here are a couple:
1 Fluctuations in population size: The mere fact that a population is large today doesn't mean that it wasn't small at some other time or that it won't decrease someday. The populations of many organisms that need water may drop to low levels during a drought, for example. When a population is or becomes exceedingly small, the phenomenon is called a population bottleneck (discussed later in this chapter).
1 Founder effects: Founder effects refers to those chance events that occur when a population is founded in a new location by a small number of individuals from a population somewhere else.
As a population shrinks (which can happen for a variety of reasons), it begins to feel the effects of genetic drift more acutely, mainly because while it's growing smaller, it's losing diversity. This loss is a bad thing, because diversity can protect a species or a group of individuals from the vagaries of fate. This discussion is important for a couple of reasons, one of which may appeal to folks in general (tree-huggers and animal-lovers in particular), and the other may appeal primarily to scientists (but is cool anyway):
1 It informs conservation efforts. Numbers aren't enough when it comes to saving animals from extinction. Diversity in the endangered population is key, and genetic drift undermines diversity.
1 It lets scientists figure out when one group diverged from another. I
know — this reason lacks the pizzazz of saving cute little furry creatures, but it's still important.
The following sections go into more details.
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