Chapter 23 Section 3

A deviation from any of the five conditions is a poteintial cause of __. New __ can alter alelle frequencies, but because mtations are rare, the change from one generation to the next is likely to be very small.

Nevertheless, __ ultimately can have a large effect on allele frequencies when it produces new alleles that strongly influence fitness in a postitive or negative way. __ can affect the frequencies of homozygous and heterozygous genotypes but by itself usually has no effect on allele frequencies in the gene pool. The three mechanisms that alter allele frequencies directly and cause most evolutionary change are __, __, and __.

Darwin's concept of __ is based on differential success in survival and reproduction: Individuals in a population exhibit variaitons in their heritable traits, and those with traits that are better suited to their environment tend to produce more offspring than those with traits that are less well suited.

We now know that selection results in alleles being passed to the next generation in proportions different from their proportions in the present generation.
o By consistently favoring some alleles over others, natural selection can cause __ (evoulution that results in a better match between organisms and their environment)

Chance events can cause allele frequencies to fluctuate unpredictably from one generation to the next, especially in small populations- a process called __.

Allele frequencies can also be affected by chance events that occur during __.

Certain circumstances can result in genetic drift having a significant impact on a population. Two exapmles are the __ and the __.

When a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from th source population, called the __.

The __ might occur when a few members of a population are blown by a storm to a new island, for example. __ -in which chance events alter allele frequenies- occurs in such a case because the storm indiscriminately transports some individuals and their alleles), but not others, from the source population.

The __ probably accounts for the relatively high frequency of certain inherited disorders among isolated human populations.

A sudden change in the environment, like a fire or fllod, may drrastically reduce the size of the population. A severe drop in population size can cause the __, so named becuase the population has passed through a restrictive __ in size. By chance alone, certain alleles may be overrepresented among the survivors, others may be underrepresented, and some may be absent altogether.

Ongoing __ is likely to have substantial effects on the gene pool until the population becomes large enough that chance events have less effect. But even if a population that has passed through a bottleneck ultimately recovers in size, it may have low levels of genetic variation for a long period of time- a legacy of the __ that occurred when the population was small.

One reason it is important to understand the __ is that human actions sometimes create severe __ for other species.

What are the four key points of genetic drift?

Which idea of genetic drift:
o Chance events can cause an allele to be disproportionately over- or underrepresented int eh next generation. Although chance events occur in populations of all sizes, they alter allele frequencies substantially only in small populations

Which idea of genetic drift:
Because of genetic drift, an allele may increase in frequency one year, then decrease the next; the change from one year to the next is not predictable. Thus unlike natural selection, which in a given environment consistently favors some alleles over others, genetic drift causes allele frequencies to change at random over time

Which idea of genetic drift:
By causing allele frequencies to fluctuate randomly over time, genetic drift can eliminate alles from a population. Because evolution depends on genetic variation, such losses can influence how effectively a populatino can adapt to change in the environment

Which idea of genetic drift:
Alleles that are neither harmful nor beneficial can be lost or become fixed etirely by chance through genetic drift. In very small populations, genetic drift can also cause alleles that are slightly harmful to become fixed. When this occcurs, the population's survival can be threatened.

__ nd __ are not the only phenomena affecting allele frequencies. Allele frequencies can also change by __, the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes.

Because alleles are exchanged among populations, __ tends to reduce the genetic difference sbetween populations. If it is extensive enough, __ can result in neighboring populations combining into a single population with a common gene pool.

When neighboring populations live in different environments, alleles transferred by __ may prevent a population from fully adapting to its environment.

True or False:
Sometimes beneficial alleles are transferred very widely.
True False

To sum up, _- like __, can introduce new alleles into a population. But because it can occur at a higher rate than __, __ is more likely than mutation to alter allele frequencies directly. And once __ or __ introduces a new allele to a population, natural selection may then cause the new allele to increse in frequency or decrease in frequency.