if Natural selection acts on variation, what are the factors that produce (mutat
ID: 101094 • Letter: I
Question
if Natural selection acts on variation, what are the factors that produce (mutations) and redistribute variation (gene drift and gene flow)? how is new genetic material introduced into breading population?
Make sure to cover the following
*Define Population genetics other forces of evolution
Two forces that redistribute alles between population
1. gene flow- define
- example
- effect on human evolution
2. gene drift - define
- example. genetic bottenocic, fonder effect
- discuss effect on human evoultion
effects
sicle cell, tay-sachs, balaned polymorphism.
Explanation / Answer
Gene flow also called migration — is any movement of individuals, and/or the genetic material they carry, from one population to another. Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries. If gene versions are carried to a population where those gene versions previously did not exist, gene flow can be a very important source of genetic variation. Evolution can also occur as a result of genes being transferred from one population to another. This gene flow occurs when there is migration. The loss or addition of people can easily change gene pool frequencies even if there are no other evolutionary mechanisms operating. For instance, if all red haired people were to leave Scotland, the next generation there would likely have very few people with this trait. The Scottish population would have evolved as would the populations into which the red haired people migrated.
Genetic drift is a mechanism of evolution in which allele frequencies of a population change over generations due to chance (sampling error).Larger populations are unlikely to change this quickly as a result of genetic drift. Genetic drift, unlike natural selection, does not take into account an allele’s benefit (or harm) to the individual that carries it. That is, a beneficial allele may be lost, or a slightly harmful allele may become fixed, purely by chance.A beneficial or harmful allele would be subject to selection as well as drift, but strong drift (for example, in a very small population) might still cause fixation of a harmful allele or loss of a beneficial one.
The bottleneck effect is an extreme example of genetic drift that happens when the size of a population is severely reduced. Events like natural disasters (earthquakes, floods, fires) can decimate a population, killing most indviduals and leaving behind a small, random assortment of survivors.The allele frequencies in this group may be very different from those of the population prior to the event, and some alleles may be missing entirely. The smaller population will also be more susceptible to the effects of genetic drift for generations (until its numbers return to normal), potentially causing even more alleles to be lost.
The founder effect is another extreme example of drift, one that occurs when a small group of individuals breaks off from a larger population to establish a colony. The new colony is isolated from the original population, and the founding individuals may not represent the full genetic diversity of the original population. That is, alleles in the founding population may present at different frequencies that in the original population, and some alleles may be missing altogether. The founder effect is similar in concept to the bottleneck effect, but it occurs via a different mechanism (colonization rather than catastrophe).
In the 1940s, J. B. S. Haldane observed that many red blood cell disorders, such as sickle-cell anemia and various thalassemias, were prominent in tropical regions where malaria was endemic .Haldane hypothesized that these disorders had become common in these regions because natural selection had acted to increase the prevalence of traits that protect individuals from malaria. Just a few years later, Haldane's so-called "malaria hypothesis" was confirmed by researcher A. C. Allison, who demonstrated that the geographical distribution of the sickle-cell mutationin the beta hemoglobin gene (HBB) was limited to Africa and correlated with malaria endemicity. Allison further noted that individuals who carried the sickle-cell trait were resistant to malaria .
Balanced Polymorphism
If natural selection eliminates individuals with detrimental phenotypes from a population, then why do harmful mutant alleles persist in a gene pool? A disease can remain prevalent when heterozygotes have some other advantage over individuals who have two copies of the wild type allele. When carriers have advantages that allow a detrimental allele to persist in a population, balanced polymorphism is at work. This form of polymorphism often entails heterozygosity for an inherited illness that protects against an infectious illness. Examples are fascinating.
Tay-Sachs Disease
Carrying Tay-Sachs disease may protect against tuberculosis (TB). In Ashkenazim populations, up to 11 percent of the people are Tay-Sachs carriers. During World War II, TB ran rampant in Eastern European Jewish settlements. Often, healthy relatives of children with Tay-Sachs disease did not contact TB, even when repeatedly exposed. The protection against TB that Tay-Sachs disease heterozygosity apparently offered remained among the Jewish people because they were prevented from leaving the ghettos. The mutant allele increased in frequency as TB selectively felled those who did not carry it and the carriers had children with each other. Genetic drift may also have helped isolate the Tay-Sachs allele, by chance, in groups of holocaust survivors. Precisely how lowered levels of the gene product, an enzyme called hexoseaminidase A, protect against TB isn't known.
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