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By the end of this section, you will be able to:
  • Describe the different types of variation in a population
  • Explain why only heritable variation can be acted upon by natural selection
  • Describe genetic drift and the bottleneck effect
  • Explain how each evolutionary force can influence the allele frequencies of a population

Individuals of a population often display different phenotypes, or express different alleles of a particular gene, referred to as polymorphisms. Populations with two or more variations of particular characteristics are called polymorphic. The distribution of phenotypes among individuals, known as the population variation     , is influenced by a number of factors, including the population’s genetic structure and the environment ( [link] ). Understanding the sources of a phenotypic variation in a population is important for determining how a population will evolve in response to different evolutionary pressures.

 This photo shows four kittens in a basket: two are gray, black, orange, and white, the third cat is orange and white, and the fourth cat is black.
The distribution of phenotypes in this litter of kittens illustrates population variation. (credit: Pieter Lanser)

17.3a patterns of natural selection

As mentioned previously, natural selection is the preferential survival and reproduction of individuals who are most well adapted to their environment. Over time, natural selection generally weeds out unfavorable alleles, and increases the frequency of favorable ones. However, sometimes it's not so simple as an "unfavorable allele" vs. a "favorable allele."

Balanced polymorphism and heterozygote advantage

Sometimes an allele provides a benefit only in certain environments, like the sickle-cell allele. Two copies of the sickle-cell allele give a person sickle-cell anemia, which causes their blood cells to be shaped like a sickle. This reduces the oxygen they can carry, and can cause severe pain and possibly an early death. You would expect this allele to not be favored by natural selection, and in most environments that would be a correct prediction. However, in places where malaria is common, having one or both sickle-cell alleles is actually a benefit. The parasite that causes sickle-cell anemia can live inside of healthy blood cells, but not so much the sickled ones. Therefore, people with both copies of the sickle-cell allele are reistant to malaria, which can be fatal. Because the sickle-cell allele protects against malaria, and because the normal blood cell allele means an individual does not have sickle cell anemia, both of these alleles are favored in areas where malaria is prevalent. Selection for two different alleles keeps both of them in the population -- neither allele is expected to be lost forever. This is called balanced polymorphism     .

Individuals who carry one sickle-cell allele and one normal blood cell allele do not exhibit sickle cell anemia. Some of their blood cells take on the sickled shape, but not enough to cause problems. These people are said to have "sickle cell trait." They are also resistant to malaria. Therefore, being heterozygous for these alleles in an area where malaria is prevalent is the best possible genotype. These individuals are unlikely to die from malaria, and they won't die from sickle-cell anemia. In these areas, the heterozygote is the most fit, a pattern called heterozygote advantage     . However, in places where malaria is not prevalent, their advantage goes away. In these areas, they are just as fit as individuals homozygous for the normal blood cell allele.

Questions & Answers

what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, General biology part i - mixed majors. OpenStax CNX. May 16, 2016 Download for free at http://legacy.cnx.org/content/col11749/1.5
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