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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

where we get a research paper on Nano chemistry....?
Maira Reply
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
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
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
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.
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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