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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 demand
TECK Reply
the amount of a good that buyers are willing and able to purchase
what is population
Amadou Reply
The people living within a political or geographical boundary.
what happens to price and quantity when demand curves shift to the right
Asha Reply
price level goes up. quantity demand increases
example- inferior goods
demand law
Its states that higher the price the of the commodity, and lower the quantity demanded
I am confused but quantity demand will increase.
No. That's the law of supply
the price goes down and quantity demanded increases
Price goes down Demand quantity increases
what happens to price and quantity when supply curve shifts left?
Asha Reply
price level will increase
quantity demand will decrease
what is inflation
Pop Reply
inflation is a general and ongoing rise in the level of prices in an entire economy.
is the pasistance increase in the price of a country economy
how does inflation affects the economy of a country? what is deflation?
deflation can simply be define as the persistence decrease in price of a countrys economy
the revenge of malthus relates "revenge" with "commodity prices". collect data for 3 commodoties and check their price evolution
Jamshi Reply
what is elasticity
dubela Reply
Elasticity is an economics concept that measures responsiveness of one variable to changes in another variable.
Computer software represents
Mboledi Reply
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plz Reply
Hey what are you trying to mean?
what is Asset
like a banana
demand is the process whereby consumers are willing and able to purchase a particular product at various price over a given period of time
Samuel Reply
The law of dinimish
Frank Reply
What is the law of dinimish
What is the law of dinimish
What is the law of dinimish
opportunity cost is to forgo something for another.
jackie Reply
what is financial market
Asheeru Reply
what is demand
Levinel Reply
Demand is an economic principle referring to a consumer's desire to purchase goods and services and willingness to pay a price for a specific good or service.
explain any three exceptions to the law of demand
Emma 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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