Evidence for balancing selection can be found in the number of alleles in a population which are maintained above mutation rate frequencies. All modern research has shown that this significant genetic variation is common in panmictic populations. It is the field experience of Darwin, Wallace and others, that natural populations in the wild are extraordinarily varied. Museum collections of single species tell the same story.
There are several ways balancing selection works to maintain polymorphism. The two major and most studied are heterozygote advantage and frequency-dependent selection.
In heterozygote advantage, or "heterotic balancing selection", an individual who is heterozygous at a particular gene locus has a greater fitness than a homozygous individual. Polymorphisms maintained by this mechanism are "balanced polymorphisms".
A well-studied case is that of sickle cell anemia in humans, a hereditary disease that damages red blood cells. Sickle cell anemia is caused by the inheritance of a variant hemoglobin gene (HgbS) from both parents. In these individuals, hemoglobin in red blood cells is extremely sensitive to oxygen deprivation, and this causes shorter life expectancy.
A person who inherits the sickle cell gene from one parent, and a normal hemoglobin gene (HgbA) from the other, has a normal life expectancy. The heterozygote is resistant to the malarial parasite which kills a large number of people each year. The heterozygote frequency is kept high because of the fierce selection against both the homozygotes.
The heterozygote has a permanent advantage (a higher fitness) wherever malaria exists.
Frequency-dependent selection occurs when the fitness of a phenotype depends on its frequency.
In "positive frequency-dependent selection" the fitness of a phenotype increases as it becomes more common. In "negative frequency-dependent selection" the fitness of a phenotype increases as it becomes less common. For example, in prey switching, rare morphs of prey are fitter because predators concentrate on more frequent morphs.
The fitness of a genotype may vary greatly between larval and adult stages, or between parts of a habitat range.
The fitness of a genotype may depend on the fitness of other genotypes in the population: this covers many natural situations where the best thing to do (from the point of view of survival and reproduction) depends on what other members of the population are doing at the time.
Philip Stanhope, 4th Earl of Chesterfield
Philip Dormer Stanhope, 4th Earl of Chesterfield PC KG (22 September 1694 – 24 March 1773) was a British statesman and intellectual.
Stanhope was born in London. He got his schooling at Trinity Hall, Cambridge. After that, he went on the Grand Tour of the continent. He became gentleman of the bedchamber to the Prince of Wales.
In 1715 he entered the House of Commons as Lord Stanhope of Shelford and member for St Germans. He left for Paris and sent the government valuable information about the Jacobite plot. In 1716 he returned to Britain and sat in the House of Commons. He liked to take part in the debates.
He took his seat in the House of Lords. In 1728 he was sent to the Hague as ambassador. He did a good job, and got the Order of the Garter in 1730, and the position of Lord Steward. After he did not support Walpole's Excise Bill, he was dismissed from his stewardship. He spent years opposing the ruling party in the House of Lords. First against Robert Walpole and then against King George II of England. In 1744, the "Broad Bottom" party, led by Chesterfield and Pitt, came into office. They ruled together with the Pelhams.
He was then sent to the Hague as ambassador a second time. The object of his mission was to persuade the Dutch to join in the War of the Austrian Succession. His mission was successful. When he got back to London, he was made Lord-Lieutenancy of Ireland. This was a position he wanted for a long time.
He took the his role seriously. He established schools and manufacturing in Ireland. In politics, he found a way to work with both Protestant and Roman Catholic parties.
He also still took his seat in the Upper House. In 1751 he helped in making the Gregorian calendar a fact. That Act of Parliament is sometimes known as Chesterfield's Act. After he slowly became deaf, he had to stop his work in the parliament.
He lived for some years at the Ranger's House, Chesterfield Walk, Greenwich, London.
In politics, he is best known for the time he was Lord Lieutenant of Ireland. As a writer, he is best known for his "Letters to his Son". Those letters were not really written to be read by the general public. His widow Eugenia Stanhope first published them in 1774. "Letters to his Godson" was published in 1890. Both sets of letters are written with great style and knowledge.
Chesterfield County, Virginia and Chesterfield County, South Carolina in the United States were named in his honour. He also plays a role in the 1757-1758 novel "The Virginians" by William Makepeace Thackery.
Things he has probably said:
Grand Tour
The Grand Tour was the name of travel through Europe by rich European (mostly British) young men between 1660 until about 1840. It was done to learn about European society and art. The idea was that what they learned on their travels could be used to help those that stayed at home. Italy was often the final country to visit. They would mostly travel with servants and a teacher called a "bear-leader".
Which places the Grand Tour would visit changed through the years. An example would be a British tourist would go from Dover, England and crossed the English Channel to Ostend, in the Spanish Netherlands/Belgium, or Calais, or Le Havre in France.
There they could rent or buy a carriage and travel to Paris. Or they could make the trip by boat, traveling up the Seine to Paris, or up the Rhine to Basel.
In Paris, the traveler could take lessons in French, dancing, fencing, and horse riding. Paris was popular because of the language and fashion of French high society.
From Paris he would go to Switzerland for a while, often to Geneva. From there the traveler would make a difficult crossing over the Alps into northern Italy.
Once in Italy the tourist would visit Turin, then might spend a few months in Florence. Other Grand Tourists would be in Florence to welcome him. A trip to Pisa then on to Padua, Bologna, and Venice.
Venice was seen as the most important place for Italian architecture, and was very important place to visit.
From Venice the traveler went to Rome to study the ruins of ancient Rome. Some travelers also visited Naples to study music. Naples was often the final destination before turning back.
From here the traveler went north through the Alps to the German-speaking parts of Europe. From there travelers visited Holland and Flanders before returning across the Channel to England.
Alnico
Alnico is a type of alloy made up of iron with aluminium (Al), nickel (Ni) and cobalt (Co). The name is an acronym from Al-Ni-Co. Alnico may also contain copper and sometimes titanium.
Alnico alloys are ferromagnetic (a mechanism by which some materials form permanent magnets). Alnico alloys also have a high coercivity (they do not easily lose their magnetism) and are used to make permanent magnets. Before rare-earth magnets were developed in the 1970s, alnico magnets were the strongest type of magnet. The development of Alnico began in 1931 when T. Mishima in Japan found that an alloy of iron, nickel, and aluminium had double the coercivity of the best magnets of that time. (the coercivity of Alnico is 400 oersted).