bild 3 lecture 6

bild 3 lecture 6 - A) Beak depth B) C) The top picture is...

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Beak depth A) B) C)
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The top picture is G. fortis , and the bottom pictures show two of the drought-resistant plants with tough seeds on which large deep-billed birds could feed during a drought. The three types of selection acting on phenotypes. The picture below shows the three types of selection that can act on phenotypes. a) b) c) These three types of selection have different effect on a population. To be effective, all of them require that the character being selected shows a significant level of heritability . That is, some of the phenotypic variability must be the result of genetic variation, and the variation must be selectable so as to change allele frequencies in the population. Directional Selection and Heritability The shift in bill depth after the drought was the result of directional selection. It was substantial and highly significant. But was it heritable? It might not have been — the change in bill depth might simply have been the result of a change in the environment. If the change was not heritable, so that there was no change in the genetic makeup of the population after the selection, then the phenotypic selection would not have produced heritable evolutionary change of the type predicted by Darwin’s theory.
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How can heritability be detected and measured? One way is shown in the graph below, in which heritability is estimated as gain (selective response) divided by reach (selection differential). One problem with measuring heritability in this way is that the environment might have improved from one generation to the next, making the mean bill depth of the offspring bigger entirely because of environmental influences. Thus, a character may appear to be heritable when it is not. To get around this problem, heritabilities are usually estimated by measuring the relationship between parents and offspring. In the case of the finches, the heritability of bill depth was estimated by calculating the regression of the offspring values on the midparent values, which yields a value from zero — no heritability — to one, 100% heritability. The heritability of bill depth turned out to be high, about 0.8, showing that genetic variation is an important component of the phenotypic variation in this character. Heritabilities for most measurable characters are lower than this.
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= 1976 parent- offspring pairs = = means Mean bill depth of pairs of parental birds that survived the drought = midparent value regression line 1978 parent- offspring pairs Note that the regression lines are steep — if genes played a smaller role, the slopes would be shallower and the relationship between midparent and offspring would be less significant. But did the shift in beak size result from natural selection that altered the frequencies of the genes responsible for bill depth? This analysis shows that there is heritability, and therefore genetic variation, for the bill depth character. But we have not ruled out a role played by a shifting environment. Two types of
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This note was uploaded on 11/05/2008 for the course BILD BILD 3 taught by Professor Woodruff during the Spring '08 term at UCSD.

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bild 3 lecture 6 - A) Beak depth B) C) The top picture is...

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