Biology 1001A Chapter Notes - Chapter 9: Qualitative Variation, Phenotype, Chromosome
27 Dec 2015
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18.1: VARIATION IN POPULATIONS
Phenotypic variation: dierences in appearance or function between
individual organisms
oIf dierence is heritable, it is passed through generations
18.1a: Phenotypic Variation
Darwin’s theory recognized importance of heritable phenotypic
variation
Micro evolutionary studies begin by assessing phenotypic variation
within population
Quantitative variation: variation that is measured on a continuum
(such as height) rather than in discrete units or categories
oUsually displayed on bar graph
oNatural selection changes the mean value or a character
oCurve can be broad or narrow, meaning more or less variation in
population
Qualitative variation: variation that exists in two or more discrete
states, with intermediate forms often being absent
Polymorphism: the existence of discrete variants of a character
among individuals in a population
oWe describe traits as
polymorphic
oDescribe phenotypic polymorphisms by calculating the
percentage/frequency
of each trait
Phenotypic variation can be caused by
oGenetic dierences
oEnvironmental factors
oInteraction between genetics and environment
o
Thus, not all genetic/phenotypic variations are perfectly
correlated
Organisms with dierent genotypes often exhibit same phenotype
oEx. Heterozygous vs homozygous dominant
Organisms with same genotype sometimes exhibit dierent phenotype
oEx. Acidity of soil in6uences 6ower colour in some plants
Only genetically based variation is subject to evolutionary change
oPhenotype of an individual organism, rather than its genotype
that is successful or not
oNatural selection operates on phenotype, not genotype
Farmers may maximize yield by fertilizing and irrigating more
productive genotype of wheat
