Post on 19-Dec-2015
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Variation
Continuous phenotypic variation
• The most common variation acted on by natural selection.
• Phenotype of individuals determined by measurement.
• Changes and patterns of variation determined statistically.
Distribution of the heights of 1,000 Harvard College students aged 18 to 25
(Adapted from Castle.)
Quantitative phenotypiccharacters: common in nature
Range of variation
Note shape
A contrast toMendelian genetics
White spotting in Dutch rabbits: a quantitative character
N = 825
The results of crosses between two strains of wheat differing in three gene pairs that determine grain color
Possible genetic basisfor some continuouscharacters
Contributing and noncontributing alleles
No dominance
Each contributing allelemakes a small contribution to the phenotypic expression
Note shape
The mathematical model of continuous character inheritiance
• Two kinds of alleles• contributing: a+, b+, c+, etc.• non-contributing: a, b, c, etc.• Genes a, b, c, etc. in different chromosomes.• Locations in chromosomes called Quantitative Trait
Loci (QTLs)• Binomial expansion emulates independent
assortment of the alleles.
Percent survival of D. melanogaster flies with 16 different combinations of chromosomes exposed to a uniform dose of DDT
Length of corolla
Determining heritability: h2 = VG/(VG + VE)
One method:
BUT!
No recombination
Recombination
Arrangement of phenotypic variation in natural populationsSome recognize subspecies Recognizable subspecies would have to be allopatric.
Problem: incongruence of characters
• Human race concept.• There is no satisfactory biological definition of a
human race!• Misconception: there are character states unique to
particular groups of humans• The characters traditionally used are quantitative
characters with continuous variation.
“Racial” characters arequantitative characterscontinuous characterse.g. skin color
Phenotypic expressionin and among populationsgenerally fits a normal distribution
A common “racial” characteristic is skin color.
Melanosomes
• All individuals have approximately the same number of melanocytes.
• Therefore, what is the basis of skin color differences?• Can “races” be recognized based on skin color?
These groups easy to identify because of non-overlapping variation.
Gaps
685 nm
Skin color in 22 human populationsSamples of malesMean +/- one standard deviation
Global Patterns of Human Variation
• Can be examined genetically• Can be examined phenotypically• Are phenotypic differences concordant with genetic
differences?
Eight classes of geneticSimilarityenzyme & blood group loci
Arrayed by increasingdifference
Distribution of eight classesof skin pigmentationintensity
Concordance ordiscordance?