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?