1. The Variability of Natural Populations

Evo. Pop. Gen.

Classical Hypothesis - Fisher and Muller

• Worked with single gene traits in natural populations

• Low population variability, low heterozygote frequency

• Alternative alleles are harmful

• Main evolutionary force- directional selection

• Species variability given mainly by between-population variation

Balance hypothesis - Dobzhansky and Wright

• Examined the chromosomal polymorphisms in Drosophila populations

• Natural populations are generally variable w/ high heterozygosity

• Dominating force: Overdominant selection

• Species variability given within-population variation

Phenotypic variation

Variability in phenotypes that exists in a population. (Ex: height, weight, and body shape, hair, eye color, and the ability to roll your tongue)

Qualitative Traits

A trait that can be described as a category (Ex: black or red coat color, horned or polled, coat color dilution)

Example: Stem color of tobacco

• Have 1 or 2 genes

• Have few genotypes

• Phenotypic categories

Quantitative traits

A characteristic controlled by multiple genes as well as the environment (Ex: IQ, blood pressure, height, and weight)

Example: Number of grains of wheat

• Have many genes

• Many genotypes

• Continuous distribution of the phenotypes

Peppered moth (biston betalaria)

• Example of a qualitative trait

• Involves two phenotypes: peppered (recessive), and carbonara (dominant)

Wing traits of brown argus (Tricia artierxes issuekutzi)

• Example of a quantitative trait

• In small populations, evolutionary forces (ex: environmental changes) have a bigger effect, causing more drift and variation

Birth mass of mice

• Example of a quantitative trait

• Each population was found to have its own phenotypic average, and its own variance (differences)

What makes up phenotypic variation?

Genotypic variation and Environmental variation

Genotypic variance

The final phenotypes of heterozygous offspring (Example: AA, Aa, aa)

Environmental variance

Causes continuous distribution in final phenotypes of heterozygous offspring (Example: all the different varieties of Aa)

Phenotypic Variance equation: 

Vp = Vg + Ve + Vge

Phenotypic variance= genotypic+ environmental+interaction

If Vge = 0

Genes and the environment do not interact

If Vge > 0

There is interaction between genes and the environment

Genetic variance equation

Vg = Va + Vd + Vi

* Interactions between genes

Additive model

Deviation from the avg. phenotype due to inheriting a particular allele

* Additive interaction between alleles: heterozygous individuals are a mid-point between the alleles

Dominance model

Deviation due to the interactions between alternative alleles at a specific locus (location on a chromosome)

* Heterozygous individuals resemble each other more