The basic framework

To understand the basics, we'll start with a numerical example using some data on Drosophila pseudoobscura that Theodosius Dobzhansky collected more than 50 years ago. You may remember that this species has chromosome inversion polymorphisms. Although these inversions involve many genes, they are inherited as if they were single Mendelian loci, so we can treat the karyotypes as single-locus genotypes and study their evolutionary dynamics. We'll be considering two inversion types the Standard inversion type, $ST$, and the Chiricahua inversion type, $CH$. We'll use the following notation throughout our discussion:

Symbol	Definition
$N$	number of individuals in the population
$x_{11}$	frequency of $ST/ST$ genotype
$x_{12}$	frequency of $ST/CH$ genotype
$x_{22}$	frequency of $CH/CH$ genotype
$w_{11}$	fitness of $ST/ST$ genotype, probability of surviving from egg to adult
$w_{12}$	fitness of $ST/CH$ genotype
$w_{22}$	fitness of $CH/CH$ genotype

The data look like this:⁶

Genotype	$ST/ST$	$ST/CH$	$CH/CH$
Number in eggs	41	82	27
	$Nx_{11}$	$Nx_{12}$	$Nx_{22}$
viability	0.6	0.9	0.45
	$w_{11}$	$w_{12}$	$w_{22}$
Number in adults	25	74	12
	$w_{11}x_{11}N$	$w_{12}x_{12}N$	$w_{22}x_{22}N$