Let us explain a few terms before we start talking about microevolution.

A population is a group of individuals of the same species living in the same area at the same time. For example:

(a population of worms in a lawn)

Haploidal organisms, for example, bacteria, have a single set of genetic information. Most of organisms are diploidal. There also exist polyploidal organisms, having three or more sets of chromosomes, for example, wheat, tobacco, and potatoes.

Diploidal organisms have two sets of chromosomes in their cell nuclei. A chromosome from one set has a corresponding one in the other set. They both form a pair of homologous chromosomes.

Homologous chromosomes encode the same type of genetic information and are of the same size and shape. Chromosomes contain genes.

A gene is a section of the double helix of DNA built from a certain number of nucleotide pairs. The nucleotides are arranged in a strictly defined sequence. A gene occupies a prescribed position in a chromosome – a locus. Variants of genes that occupy the same positions in homologous chromosomes are called alleles.

Alleles decode different variants of the same trait. A hypothetical example: there could be three alleles of a gene encoding the color of an iris in a given population: allele decoding brown, blue and green.

Genes of all the individuals in the whole population form a gene pool.
(A made-up example) A mouse is diploidal, so all its traits, such as the color of its fur, for example, are double encoded. Let us assume that in a population of mice there are two alleles of the gene responsible for the fur color – white W and black B. All possible combinations of these alleles, WW, BB, and WB, are all possible genotypes. The dominating gene will determine the color of a mouse. There will be two phenotypes in this population: a white mouse and a black mouse. If the white gene is dominating, a mouse with the WB or WW genotype will be white and a mouse with the BB genotype will be black.

The above example is simplified. Genes can encode more than one trait and a trait can be encoded by more than one gene. There are also other than dominance and recessivness dependencies among alleles when determining a trait.