Evolution is the changes in organisms over generations as a result of genomic variations.

Natural selection is the non-random increase in frequency of DNA sequences that increase survival and the non-random reduction in the frequency of deleterious sequences.

There are three different types of natural selection: stabilising, directional, and disruptive selection. And they all result in different changes in phenotype frequency.

In stabilising selection, an average phenotype is selected for and extremes of the phenotype range are selected against.

In directional selection, one extreme of the phenotype range is selected for.

In disruptive selection, two or more phenotypes are selected for.

There are two types of gene transfer — horizontal gene transfer and vertical gene transfer.

Horizontal gene transfer is where genes are transferred between individuals in the same generation.

This is the type of gene transfer typical of prokaryotes.

Vertical gene transfer is where genes are transferred from parent to offspring as a result of sexual or asexual reproduction.

This is the type of gene transfer typical of eukaryotes.

Natural selection is more rapid in prokaryotes.

Prokaryotes can exchange genetic material horizontally, resulting in faster evolutionary change than in organisms that only use vertical transfer.

A species is a group of organisms capable of interbreeding and producing fertile offspring, and which does not normally breed with other groups.

Speciation is the generation of new biological species by evolution as a result of isolation, mutation and selection.

As long as a population has the opportunity to interbreed and exchange genes, they remain one species.

This is why isolation barriers are important as they prevent the gene flow between sub-populations during speciation.