Molecular evolution is the process of change in the sequence composition of molecules such as DNA, RNA, and proteins across generations. It uses principles of evolutionary biology and population genetics to explain patterns ill the changes of molecules. One of the most useful advancements in the development of molecular biology is proteins and other molecules that control life processes are conserved among species. A slight change that occurs over time in these conserved molecules (DNA, RNA, and protein) are often called molecular clocks. Molecules that have been used to study evolution are cytochrome c (respiratory pathway) and rRNA (protein synthesis).

In this type, hybrids are formed due to interspecific crosses but they are sterile due to the failure of the chromosomes to segregate normally during meiosis, example Mule (interspecific cross between a horse and donkey). It is a post zygotic isolation mechanism.

(a) The diagram indicates gene flow
(b) Movement of genes through gametes or movement of individuals in (immigration) and out (emigration) of a population is referred to as gene flow. It can be a strong agent of evolution.

The term evolution is used to describe heritable changes in one or more characteristics of a population of species from one generation to the other.