G. H. Hardy and W. Weinburg applied Darwin’s theory to show that although genetic change happens at each parent/child exchange, the composition of a genepool is not changed.  They showed this is the case in a population for which the following 5 conditions may be found- 

1. No extreme characteristics occur
2. All offspring are equal and have equal ability to survive and reproduce.
3. There is no order in mating.
4. The population is large enough that chance is unlikely to alter occurrence or proportions.
5. There is no migration of individuals into or out of populations.

• p² + 2pq +q²=1

The equilibrium has proven to be a valuable contribution to population genetics and also to the understanding of heredity, which now has helped to form the modern evolutionary theory. 

In the 1920’s and 30’s, three men - R.A. Fisher, J.B.S. Haldane and Sewall Wright formulated a more modern version of Darwin’s theory.  Their theory is as follows: a population of inbreeding plants or animals can be called a genepool.  Sexual reproduction keeps genes in a particular pattern and any new genes appearing are, therefore, the result of mutations.  The mutant will reproduce, and the gene will become more frequent within the species.  The new genes may cause changes to the body which may make the organism better at surviving and reproducing.  Evolution could therefore be defined as the process for which gene-frequencies change within the population.  This change in frequency may affect immigration, emigration, natural selection and random drift throughout populations. 

Natural selection therefore changes the path of survival of a gene within the genepool.  When environmental changes occur, an evolutionary change occurs as well. 

This more complicated theory enables us to form many new ideas related to evolution and, in the years since, a more complete story of the evolution of plants and animals has been formed.