In the first decade of twentieth century, Hugo de Vries based on his work on evening primrose brought forth the idea of mutations – large difference arising suddenly in a population. He believed that it is mutation which causes evolution and not the minor variations (heritable) that Darwin talked about. Mutations are random and directionless while Darwinian Variations are small and directional. Evolution for Darwin was gradual while Hugo de Vries believed mutation caused speciation and hence called it saltation (single step large mutation).
In a given population one can find out the frequency of occurrence of alleles. This frequency is supposed to remain fixed and even remain the same through generations. This principle says that allele of a gene or a locus frequencies in a population are stable and is constant from generation to generation. The gene pool (total genes and their alleles in a population) remains a constant. This is called genetic equilibrium.
Sum total of all the allelic frequencies is 1 (p + q = 1). Hence, p2 + 2pq + q2 = 1. This is a binomial expansion of (p +q)2. When frequency measured, differs from expected values, the difference (direction) indicates the extent of evolutionary change.
Five factors are known to affect Hardy-Weinberg equilibrium. These are gene migration or gene flow, genetic drift, mutation, genetic recombination and natural selection.
If the same migration occurs by chance, it is called genetic drift. Sometimes the change in allele frequency is so different in the new sample of population that they become a different species. The original drifted population becomes founders and the effect is called founder effect. A critical analysis makes us believe that variation due to mutation or variation due to recombination during gametogenesis, or due to gene flow or genetic drift results in changed frequency of genes and alleles in future generation.
Natural selection can lead to stabilization (in which more individuals acquire mean character value), directional change (more individuals acquire value othe than the mean character value) or disruption (more individuals acquire peripheral character value at both ends of the distribution curve).