Simplify the following text for a Grade 10 native English-speaking person.
Original: In 1937 he published one of the major works of the modern evolutionary synthesis , the synthesis of evolutionary biology with genetics , entitled Genetics and the Origin of Species , which amongst other things defined evolution as `` a change in the frequency of an allele within a gene pool '' .
Simplified: In 1937 Dobzhansky published one of the major works of the modern evolutionary synthesis , entitled Genetics and the Origin of Species . He defined evolution as `` a change in the allele frequency within a gene pool '' .