A noted geneticist and evolutionary biologist, Theodosius Dobzhansky is renowned for his work in bridging Darwinian evolution with Mendelian genetics and paving the way for the science of the modern evolutionary synthesis.
In 1937, Dobzhansky published a book on evolutionary theory called "Genetics and the Origin of Species". In this book, he laid the foundation for modern synthetic theory of evolution, by explaining the role of genetics in evolution by natural selection. Around the time this book was published, some researchers had begun to consider Darwin's evolutionary theory rather dated. It didn't, after all, appear to have an answer for the new genetic discoveries that were being made.
Dobzhansky studied population genetics in fruit flies and showed that the diverse genes in successfully evolved species helped them to adapt to different environments and conferred them with the genetic diversity that helped them to survive. Through his work he was able to show a single genetic mutation or change proceeded to affect the rest of the genetic arrangement and how this then led to an evolutionary change in the species. He explained that natural mutations occurred in species all the time, some harmful, some giving rise to gradual genetic changes. When individuals in a population with these mutational changes mated solely with one another, they gave rise to a new species with a genetically distinct identity.
Dobzhansky was one of the first Drosophila researchers to conduct field research on fruit flies. He travelled to Brazilian rain forests as well as around Mexico, Arizona and California in his search for different fruit fly species. He discovered that there were genetic similarities between different fly species from the same region. The same fly species from a different region were different genetically.