The biologist who is known as the Darwin of the 20th century is Theodosius Dobzhansky. He was a geneticist and evolutionary biologist who made significant contributions to the field of evolutionary biology.

Early Life and Education
Dobzhansky was born in Ukraine in 1900 and later moved to the US, where he completed his education. He received his Ph.D. in genetics from Columbia University in 1927.

Contributions to Evolutionary Biology
Dobzhansky's most significant contribution to evolutionary biology was his concept of "evolutionary conservatism." He argued that even though species change over time, they still retain fundamental characteristics that have been conserved over long periods of evolutionary time.

He also made significant contributions to the study of the genetics of natural populations. He conducted experiments on fruit flies, which helped to elucidate the mechanisms of genetic variation and natural selection.

Dobzhansky was a vocal advocate of the theory of evolution and wrote extensively on the subject. His most famous book, "Genetics and the Origin of Species" (1937), was instrumental in the development of the modern synthesis of evolutionary theory.

Legacy
Dobzhansky's work had a profound influence on the field of evolutionary biology. His ideas on evolutionary conservatism and the genetics of natural populations are still widely studied and discussed today. He was also a prominent figure in the public debate over evolution, and his advocacy helped to promote the acceptance of evolutionary theory in the US and around the world.

Conclusion
In conclusion, Theodosius Dobzhansky was a biologist who made significant contributions to the field of evolutionary biology. His ideas on evolutionary conservatism and the genetics of natural populations are still relevant today, and he played an important role in the public debate over evolution. He is rightly regarded as the Darwin of the 20th century.

Ernst Mayr is known as Darwin of 20th century. He is known as the father of 20th century as he contributed a lot to the science. He was a leading evolutionary biologist in his time. He promoted and dispersed Darwin's hypotheses.

So, the correct answer is 'Ernst Mayr'