Mayr's work contributed to the conceptual revolution that led to the modern evolutionary synthesis of Mendelian genetics and Darwinian evolution and to the development of the biological species concept. He is known for his theory of allopatric speciation (speciation by geographic isolation) which is considered the basis of the theory of punctuated equilibrium.
Also, he was writing about systematics (the modern version of taxonomy) long before anyone had a clue about what he was talking about, something he's quite aware and proud of:
Now, in a way I'm sometimes surprised at how advanced I was in my 1942 book Systematics and the Origin of Species. That was quite a bit ahead of its time. I had no teacher who was that much ahead. How I could see things in such a modern way I still don't understand. But I did.Mayr is also an outspoken historian and philosopher of biology who rejects a lot of the reductionism that characterizes much of biology today, namely molecular biology and the infatuation with individual/isolated genes (he was critical of J. B. S. Haldane's "bean-bag genetics" way back when, and his work runs counter to a lot of Dawkinsian neo-Darwinism). You've got to look at the entire genotype when it comes to evolution, argues Mayr. When it comes to molecular biology, he says:
The funny part is that molecular biology has a remarkably small impact on the theory structure of biology. At least that's the way it looks to me. Of course, they can point out that the genetic code has shown that life as it now exists on the earth could have originated only a single time, otherwise it wouldn't be the same code that it is. And of course there are several other things that molecular biologists have contributed. But none of them really touched the theory structure of the Darwinian paradigm, in my opinion.And when asked by SciAm, "What do you think the major questions, or even a single question, for a young researcher today is. Where would you point that person?," Mayr responded:
Well, you know, the genotype. I'll mention something that nobody ever mentions. Let's say you have now a genotype that makes a certain protein. And that protein, and you can see this in every issue of Science practically, that protein is a very complex structure, incredibly complex. Now how that step from a group of amino acids to that polypeptide [happens] is an enormous jump. I think right now everybody leaves it alone because nobody yet has figured out just exactly how to attack this problem.