(Disclaimer: I'm not a lawyer)
In most countries, copyright is based on the year of death of the author, not the publication date of the work. To quote Wikipedia:
In most other countries that are signatories to the Berne Convention, copyright term is based on the life of the author, and extends to 50 or 70 years beyond the death of the author. (See List of countries' copyright length.)
Dobzhansky died in 1975 so it will take until 2045 in most countries until his works are public domain. Fisher died in 1962 so it'll take until 2032.
George Orwell, on the other hand, died in 1950, so his works are in the public domain here in Australia where the limit is 50 years (look at ebooks@Adelaide), but not in the public domain in the US, where the limit is 70 years.
Actually, Fisher's work should be in the public domain in Australia (1962+50=2012), The Genetic Theory of Natural Selection is on archive.org, but only the first 1930 edition, not the 1958 second edition (link via Razib Khan).
Maybe no-one bothered to put together a 'nice' edition yet? Edit: No wait, I misunderstood - Australia's 50 years expiration only applies if the author died before 1955 (as in Orwell's case), after that it's 70 years like in the US, so Fisher's work is not in the public domain in Australia.