There are two theories about the way that Indo-European languages spread - which one is correct?

Transcript -- Revolution or evolution?

Can Darwin's theory of evolution be applied to languages? If so what are the analogues for natural selection and species diversification? What truths does this approach reveal and what problems does it throw up? In this album Professor Mark Pagel of Reading University and Quentin Atkinson, an evolutionary biologist at Oxford, discuss the pitfalls and the up-sides to approaching language through a Darwinian model. Focussing on Indo-European languages, they show how mathematical and statistical models can be used to study the development of both particular words and of grammatical terms. Looking to the future they speculate on how language will develop in the new globalised culture. 
The tracks on this album were produced by The Open University in collaboration with the British Council. They form part of Darwin Now, a global initiative celebrating the life and work of Charles Darwin and the impact his ideas about evolution continue to have on today’s world. © British Council 2009.