That's the conclusion reached in a paper published last year in Biology Letters: Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land.
The authors conclude:
The data support the growing evidence that, except following mass extinctions, tetrapod diversity was primarily achieved by unrestricted expansion into empty ecospace, that is by the filling of unrealized modes of life, and multiplying into already realized modes. As taxonomic diversity has increased, there have been incentives for tetrapods to move into new modes of life, where initially resources may seem unlimited, there are few competitors and possible refuge from danger. And as ecological diversity increases, taxa diversify from their ancestors at a much greater rate among faunas with more superior, innovative or more flexible adaptations.In laymen's terms, they're saying that evolution is driven not by competition, but by the lack thereof, when species escape direct competition by moving into new, unoccupied niches.
The dearth of evidence supporting large-scale biotic replacements driven by competition is nothing new; as the paper notes, Stephen Jay Gould noticed this back in1980, and he wasn't alone. And it's long been understood that when selection pressures are eased (say, by moving to another ecospace), all species will over-reproduce (which probably increases adaptive opportunities). What's new here is the bold claim that this expansion of opportunities, rather than the constraint of competition over resources, appears to be a prime mover of tetrapod evolution.
The paper has since been cited only six times on Google Scholar, including in one paper published just today. So, it's still too early to tell whether its assessment will have a major impact in our understanding of evolution. But the hypothesis instantly appealed to me, so I am going to keep an eye on it.
Reading it, my first thought was that someone ought to test the hypothesis by applying it to hominin evolution, and see whether the diversification of australopiths and their descendents in the Plio-Pleistocene follows the same pattern. That's when hominins moved out of the forests and onto savannah and woodland biomes... niches that had no prior hominoid primate presence. If it held up at this smaller scale, the hypothesis would receive some firmer experimental support, and also potentially shed some light on just what it was, precisely, driving the rapid hominin evolution of this time period.
That question, in case you didn't know, is the big mystery of human evolution, particularly the evolution of our freakishly large brains. Could our big brain be a consequence of the sudden expansion of hominin eco-space, a brain born out of opportunity rather than struggle? It's certainly romantic to think so. But romance won't cut it in science.
As far as I can tell, no one has run such a test yet. I'd like it to be me, but I currently lack the resources and full training to do it properly. I'll just have to keep my eyes peeled and hope that no one gets to it before I do.