According to current consensus, the Finnish population has gone through several bottlenecks. There might have been just two surviving paternal lineages from the bottle neck 4000 years ago. And after that, there were plenty of times when only a 1/3 or less survived the famines related to wars and plunders.

So, interestingly, Finnish DNA is an isolate but highly diverse within itself.

See e.g https://www.nature.com/articles/ejhg2016205