Cofty, magnificent! Thankyou.
I was thinking of an event currently happening before our very eyes. Ash die-back is the species succumbing on a huge scale to the fungus Chalara fraxinea. This is resulting in huge swathes of ash trees withering to nothing.
However, some ash plants have an immunity to this fungus. The result will be that the remaining ash gene pool will have been selected for survival from this fungus. There will have been a huge scale short timespan event leaving a stronger sub-species. Evolution as we watch.