From ScienceDaily: Evolutionary Process More Detailed Than Previously Believed, Study Shows
In observing yeast cells, scientists have realized that evolution doesn't happen in a straight line. The common model is that more useful mutations will disseminate through a population before developing further adaptations. However, Dr. Katy Kao at Texas A&M University and Dr. Gavin Sherlock at Stanford have realized that "evolutionary process [is] much more dynamic than initially thought." They witnessed separate yeast populations with different beneficial mutations compete with each other. One population that was almost eliminated for being less competitive was able mutate just in time to save itself.
Pretty interesting. With all those pictures of the lineage of species showing straight or forked lines, I definitely thought species moved as a whole or just eventually became other species. Now, it seems silly to think that there could only be one mutation at a time.