What happened to the greenish warbler birds as they migrated from the southern portion of their range up around the tibetan plateau? a. they evolved separately as they moved north and became two separate species. b. the original species died off, but the ones that were able to adapt to a northern climate evolved into a new species. c. as a whole the species was able to adapt to a colder climate, but no significant changes occurred. d. they were unable to adapt and evolve to ensure their survival and became extinct.