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…into numerous ecological niches?
Under times of extreme ecological distress such as this, how far would, say, five million years of evolution get you? How much diversification.
This is research for a novel. Thanks!

Meaning, does evolution occur with a purpose? I understand that there is no true purpose as to why evolution takes place, but, to my knowledge, it can be labeled as a process. For example, the panda bear with the “thumb” (a.k.a. the sesamoid bone). Would the panda acquire that trait that would cause the bone to evolve if he had not migrated away from his original niche? The environment does stem to the members of the species adapting some physical traits, but their offspring will not automatically be born with those traits. Thus, what I am essentially asking is does evolution occur for a reason? Does variation play a significant role since without variation, all species members of a species would be the same and if they were all identical and they shared traits selected for, then that would cause that group of species to disappear? Therefore, evolution occurs to prevent extinction? Am I right? I know this is kind of confusing, but some please, please help me!!

Which group of animals evolved to fill the niches left following the extinction of the dinosaurs?
Answer
a.
birds
b.
mammals
c.
fish
d.
amphibians

I’m planning to write a post-apocalyptic story about the adventures of animals living after humanity destroys itself in a nuclear war. I’ve decided to have many of the general groups of invertebrates, amphibians, and reptiles survive in my story, but I’m having more trouble deciding on what types of fish, birds, and mammals would live.
The fish that I decided that I would use so far are the ones like lobe-finned fish, sharks, rays, jawless fish, seahorses, and mummichogs, but I can’t think of much else, because don’t know the exact nature of the impact that an event like this would have on modern marine life. With mammals I’ve got species like rodents, shrews, bats, pigs, wolves, and honey badgers, but again, I’m stuck. Birds are the ones that are giving me the most trouble, crows are actually the only birds that I’ve really decided on so far. So what other kinds of birds are likely to be post-apocalyptic survivors?
Also, what types of animals would likely evolve to replace large land predators if those were to go extinct? It’s just that I once read an old speculative biology book about how animals may evolve 50 million years in the future, and it had rats replace the large predators, but I also read on the Internet that there were more likely candidates to fill in that niche, but I don’t know exactly which.

a. melting of glaciers and permafrost
b. loss of high altitude species
c. increased productivity of food crops in the short term
d. increased drought and forest fires
e. all of the above are true
2) All of the following characteristics would make a species more prone to extinction except
a. low population density
b. specialized niche
c. short generation times
d. small population size
e. low reproductive rates

Which statement best describes extinction?
A) Extinction is very rare and was not seen at all until the last few hundred years.
B) Extinction is most common for generalist species that have large niches.
C) Extinction is always caused by human interferences to an ecosystem.
D) Extinction events have happened throughout all of Earth’s long history.
E) Extinction increases Earth’s biodiversity over long time periods
I am torn between D and E. It could be E because my text book says “The balance between the evolution of new species and extinction of existing species is what creates earth’s biodiversity.” So I don’t know which one best describes extinction. What do you think?