How can evolution explain how organisms have survived over geologic time?

Answer 1

I presume you mean to ask "Why is it that some organisms are unchanged even after geologic time periods while others have been greatly altered?"

Entire books can (and have) been written about this subject.

Evolution - despite what some people think of it - does not *require* change to occur. In fact, in a stable environment it tends to discourage it. However, if change occurs and works better than what is already present, evolution encourages the change to remain, eventually replacing or diverging from the former (unchanged) population.

If the environment in which an organism exists is relatively stable over geologic times and the organism is well adapted to said environment, most changes will be discouraged by evolution. This is because if the organism is well adapted to the (stable) environment, then any changes from it are likely to be away from well adapted - and thus a disadvantage. Those with the change likely to become a snack (due to a change that makes it easier for predators to find them) or malnourished (due to a change that made it harder to find / consume food), or otherwise at a disadvantage. Such creatures are less likely to live to be old enough to breed, and if by chance they do breed, their children are themselves unlikely to live long enough to breed - and so forth each generation until the changed sub-species is extinct.

Most creatures that are unchanged after geologic times live in very very stable environments - the bottom of a sea (ceolecanth), the tropic jungles, and so forth. If the area is isolated it helps, as it discourages change (lack of new predators / prey entering the area, such as with Australia, madagascar, New Zealand, and so forth.

The tropic jungle comment may seem strange, but if you consider it you will see that the temperature is the same year round, rainfall is similarly consistent, and so forth. The vegetation can be thick enough and the biodiversity dense enough (all niches filled) to slow the spread of new creatures somewhat.

Evolution only becomes an encourager of change when change is entered into the system from another source - changing seasons, changing competitors / predators / prey, changing vegetative patterns, changing food sources, new niches opening, and so forth. Then evolution rather rapidly encourages new adaptions.