Also Punctuated equilibrium comes into play, it works basically like this...
A small geographically and genetically isolated population (A)evolves faster than a large population where genes must take longer to reach fixation (B)...
At some point the population (A) overcomes the geographical isolation and being more evolved to suit the environment supplants the population (B)....
What we see in the fossil record unless we get very lucky and dig in the area of geographic confinement is an abrupt transition from the species that comprised population (B) and the initial population of (A) to the species which comprised the population (A) at the time of geographic breakout....
This combined with the rarity of fossilised organisms accounts for the lack of intermediary fossils....