Originally posted by wildgrass
Why would that be probable?
If a species is well-adapted, the conditions should select against any significant genetic changes.
Firstly, the abundance of life demonstrates that for any given environment many possible forms can be 'well-adapted' to the conditions. Genes come in all shapes and sizes. Some are highly conserved because even minor changes are catastrophic, others may have many variants without causing major problems. Genetic drift is normal for a large proportion of the genome.
Even if basic appearance of an animal doesn't change significantly, minor changes exist and more subtle changes happen too, ones that cannot be seen from fossilized remains.
Lets take the crocodile that some might say has been around for millions of years relatively unchanged. Yet look around the world and you will find many different species of crocodile, and as you look closer you will find very significant differences between them. Some for example are adapted to saltwater living. You will probably find that crocodiles are as genetically dissimilar from each other as the great apes.