Quick, presumably basic question regarding the evolution of a specific species.

 

Why is grass still grass? (and by the same token a single cell organism still single celled organism)

 

Because amirite in presuming the latter in the evolution scale almost inevitably replaces its source. If grass was to evolve and survive why are there no jumanji style things alive?

A mutation will only become prevalent and eventually fixed if it confers a selective advantage on its bearer - i.e. the mutation allows you to have more progeny than your cogeners. This means that for a particular niche, there are likely to be optimal phenotypes. Crossing the saddle between two optima might involve too much negative selection for it to be possible, hence stabilizing /purifying selection will retain the predominate, successful phenotype.

http://en.wikipedia.org/wiki/Stabilizing_selection

 

or in simpler terms, the current grass phenotype is better than any of the random phenoptyic deviations from it that evolution has afforded the grasses.

 

That said we've got phenotypes ranging from 30m tall bamboo, to obligate aquatic forms, to lawns - adaptations like blades, spears aerial roots giant flowerheads, c3 and c4 pathways, so there is a pretty substantial diversity of successful phenotypes in the grasses.

http://en.wikipedia.org/wiki/Poaceae