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== Challenges to stasis ==
Although on the whole stasis does appear to be a pervasive pattern in the fossil record, there are exceptions that provide evidence for more infrequent gradual change, with some caveats.  Some of the prominent instances of gradualism come from fossil foraminifera (Gould and Eldredge, 1977).  When considered over geological time some tiny foram tests do appear to change gradually; however, one potential difficulty with such studies is that they can usually only consider fossil forams from one or a few sediment cores, while the geographic ranges of some of these foram species extended across the entire Pacific Ocean.  This made it hard to determine if such morphological changes reflect shifts in the entire species, or in only smaller parts of the species ranges.  Ultimately, whether change is consigned to a single geographically restricted population or occurs across the entire species is given added relevance when we consider the mechanisms that cause stasis (discussed more fully below).

Two of the studies that have most prominently challenged stasis are Gingerich’s (1976) work on fossil mammals from the Bighorn Basin of the western United States and Sheldon’s (1987) (Fig. 6) work on Ordovician trilobites from Wales.  Both studies concluded that species can show gradual shifts in morphology throughout their history.  Each of these studies did only sample a part of the known species’ geographic range; again, this means the changes observed might just be shifts within single populations, not the entire species.  

[[Image:Sheldontrilos.gif|thumb|400px|right|Patterns of gradual change in Ordovician trilobites from Sheldon (1987), used with permission, Macmillan Publishers Limited: ''Nature''.]]