Two evolutionary arguments also support the utility of
bacterial shape. Firstly, shape has a vector through evolutionary
time—rod-like organisms having arisen first and
coccoid forms being derivatives at the ends of evolutionary
lines [7–11]. Progressive development of a trait
implies that selective forces are operating. Secondly,
prokaryotes with different genealogies may converge
morphologically, indicating that a similar shape may
confer advantages in certain environments. So, for
example, although they have a non-peptidoglycan-based
cell wall, the Archaea exhibit a range of morphological
forms similar to that of the bacteria [12]. The simplest
conclusion is that morphological adaptation serves an
important biological function