TABLE 1.

Selective forces, bacterial shapes, and possible rationales

Selective force	Shape example	Possible rationale
Nutrient limitation	Smaller cells	Greater surface-to-volume ratio
	Filaments	Increased total surface area
	Prosthecae	Increased total surface area
	Extremorphic	Storage capacity of giant cells
	Pleomorphic	?
Cell division	Geometric symmetry	Equal segregation to daughters
	Uniform width	Cell division apparatus
Attachment	Rods	Cell-to-cell, fluid shear
	Filaments	Resistance to fluid shear
	Prosthecae	Elevate in aqueous environment
	Miscellaneous	Biofilms
Passive dispersal	Small cells	Effect of Brownian motion
	Cells of various widths	Different flotation requirements
	Small cells	Flow through geological strata
	Larger cells	Entrapped in geological strata
Active motility	Larger rods	Effect of Brownian motion
	Medium rods	Efficiency of general motility
	Rods of various widths	Chemotaxis, different gradients
	Rods of various lengths	Motility near solid surfaces
	Helical rods	Motility in viscous solutions
	Rods or filaments	Gliding by slime extrusion
	Rods or cocci	Pilus-directed twitching
Polar differentiation	Rods or filaments	Stable multiprotein complexes
Predation	Smaller cells	Escape predator contact/capture
	Larger cells	Too large to capture or digest
	Filaments	Too large to capture or digest
	Prosthecae	Too large to capture or digest
	Helical rods	Escape predator internalization
Differentiation	Rod to coccus	Slow-growth conditions
	Rod to filament	Low-nutrient conditions
	Bifids (Y shapes)	More polar-localized complexes
	Swarm cells	Increased motility, attachment
	Filamentation	Defense during pathogenesis
	Miscellaneous	Multicellular adaptations externally imposed (?)