Changes (albeit over thousands of years) in lake level would impose a new selectional pressure and drive morphological changes, such as alterations in shell structure, in the species present.
The case for punctuated equilibrium falters still further because the types of shell shape change seen in the Lake Turkana sequence might not be species alterations after all. Present-day fresh-water mollusk species show remarkable plasticity of shell from when they grow in various water conditions.
Differences in various water hardness. Bottom substrates and other environmental influences can trigger phenotypic variability within the populations of a single species, such as Britain’s common pond snail, Lymnaea peregra, with the result that shell from can vary widely