“I can see no difficulty in natural selection preserving and continually accumulating variations.”
“An extraordinary amount of modification implies an unusually large and long continued amount of variability, which has continually been accumulated by natural selection for the benefit of the species.”
As natural selection acts continuously, the process is to continus without limits. Darwin explains –
“What limit can be put to this power, acting during long ages and rigidly scrutinising the whole constitution, structure, and habits of each creature, favouring the good and rejecting the bad? I can see no limit to this power, in slowly and beautifully adapting each form to the most complex relations of life.”
Contrary to Darwin’s premise on the continuous action of natural selection without limits, however in an article in the journal Evolution,* the North American freshwater fish Centrarchidae diversity has been found to be constrained.
“The decline in fitness with increasing distance from an adaptive peak can constrain morphological divergence between sister lineages… These findings indicate that piscivory has curtailed the diversification of feeding morphology in the centrarchids”
Evidence from this freshwater fish fails to demonstrate that natural selection acts continuously without limits as hypothesized by Darwin.
* Evolution 63, 1557 (2009)