Natural selection always requires thousands or millions of years to produce measurable evolutionary change.

Evolutionary theory was born in deep time. Fossils and geological succession emphasized changes unfolding across immense intervals, so natural selection was often pictured as a process too slow to watch directly.

But evolutionary speed depends on generation time, heritable variation, and the strength of selection. Microbes exposed to antibiotics and insects exposed to pesticides supplied dramatic examples: allele frequencies could change rapidly when survival differences were large.

Long-term field studies then measured comparable shifts in wild vertebrates. Traits in birds, fish, and other animals changed detectably across only a few generations as environments and selective pressures changed.

Experimental evolution made the process even easier to observe by following populations under controlled conditions and measuring genetic change through time.

None of this means every evolutionary transformation is fast. Large anatomical changes, ecological transitions, and speciation can require long periods, and short-term responses can reverse when environments change.

The correction is to always. Natural selection can produce measurable evolutionary change within years or decades; deep-time evolution and rapid contemporary evolution are different timescales of the same process.