Adaptation of a species to its enviroment which leads to newer species.
With complexer organisms that takes millions of years
However, put Escherichia Coli in a culture and let it fill most of the culture. add Peniciline to the culture. 99.99999...% will die. But there is a 0.000...1% which accidently had a genetic mutation which makes it produce a certain enzyme that blocks the function of peniciline in the cell, preventing it to do its work.
From these surviving E.Coli, an entire culture will grow that has the peniciline resistance.
THE intelligent design question, is "how do you explain an eye", as an eye is an incredibly complex organic structure.
You could be lazy, and respond "why are we full of nonsense coding dna ? Why are we full of nonactive dna sequences ?
Or you could take a biology book and make a timeline, explaining how you start with a simple thing, and becomming more complex by chance(genetic mutation), allowed this to outcompete the existing kind