SURVIVAL OF THE FITTEST
what dose it mean to be "fit"
the availability of smaller seeds decreases. the population of birds with smaller weaker beaks declines until eventually they die off. this leaves only the birds with thicker stronger beaks surviving.
in this situation the thicker stronger beak is the helpful trait because it makes it easier to eat the seeds available in this environment
if a species struggles to survive in an environment, over hundreds of years it can adapt to make it easier to survive and reproduce