The Trachea
is your answer
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D) allele frequencies of some traits change in the population
Alleles can be phenotype (how they appear) and natural selection is when an organism changes in a way that helps them survive.
It can be the way their beaks are formed that gives them a better chance of catching food among other stuff (Darwin's investigation)
Or how they change their color to adapt to the sudden change in tree coloration (Pepper moths)
