Answer :

Different members of a species may have different traits, and some will be better than others in helping the organism survive. Basically, the traits that help one survive will let it live on to pass that trait to its offspring, but the organisms with traits that make life harder or possible will not live to be passed on.
So with natural selection, NATURe SELECTs the best-fit organisms of a species to live. Over time as the bad traits die off and the good ones prosper, that's them evolving

So in an enviroment the ones with a better adaptations will survive and pass on thier traits to the next generation. If a group of rabits have smaller feet and hop faster than other jack rabbits, then they will survive when chased by predators, and pass on thier traits, while others would get eaten. Over time the generations would have more rabbits with small feet and faster speeds until all rabbits have that trait.  

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