Gray Tree Frog (hyla versicolor) and Cope's Gray Tree Frog (hyla chrysoscelis).
They look identical but h. chrysoscelis has 24 chromosomes and h. versicolor has 48 chromosomes. H. versicolor evolved when two tetraploid h. chrysoscelis individuals met and bred. The two species have different calls and that is the only way for a layman to tell them apart. They do not interbreed, obviously.
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