Embryos of different species look very similar, which shows that the organisms share a ancestor.

★ Embryos of different species look very similar, which shows that the organisms share a common ancestor.

Explanation:

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genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).