Which of the following best defines a gene pool

The answer is that a "gene pool is set of all genes, or genetic information, in any population in a species" Gene pools are always found in different species or any organisms.

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Eukaryotic cells are theorized to have evolved from prokaryotes called the endosymbiotic theory. this explains that the most primitive eukaryotic cell engulfed a prokaryotic cell (by the process of phagocytosis) that is capable of cellular respiration and another prokaryotic cell that is capable of photosynthesis. these prokaryotic cells eventually became organelles and these organelles are the mitochondria and the chloroplast