Cauliflower, cabbage, broccoli, savoy cabbage, kale, Brussels sprouts, and many other vegetables are all the exact same plant, Brassica oleracea. The only difference is how they are cultivated. 1)
Today we have over 5,000 different types of potatoes. However, they all are derived from the original species of wild potatoes from a small region of the Andes. All ancestral potatoes contain high levels of the glycoalkaloids toxin, and eating them in high quantities could lead to paralysis and even death. Andean inhabitants ate the potatoes with a clay saucer, which helped to neutralize toxins and allowed the domestication of potatoes around 7,000 to 10,000 years ago. Even today's potatoes contain small amounts of toxins, but they are negligible to human health. However, when raised or stored incorrectly, they can still become poisonous. 2)
Tomatoes are fruits, specifically a type of edible berry. Tomatoes (Solanum lycopersicum) are closely related to potatoes (Solanum tuberosum) and eggplant (Solanum melongena); however, we eat different parts of the plants (tubers vs fruits). Like other members of Solanum family, tomatoes contain toxins, including tomatine and solanine. However, the amounts are very low. 3)