Why organisms reproduce

A cat has kittens, not puppies. Offspring look like their parents. But unlike nutrition or breathing, reproduction is not needed to keep one individual alive. It keeps the species going: without it, a kind of animal or plant would die out.

The instructions for building a body are in the DNA in the nucleus. DNA is the blueprint: it tells the cell which proteins to make, and so how the body is designed. To reproduce, a cell makes a copy of its DNA, makes extra cell parts, and then divides in two.

"Why copy the DNA instead of splitting it?" If you tear a book into two halves and give one to each friend, neither of them has the whole book. Photocopy it, and both have everything. The cell does the same: each new cell gets a full copy.

No copying is perfect. Think of a phone passcode: 1-2-3-4 opens the phone, but 1-2-4-3 doesn't. A small change can make a real difference. Small changes in copying give variation: the offspring are similar, but not exactly the same.

"Is variation good or bad?" Both, but it helps the species. Each species lives in its own niche (its place and way of life). Accurate copying keeps it well suited to that niche. But niches can change. Suppose bacteria live in cool water and the water warms up because of global warming: most die, but the few variants that can stand heat survive and multiply. Variation doesn't protect every individual, but it keeps the species alive.